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An absorptive device for broadband low-frequency sound with ventilation is essential but challenging in acoustic engineering, which is subjected to the narrow-band limitation and difficulty of balancing high-efficiency absorption and excellent ventilation. Here, we have theoretically and experimentally demonstrated an ultra-sparse (with filling ratio of 53.7%) broadband metamaterial absorber which can efficiently absorb (absorptance >90%) sound energy ranging from 307 to 341 Hz, while enabling air to flow freely. The broadband absorber is constructed by parallel coupling four ventilated metamaterials absorbers (VMAs) showing different operating frequencies. Each VMA is composed of three folded Fabry–Perot resonators as paste components, which are patched subsequently to the walls of a waveguide and correspondingly act as dark, middle, and bright modes following the coupled mode theory. In the VMA, the dark mode is highly over-damped to absorb sound energy, while the bright mode is highly under-damped to be an effective acoustic soft boundary, and the middle mode in-between should be slightly over-damped to strengthen the absorptions. Further investigation demonstrates that broadband high-efficiency absorption is robust against oblique incident angles. The proposed VMA provides a clear scheme for efficiently absorbing low-frequency sound while allowing free air flow simultaneously, which may prompt versatile applications in noise control.

On the quest towards efficiently eliminating noises, the development of a subwavelength sound absorber with the capability of free ventilation remains challenging. Here, we theoretically propose and experimentally demonstrate an asymmetric metamaterial absorber constructed by tuned Mie resonators (MRs) with unbalanced intrinsic losses. The lossy MR layer is highly dissipative to consume the sound energy while the lossless one acts as an acoustically soft boundary. Thus, the absorber presents quasi-perfect absorption (95% in experiment) for sound waves incident from the port nearer the dissipative MR and large-amount reflection (71% in experiment) from the opposite port. Moreover, the fluid dynamics investigation confirms the superior character of free air circulation owing to the ultrasparsity (volume filling ratio as low as 5%) of the absorber and its robustness to the velocity of airflows. Due to the multiple-order resonant modes of MR, we further demonstrate the flexibility of a methodology to extend asymmetric absorptions into multibands. Coupled mode analysis is employed to reveal the physical mechanism and further indicates that sparsity can be tuned by attentively controlling the reference leakage factor and intrinsic loss.

In PT-symmetric gratings, asymmetric diffraction can be generated by modulating the ratio between imaginary and real refractive indices. In this paper, a rigorous coupled-wave analysis (RCWA) has been developed to analyze the diffraction properties of acoustic PT-symmetric gratings with two kinds of modulating approaches, including modulating the effective modulus and the effective density. Asymmetric diffraction with both Bragg incident angles and perpendicular incident angles is discussed by using the RCWA method. Results show that the modulation ratio for the diffraction vanishing point changes with the modulation amplitude differently for two kinds of modulating approaches. Moreover, the sound energy will be weaken or be enhanced at Bragg incident angles depending on the sign of incident angles and the modulation ratio.

The sound energy from marine mammal populations vocalizing over extended periods of time adds up to quasi-continuous “choruses,” which create characteristic peaks in marine sound spectra. An approach to estimate animal distribution is presented, which uses chorus recordings from very sparse unsynchronized arrays in ocean areas that are too large or remote to survey with traditional methods. To solve this under-determined inverse problem, simulated annealing is used to estimate the distribution of vocalizing animals on a geodesic grid. This includes calculating a transmission loss (TL) matrix, which connects all grid nodes and recorders. Geometrical spreading and the ray trace model BELLHOP [Porter (1987). J. Acoust. Soc. Am. 82(4), 1349–1359] were implemented. The robustness of the proposed method was tested with simulated marine mammal distributions in the Atlantic sector of the Southern Ocean using both drifting acoustic recorders [Argo (2018). SEANOE] and a moored array as acoustic receivers. The results show that inversion accuracy mainly depends on the number and location of the recorders, and can be predicted using the entropy and range of the estimated source distributions. Tests with different TL models indicated that inversion accuracy is affected only slightly by inevitable inaccuracies in TL models. The presented method could also be applied to bird, crustacean, and insect choruses.

Both experimental and theoretical studies are conducted in this work to evaluate aeroacoustics damping performances of off-axial double-layer in-duct orifices at low Mach (≤0.0041) and Helmholtz number. The damping performance is characterized by using power absorption coefficient α. It denotes the fraction of incident sound energy being absorbed The effects of 1) mean duct flow (behaving like a bias flow) Mach number M a , 2) the porosities σ 1 and σ 2 of the front- and back-plates (with respect to the flow direction), 3) the off-axial distance dv (normalized with the plate radius) of the orifice relative to the plate’s centre point are experimentally examined first. Varying d v 1 and d v 2 is found to have an impact on the noise damping performance, especially at a higher Ma. Power absorption coefficient α is shown to be increased by 15%, when dv1 = 30% and dv2 = 0 in comparison with that when dv1 = dv2 = 0. In addition, the aeroacoustics damping performances of the double-layer in-duct orifices are compared with that of a single-layer one. The double-layer perforated plates with a smaller diameter orifice (D1,2 ≤ 6 mm) are found to involve with a larger α than that of single-layer orifice plates over a broader Helmholtz number range. Finally, a simplified 1D theoretical model is derived to simulate the experimental measurements. It is based on Rayleigh conductivity. It is found that a qualitatively good agreement is achieved for single-layer in-duct orifice between the theoretical predictions and the experimentally measured ones. As far as double-layer in-duct orifices are concerned, there is some discrepancy between the theoretical and experimental results.

In this work, acoustic damping performances of double-layer in-duct perforated plates are studied at low Mach (Ma) and Helmholtz number (He) to evaluate the effects of (1) M a, (2) the porosities (i.e., open-area ratio) σ1 and σ2 of the front and back plates, and (3) the axial distance Lc between these two plates. The orifices’ damping is characterized by sound absorption coefficient α denoting the fraction of incident sound energy being absorbed. For this, a quasi-steady acoustic model is developed first and experiments are then conducted. When M a = 0, α is experimentally found to oscillate with He, whatever the porosities of σ1 and σ2 are set. However, when M a is increased to and above 0.037, the power absorption troughs, i.e., local αmin of the double-layer plates with σ1,2 ≤ 9% are more separated and shallower. Furthermore, when σ1 = 9% or σ2 = 9%, the damping performances are quite different in terms of the local αmax peaks and their number. In addition, increasing L c with respect to the downstream pipe length Ld gives rise to an increase of αmin and αmax by 10%. Finally, the double-layer plates are shown to involve a larger α than that of single-layer one over a broader He range.In this work, acoustic damping performances of double-layer in-duct perforated plates are studied at low Mach (Ma) and Helmholtz number (He) to evaluate the effects of (1) M a, (2) the porosities (i.e., open-area ratio) σ1 and σ2 of the front and back plates, and (3) the axial distance Lc between these two plates. The orifices’ damping is characterized by sound absorption coefficient α denoting the fraction of incident sound energy being absorbed. For this, a quasi-steady acoustic model is developed first and experiments are then conducted. When M a = 0, α is experimentally found to oscillate with He, whatever the porosities of σ1 and σ2 are set. However, when M a is increased to and above 0.037, the power absorption troughs, i.e., local αmin of the double-layer plates with σ1,2 ≤ 9% are more separated and shallower. Furthermore, when σ1 = 9% or σ2 = 9%, the damping performances are quite different in terms of the local αmax peaks and their number. In addition, increasing L c with respec...

Active noise control (ANC) is used to reduce undesirable noise, particularly at low frequencies. There are many algorithms based on the least mean square (LMS) algorithm, such as the filtered-x LMS (FxLMS) algorithm, which have been widely used for ANC systems. However, the LMS algorithm cannot balance convergence speed and steady-state error due to the fixed step size and tap length. Accordingly, in this article, two improved LMS algorithms, namely, the iterative variable step-size LMS (IVS-LMS) and the variable tap-length LMS (VT-LMS), are proposed for active vehicle interior noise control. The interior noises of a sample vehicle are measured and thereby their frequency characteristics. Results show that the sound energy of noise is concentrated within a low-frequency range below 1000 Hz. The classical LMS, IVS-LMS and VT-LMS algorithms are applied to the measured noise signals. Results further suggest that the IVS-LMS and VT-LMS algorithms can better improve algorithmic performance for convergence speed and steady-state error compared with the classical LMS. The proposed algorithms could potentially be incorporated into other LMS-based algorithms (like the FxLMS) used in ANC systems for improving the ride comfort of a vehicle.

The soundscape of the Northeast Pacific Ocean is studied with emphasis on frequencies in the range 63–125 Hz. A 34-year (1964–1998) increase and seasonal fluctuations (1994–2006) are investigated. This is achieved by developing a simple relationship between the total radiated power of all ocean sound sources and the spatially averaged mean-square sound pressure in terms of the average source factor, source depth, and sea surface temperature (SST). The formula so derived is used to predict fluctuations in the sound level in the range 63–125 Hz with an amplitude of 1.2 dB and a period of 1 year associated with seasonal variations in the SST, which controls the amount of sound energy trapped in the sound fixing and ranging (SOFAR) channel. Also investigated is an observed 5 dB increase in the same frequency range in the Northeast Pacific Ocean during the late 20th century [Andrew, Howe, Mercer, and Dzieciuch (2002). ARLO 3, 65–70]. The increase is explained by the increase in the total number of ocean-going ships and their average gross tonnage.

This study explored the effect of building facade on indoor transportation noise annoyance in terms of frequency spectrum and expectation for sound insulation. The stimuli for transportation noise (road, rail, and aircraft noise) consisted of the original sound recorded outdoors, and the indoor sounds involved sounds filtered by filters using frequency responses of the insulation of the facades. The first experiment, which was conducted to reveal the effect of the frequency spectrum, compared the relative annoyance due to indoor noise and outdoor noise, both presented at 45 and 65 dB (LAeq). The results showed that in most cases, the indoor noise with lower sound energy in the high-frequency range produced lower annoyance than the outdoor noise for equal sound levels. In the second experiment, the participants were asked to rate the annoyance due to transportation noises presented at sound levels ranging from 35 to 75 dB (LAeq) in a specific imagined hearing situation (indoors or outdoors). Contrary to the first experiment, the results showed that in most cases, the annoyance due to indoor noise in an imagined indoor hearing situation, where sound insulation of the facade is expected, was higher than that due to outdoor noise in an imagined outdoor hearing situation for equivalent sound levels. The results of the two laboratory experiments demonstrate that the building facade has influences on the indoor transportation noise annoyance due to the frequency spectrum and the expectation for sound insulation.

Noise produced by scientific equipment during fisheries surveys is largely unstudied, though these sound sources may have an effect on the organisms of interest and on their resultant stock assessments. This paper describes acoustic signatures of two underwater mobile vehicles and accompanying research ships used to survey demersal fishes, and discusses the acoustic contributions of the survey equipment to rockfish habitat. Increases in noise over ambient levels were high, but the majority of sound energy was generated by communication and navigation instrumentation on the ships and survey vehicles, and was out of the expected sensitivity range for fish hearing.

The acoustic black hole (ABH) effect is realized in thin plate structures with a decreasing thickness according to a power-law function, and offers potential applications for structure vibration damping enhancement and free-field noise radiation suppression. In this paper, a wavenumber domain method (WNDM) is proposed for the analysis of vibro-acoustic coupling and internal noise reduction mechanism of a pentahedral cavity enclosed by a flexible plate with a two-dimensional ABH indentation, subject to a point force excitation. The system response of the ABH plate-cavity is computed by a validated finite element model. The relationship between the space-averaged sound energy inside the cavity and the spectra of the structural displacement and the acoustic mode of the cavity is established. This allows revealing a dual physical mechanism behind the observed noise reduction: amplitude reduction and mismatching between the wavenumber spectra of the plate displacement and the acoustic field, which results in a weakened vibro-acoustic coupling. An additional configuration with an ABH embedded in an irregular pentagonal wall of the cavity is examined. Despite the increasing complexity in the geometry of the coupling interface and its coupling with the cavity, numerical analyses confirm the generality of the observed physical phenomena and the applicability of the proposed WNDM to more complex system configurations.

This work investigates the acoustical intensity generated by the sound radiated from rectangular baffled plates in eight cases with distinct combinations of classical boundary conditions. The primary objective is to identify the regions of these plates that effectively contribute to the radiated sound power into the far-field, especially when the driven frequency occurs below the critical coincidence frequency. This identification is done by filtering the non-propagating waves, both using the (analytical) supersonic intensity method and the (numerical) useful intensity model. The spectral decomposition of the sound energy operator is applied, becoming possible the dimensional reduction of this operator to remove the components that do not propagate. Brief theoretical formulations, both for the plates vibration and the resulting acoustical field, are discussed. The closed form solution of the normal velocity field for the eight cases is given. Then, the supersonic intensity is estimated. On the other hand, a summary of the useful intensity technique is discussed and it is computed for the same cases. In the numerical examples, the comparison of the supersonic intensity, the useful intensity and the classical acoustic intensity is shown. The edge and corner modes are clearly identified and the useful intensity plots showed to fit well with the supersonic intensity ones.

The ocean mesoscale vortex can make uneven distribution of sound energy, which has a significant impact on underwater acoustic equipment, weapons and submarine warfare. Based on the observed data of 2014 West Pacific sea oceanographic survey, the influence of the properties, strength, position and depth of the mesoscale vortex on the acoustic propagation characteristics is simulated by the parabolic equation. Our observed vortex is a typical cyclone-type cold vortex in the Kuroshio extension body, and the cold water turns on intensely at the center of vortex. The effect depth of vortex is 500 m, and then, the parabolic equation of acoustic model is used to analyze and simulate the impact of mesoscale cold-core vortex on acoustic transmission loss. Results show that the horizontal perturbation of the surface velocity caused by mesoscale vortex has a great influence on the spatial distribution of water sound field. The cold vortex makes the position of the convergence zone move forward, with the width reduced, and the gain efficiency enhanced. When sound propagates outside of the cold-core vortex, compared with inside of the cold-core vortex, the assembled areas are backward, with the width increased, and gain effect weakened. The non-uniform hydrological environment caused by the ocean mesoscale vortex is a key modulating factor of the abnormal variation of acoustic energy in spatial distribution.

The noise suppression problems within ducts are widespread in aero engines, gas turbines, blowers and various mufflers. It has been well demonstrated that the Cremer impedance can maximize the propagating damping by degenerating the lowest order mode pair into a single mode, and consequently maximize the sound attenuation in both circular and rectangular ducts, however, the Cremer impedance of annular ducts is still unsettled. Therefore, the Cremer concept is further developed to theoretically optimize the acoustic impedance for annular ducts with two degrees of freedom, i.e. inner and outer liner impedances. Through decoupling successfully the radial wavenumber and impedance, the branch point equations are derived in the presence of grazing flow to solve the triple or double eigenvalues, which can be used to design simultaneously the optimal inner and outer impedances. A parametric study is conducted to investigate the effect of the Helmholtz number (kb), the circumferential mode order (m), the radius ratio (RR) and the liner configurations on the optimum radial wavenumber, Cremer impedance and sound transmission loss (TL). Some conclusions are drawn. First, the double eigenvalue solutions can give optimum impedance under more general conditions than triple eigenvalue solution. At small RR, the solutions of outer wall approximate to Cremer impedance of circular ducts, and at large RR and m = 0, the double eigenvalue solutions of both inner and outer walls are identical to Cremer impedance of rectangular ducts with zero transverse mode. Second, with the decrease of kb and the increases of m and RR, the optimum TL exhibits a general tendency of growth. Third, higher TL gain leading by inner liner can be achieved for larger RR and lower m, and whether the inner liner works effectively depends on the sound energy densities near inner and outer walls. Fourth, the RR and kb have larger impacts on the optimum TL than m, thus needing to be given priority to consider in design, the utilization of inner liner needs to comprehensively balance the effectiveness, economical efficiency and the realizability.

This paper applies ideas originally developed in the field of acoustics of multiscale porous media to the analytical modeling of dissipative silencers. Analytical models for the long-wavelength acoustical properties of different types of dissipative silencers, such as pod silencers, lined ducts, splitters, and bar silencers, are introduced in this work. Some of these models account for the possibility of using different porous materials in a given silencer. For instance, a model for a splitter silencer comprising baffles made of different porous materials is derived. Examples of the sound transmission behavior of the said types of silencers are presented to evidence that the dissipation of sound energy is largely influenced by pressure diffusion occurring in the porous constituents. The predictions of the analytical models are in good agreement with finite element calculations.

This theoretically supplemented experimental research effort is devoted towards demonstrating the feasibility of using heat flux driven transducers, in order to annihilate undesirable acoustic fields directly at the source boundary. Using commonly available materials, a thermophone is produced and deposited on a conventional vibro-acoustic loudspeaker. Unifying the sound energy input from heat and work related processes, a holistic model is derived from first law of thermodynamics - simulating pressure fields at different locations. In the case of thermo-acoustic sound production, the formulation is complemented by an efficiency model based on Fourier heat conduction in a slab with internal unsteady generation. The ensuing sound pressure level emanating from the two sources is predicted for a broad range of measurement locations and relative excitation phase angles. The theoretical estimates are corroborated by experimental results under different conditions. For the prescribed experimental parameters, a heat-flux transducer is shown to successfully diminish the pressure wave generated by a conventional loudspeaker to ambient noise levels.

Istanbul’s Hagia Sophia is a monumental structure with multiple sub-spaces coupled to one another through arches. Its architectural elements have undergone alterations as its function has changed from that of a church to a mosque, a mosque to a museum, and back to a mosque. This study makes use of Hagia Sophia’s rich formal and material characteristics to conduct a comprehensive investigation of room acoustics coupling. The methodology involves the application of the diffusion equation model (DEM) for sound energy flow analysis. Energy flow decays and energy flow dips are examined for almost 1000 receiver positions distributed throughout the various sub-spaces of the building. Ray-tracing (Ray-t) simulations are used to support the energy flow decay analysis conducted using DEM. The Ray-t data are subjected to Bayesian analysis to identify the decay parameters and the degree of acoustical coupling. Among the many variables, the source-receiver distance and positioning within different sub-spaces appear to be the underlying determinant of multi-slope sound decay pattern. On the other hand, the cases of multi-slope decays identified within the structure tend to weaken and single-slope cases increase when the overall absorption area increases in the mosque state due to the carpeted floor.

Perforated plates are widely used in many practical burners to attenuate noise emissions. In this study, the acoustic absorption capability of dual perforated plates (DPPs) with different porosities and aperture diameters was evaluated and tested in an impedance tube, and the damping performance of the DPPs located in a liquid fuel combustor inlet section was experimentally studied. The DPPs have an adjustable first cavity and can mitigate the thermoacoustic oscillations within a wide sound absorption bandwidth. The DPPs can absorb 95% of the specified incident sound energy. The combustion results indicated that the installation of DPPs at the inlet section has two effects: sound attenuation and the frequency shift of the combustor. The maximum reduction in dynamic pressure and CH* chemiluminescence intensity inside the chamber were 14 and 19 dB, respectively. When the primary air flow rate deviates from the optimal value, the DPPs can reduce the pressure amplitude in the combustion chamber by almost 80%. In general, this study may promote the application of DPPs under bias flow for the stabilization of spray combustion.

Around mid-1970, Schroeder introduced a mathematical theory to generate sequences, which can compute the “depth of the Schroeder diffuser”. A Schroeder diffuser is a design consisting of a number of wells of different “depths,” which propagates sound energy in different directions. The Schroeder diffuser provides the ability to create an optimal scattering of the same sound energy in different diffraction lobes. This paper aims to exploit this property and optimize well-depth sequences from Schroeder diffuser using machine learning techniques. With the advent of machine learning and parallel computing, the optimization of sequences and the iterative process to select the optimal pattern of sequence can be done more efficiently. Hence, the primary focus of this paper is to come up with a machine learning model that predicts the scattered reflections from a random well-depth sequence surface and evaluates the quality of the scattered reflections using a diffusion coefficient. The model tries to achieve optimum diffuser properties by adjusting the sequence. The boundary element method can be used for computing the diffusion coefficient. The diffusion coefficient parameter determines the quality of the diffuser and can be calculated at ⅓ Octave frequency bands.

The acoustical radiosity model (ARM) is a typical algorithm in geometrical acoustics to simulate the sound field in a room of ideally diffusely reflecting boundary. Even in such a room, the sound field decay is a complex process, as the relaxation of the sound field observed in simulations has shown. Based on the Laplace transform of the acoustical radiosity equation, this paper gives a set of properties of the ARM. It shows the system has a series of real or complex conjugate L-eigenvalues and corresponding L-eigenfunctions. Under the relaxation condition, the sound energy decay on the room boundary, generated by finite initial excitation, can be expanded as a summation of decay components, which are composed of real and/or complex conjugate decay modes. Each decay mode is a decaying and oscillating L-eigenfunction corresponding to an L-eigenvalue. The real part of the L-eigenvalue is the exponential decay rate, and the image part is the angular frequency of the oscillation. The reverberant sound field inside the room space has a similar decay structure to the boundary. As an example, the decay structure in a sphere is analyzed. The relaxation of the sound field is explained by the geometrical significance of the sound field decay.

The surface coupling approach is employed to predict the sound field in multiply connected spaces (rooms) with internal noise sources. The coupling is achieved by developing sound pressure and particle velocity at the interfaces between rooms in simple interface functions. Plane surface harmonics can be employed as interface functions where rectangular interfaces are concerned. Using the blocked sound pressure and impedance across the interface surfaces as acoustical descriptors, each room can be identified separately. The acoustical continuity conditions can then be applied to obtain the resulting sound field. Using this approach, the prediction of the sound field created by point sources in three connected parallelepipedic rooms was successfully carried out. The parametric study shows that the normalized harmonics wavenumber equal to 2 times the normalized sound wavenumber enables acceptable sound prediction. Further application was done by predicting sound energy flow of three monopole sources in a rectangular space coupled with a semi-infinite field.

The aim of this work is to study the sound field distribution in an experimental scale-model of two coupled rooms. An automatic scanning mechanism moves a microphone in small grid steps to measure room impulse responses at each grid point. The measurements cover the entire two-dimensional area of the coupled rooms. Sound energy distributions can be analyzed in the form of animated visual displays, revealing sound propagation across the coupling aperture and inside each room. This paper describes the measurement results, and the analysis method, which offer deep insights into the temporal development of a sound field in coupled spaces.

The work of this paper is backgrounded by prediction or evaluation and control of mechanical self-noise in sonar array cavity. The vibratory power flow transmission analysis is applied to reveal the overall vibration level of the fluid-structural coupled system. Through modal coupling analysis on the fluid-structural vibration of the fluid-filled enclosure with elastic boundaries, an efficient computational method is deduced to determine the vibratory power flow generated by exterior excitations on the outside surface of the elastic structure, including the total power flow entering into the fluid-structural coupled system and the net power flow transmitted into the hydroacoustic field. Characteristics of the coupled natural frequencies and modals are investigated by a numerical example of a rectangular water-filled cavity with five acoustic rigid walls and one elastic panel. Influential factors of power flow transmission characteristics are further discussed with the purpose of overall evaluation and reduction of the cavity water sound energy.

An acoustic metasurface (AMS) is proposed comprising a panel containing a micro slit of variable depth coupled to a thin cavity of symmetrical coiled spaces. Theoretical and numerical analyses are used to demonstrate that this metasurface has an effective low frequency sound absorption capability (i.e., between 100 Hz and 600 Hz) with a deep-subwavelength thickness. The mechanism underlying the sound absorption is explored by the different ways in which energy is dissipated in the absorber. It was observed that the dissipation of viscous energy takes place in the narrow region of the micro slit and represents 99.7% of the total energy dissipated while the thermal dissipation is 0.3% and occurs in the inner coiled spaces. Furthermore sound absorption of the AMS was investigated experimentally in an impedance tube for normal incidence. The results of the experimental work show that, by varying the depth of micro slit simultaneously with increasing the number of symmetrical coiled spaces, the peak frequency of the absorption can be shifted to a lower frequency without changing the total absorber thickness. A control of 93% of the sound energy, with a relative bandwidth of 50.4% in the frequency range of interest, was acquired with the proposed absorber presenting a thickness of λ / 48 . Furthermore, a coupled system was also evaluated experimentally which revealed the broadband absorption to be consistent with the theoretical model.

The dissipative splitter silencer is a common means for broadband noise attenuation in flow ducts such as ventilation and gas turbine systems where many higher order modes are cut-on. To predict the effect of a splitter silencer, analytical approaches have obvious advantages over the numerical methods on computation efficiency, but the development of analytical approaches are often based on many simplifications like the infinite length assumption. This paper presents a three-dimensional analytical method to solve the sound propagation in a duct with a finite length splitter silencer in the presence of mean flow. In the developed method, the convergence problem, which is often encountered by using the existing mode-based methods for high-frequency computations of soft-wall eigenmodes, is overcome by applying the equivalent surface source concept. With very low computational cost, the present method is validated to give consistent transmission loss predictions with the numerical results and experimental measurements available in the literature. Further, the performance of a splitter silencer is studied with different higher order incident modes, and reflection versus dissipation analysis is conducted by using the developed method. It is found that the sound attenuation is dependent on the sound energy spatial distributions of the incident sound modes, and the dissipation dominates the transmission loss behavior of the splitter silencer.

The acoustic diffusion equation model has been applied within a wide range of enclosure types to simulate diffuse sound fields. Recently, focus has been drawn to its applicability in reverberation chambers to simulate sound fields among a heterogeneous distribution of absorbing materials, particularly those that are highly absorptive. Where quasi-diffuse or non-diffuse sound fields are generated in the presence of highly absorptive material, the acoustic diffusion equation model has convenient capabilities to simulate sound energy flow. Furthermore, it has continued to show promise in characterizing the absorption coefficients of sound absorbing material. Challenges for reproducibility in reverberation room measurements have been well-documented and where traditional chamber-based measurement have shown to be inconsistent, this work will evaluate the acoustic diffusion equation model’s suitability as an alternative and accurate method to characterize absorption coefficients.

Unsteady heat release coupled with pressure fluctuation triggers the thermoacoustic instability which may damage a combustion chamber severely. This study demonstrates an electroacoustic control approach of suppressing the thermoacoustic instability in a Rijke tube by altering the wall boundary condition. An electrically shunted loudspeaker driver device is connected as a side-branch to the main tube via a small aperture. Tests in an impedance tube show that this device has sound absorption coefficient up to 40% under normal incidence from 100 Hz to 400 Hz, namely over two octaves. Experimental result demonstrates that such a broadband acoustic performance can effectively eliminate the Rijke-tube instability from 94 Hz to 378 Hz (when the tube length varies from 1.8 m to 0.9 m, the first mode frequency for the former is 94 Hz and the second mode frequency for the latter is 378 Hz). Theoretical investigation reveals that the devices act as a damper draining out sound energy through a tiny hole to eliminate the instability. Finally, it is also estimated based on the experimental data that small amount of sound energy is actually absorbed when the system undergoes a transition from the unstable to stable state if the contrpaol is activated. When the system is actually stabilized, no sound is radiated so no sound energy needs to be absorbed by the control device.

Exhaust turbocharger is one of the most important components of diesel engine. The surge of such an exhaust turbocharger could cause serious damage or even failure during the operation. Therefore, it is of great significance to detect signs when surge initially occurs. In this article, the characteristic frequencies in sound pressure and structural vibration are investigated to detect the surge of a turbocharger. An extensive data analysis in both time and frequency domain will be made to find out the characteristics of the surge phenomenon for such an exhaust turbocharger. It is observed that the rotor of such a turbocharger will be in resonance condition when the surge occurs. In addition, a method based on the sound energy in the characteristics frequency is proposed to diagnose and predict surge of the compressor. This method can efficiently alarm the surge of such turbocharger without installing the pressure sensor which will damage its structure.

During music rehearsals, students and teachers are regularly exposed to unsafe sound levels; hearing loss is a major concern among musicians. By providing real time feedback to music instructors, we can persuade them to modify their teaching methods to achieve safe listening conditions. To accomplish this, we created a user-friendly dashboard that displays the cumulative sound energy being collected in real-time. Since there are few standards for limiting music exposure, we used NIOSH standards for industrial noise exposure as the permissible exposure limit (PEL). Circular dials on the dashboard were programmed to display the amount of exposure relative to the PEL. To aid in identifying the type of exposure, each dial represents the amount of sound accumulation in a band of frequencies relative to typical human speech: below (15Hz–85 Hz), at (85Hz–252 Hz), and above (242Hz–24 675 Hz). The dashboard also displays an equalizer representing the instantaneous microphone input. After each session, the cumulative exposure data can be sent out in email format. Inexpensive and convenient to use, this dashboard will enable music instructors to make informed decisions on how to best modify their teaching approaches to reduce the risk of hearing loss.

It has been more than a couple of decades since the interest in coupled volume systems started to grow rapidly among researchers. Basically, due to their increasingly popular applications in music halls for providing a rich acoustical environment either to fulfill the competing criteria of clarity and reverberance or simply to tune the auditoriums for multi-functional use. A lifelong experience with coupled spaces is the main motivation behind this paper. The journey starts with further developing the theoretical knowledge through scale model experiments. The results of which are utilized to observe the effects of the aperture size and shape on non-exponential sound energy decay by applying Bayesian analysis. The investigations have continued by field tests of real-size architectural monuments, specifically, worship spaces with multi-volume and domed typology. The relation of non-exponential energy decay and architectural parameters, for different scenarios or original uses of historical buildings, are searched through energy flow decays by applying the diffusion equation model. Simultaneously, all of these findings from academic research have been a source to produce acoustical design solutions for either small sized recording studios or large-scale multi-purpose auditoria in landmark buildings, all of which are briefly exemplified within this study.

When sound barriers are installed on both sides of a noise source, degradation in performance is observed. Barriers having negative-phase-gradient surfaces successfully eliminate this drawback by trapping sound energy in between the barriers. In contrast, barriers can also be designed to "dredge" the energy flux out. An extended model considering higher-order diffractions, which resulted from the interplay of the induced surface wave and barrier surface periodicity, is presented. It is found that the sound dredging barriers provide a remarkable enhancement over the trapping ones, and hence have the potential to be widely used in noise control engineering.

This paper outlines an approach for obtaining microphone array geometry for use in traffic noise analysis. The designed array has a smaller number of microphones compared to existing solutions, thereby reducing the cost of system realization. The array geometry is irregular, consists of 24 microphones, and was obtained by an optimization procedure that minimizes beampatterns sidelobes. Microphone position optimization was performed in the frequency band from 300 Hz to 2000 Hz, defined by traffic noise characteristics. The designed array is used in ongoing research dedicated to the analysis of angular distribution of incident traffic noise sound energy impeding a facade in urban environment.

This paper explores a promising novel approach by integrating nonlinear energy sink (NES) and giant magnetostrictive material (GMM) to realize vibration control and energy harvesting. The vibration-based apparatus consisting of a NES, a Terfenol-D rod, and a linear oscillator (the primary system) is proposed. The mathematical model of the prototype under displacement driven has been established and simulated by utilizing the Runge-Kutta algorithm. The exhibited responses and the obtained electric energy are computed. Furthermore, the Fast Fourier Transform (FFT) of the resonant responses is performed. The distribution of the input energy is calculated to evaluate the designed structure. The instantaneous transaction of the energy is then examined by considering the energy transaction measure (ETM). Lastly, a parametric study is conducted for further optimization. The numerical simulations demonstrate that the nonlinear pumping phenomena occur, that is, the target energy transfer (TET) that leads to a very efficient vibration suppression. In addition, the results also illustrate that the localized vibration energy can be converted into magnetic field energy due to the Villari effect and then transformed into electric energy.

Stage acoustic parameters aim to quantify the amount of sound energy reflected by the stage and hall boundaries and the energy decay over time. In this research, the effect of orchestra presence on parameter values is investigated. The orchestra is simulated by dressed mannequins, which have been compared with humans with respect to acoustic properties. Impulse response measurements were performed in a concert hall, a theatre, a rehearsal room, and in two orchestra pits. Conditions were empty stage floors, stage floors with music stands and chairs only, and floors occupied by the mannequin orchestra. Results show that the direct and reflected sound levels and the energy decay are significantly affected by the orchestra compared to an empty stage or a stage with chairs and stands only. Both the direct sound and early reflected sound levels are reduced by the orchestra with the distance. The late reflected sound level is reduced considerably more than can be expected based on Barron's revised theory. It can be concluded that measurements on a stage without the orchestra being present results in significant differences. A practical method is presented to perform a "musician friendly" stage acoustic measurement with a real orchestra.

This study proposes an alternative to the classic wedge design for anechoic chambers, which is the uniform-then-gradient, flat-wall (UGFW) structure. The working mechanisms of the proposed structure and the traditional wedge are analyzed. It is found that their absorption patterns are different. The parameters of both structures are optimized for achieving minimum absorber depth, under the condition of absorbing 99% of normal incident sound energy. It is found that, the UGFW structure achieves a smaller total depth for the cut-off frequencies ranging from 100 Hz to 250 Hz. This paper also proposes a modification for the complex source image (CSI) model for the empirical simulation of anechoic chambers, originally proposed by Bonfiglio et al. [J. Acoust. Soc. Am. 134 (1), 285–291 (2013)]. The modified CSI model considers the non-locally reactive effect of absorbers at oblique incidence, and the improvement is verified by a full, finite-element simulation of a small chamber. With the modified CSI model, the performance of both decorations with the optimized parameters in a large chamber is simulated. The simulation results are analyzed and checked against the tolerance of 1.5 dB deviation from the inverse square law, stipulated in the ISO standard 3745(2003). In terms of the total decoration depth and anechoic chamber performance, the UGFW structure is better than the classic wedge design.

This study investigates reliable models and methods to be applied in sound field analysis of multi-domain structures. The case structures are two monuments, namely, Süleymaniye Mosque and Hagia Sophia in İstanbul. These are both multi-volume spaces with many smaller sub-volumes coupled to each other by coupling apertures in form of arches. A key concern of the study is to examine energy flow decays and understand the mechanism of multi-slope sound energy decays. The methodology involves diffusion equation model (DEM) application in a finite-element scheme for sound energy flow analysis. Energy flow decays, energy flow dips, and spatial flow vectors are examined for single versus multi-domain DEM solutions. It is concluded that specification of different domains with individual diffusion coefficients is a critical setting such that, if not assigned correctly, may mislead the results. The energy flow vector analysis has enabled us to comprehend the architectural features in relation to such energy flow decay dip occurrence. The computational efficiency of DEM is also discussed. The DEM application in this study has proved to be a powerful and practical method in room acoustics applications, specifically for multi-rate decay investigations.

A theoretical, numerical and experimental investigation of a low-frequency acoustic absorber (100–600 Hz) is reported herein. The acoustic metamaterial is based on a micro-perforated panel coupled to a multi-cavity of coiled-up spaces that is similar to a symmetrical labyrinth. On considering the visco-thermal losses, the effect of increasing the number of symmetrically coiled-up spaces that determines the peak position of the sound absorption and allows a greater sound energy absorption is discussed theoretically and proven through numerical analysis (FEM). Prototypes were manufactured using 3D printing technology and evaluated in an impedance tube. The results for the sound absorption coefficient acquired in the desired frequency range were greater than 91% with relative bandwidth above 35%, in agreement with the analytical model. Therefore, it is demonstrated that the proposed absorber presents a scale of deep-subwavelength since its total thickness is 0.033λ.

This research develops an underwater acoustic metamaterial, which can serve to absorb broadband low-frequency underwater sound under high hydrostatic pressure ranging from 200 to 2000 Hz. Every unit cell includes 12 harmonic oscillators as multi-scatterers in the proposed acoustic metamaterial, which induces lots of local resonance modes for absorbing sound energy in the investigated frequency range. The sound absorption band can be greatly widened in the low frequency range due to strong coupling and multiple scattering effects among harmonic oscillators. The effectiveness of the prototype is experimentally demonstrated in a standing wave tube under different hydrostatic pressure conditions. The effects of high hydrostatic pressure on local multi-resonance mechanism are investigated experimentally. Experimental results show that the average sound absorption coefficient of the proposed acoustic metamaterial achieves 0.78 in the frequency range [600 Hz, 2000 Hz] under 0.5 MPa. Furthermore, the proposed acoustic metamaterial can achieve better sound absorption performance than the pure viscoelastic polymer under high hydrostatic pressure.

In urban communities, parallel barriers are commonly erected for controlling environmental noise such as traffic and construction noise. However, owing to the multiple reflections between the parallel barriers, their performance may be worse than that of a single barrier. To improve the performance of parallel barriers, a small piece of flush-mounted panel backed by a slender cavity in an otherwise rigid wall of barriers is proposed. With the excitation of the incident wave from a sound source inside the parallel barriers, the flexible panel vibrates, and sound is radiated out to undergo acoustic interference with the sound field bounded by the parallel barriers. Consequently, the sound energy in this space and diffraction at the barrier top edge are reduced over a broad band in the low-frequency regime. A theoretical model for tackling the vibro-acoustic coupling between the sound field of the open cavity and the vibrating panel is established to investigate the noise reduction mechanism in the shadow zone. With optimal structural properties of the panel, an additional averaged insertion loss of approximately 5 dB can be achieved at 80–1000 Hz. The theorical results, which are experimentally validated, pave the way for the potential applications of the flexible panel devices (FPDs) for improving the noise reduction of parallel barriers.

Due to the limited understanding of spatial sound field and contribution of pantograph, a deep analysis is presented to extract understanding the aerodynamic noise characteristics of high-speed trains pantograph system. The near-field and far-field noise are predicted by the acoustic perturbation equations and Ffowcs Williams-Hawkings equation, respectively. The spatial sound propagation is analyzed by proper orthogonal decomposition and cluster-based reduced-order modelling. The flow field results predicted by large eddy simulation show that the flow behind pantograph is governed by hierarchical structures, which occurs to be featured with three layers as well as periodic evolution. The dipole source is dominated in far-field radiated noise while the quadrupole source is negligible, since the intensity of the quadrupole source is less than that of the dipole source. The contribution rates of base-frame, pan-head, groove, upper-arm, horn, lower-arm and rod-insulator are higher than the other components, and the radiated sound energy of them accounts for approximately 90% of the total energy. The noise contribution of pan-head exceeds 10% at the frequency of 1000 Hz. Base-frame is the largest contributor, and more than 6% of noise contribution occurs at the frequency of 630 Hz. The spatial sound propagation includes two major aspects: one is mainly reflected upwind from groove, and the other is propagated with the center of groove. The results highlight the possibility to develop a new design of high-speed trains pantographs, in order to obtain an aerodynamic noise reduction.

The use of sound pressure levels is not adequate to assess and predict environmental sound quality at different locations. Moreover, there is a lack of studies investigating the psychoacoustic metrics with spatial variation. This study aims to investigate (1) the effects of spatial factors (distance and sound emission orientation) on three physical acoustic metrics (root-mean-square sound pressure (Prms), unweighted sound pressure level (LZ), and A-weighted sound pressure level (LA)) and two psychoacoustic metrics (total loudness (N) and sharpness (S)); and (2) the role of the metrics and their statistical parameters in characterizing acoustical properties, through measurements of steady directional high-frequency sound at different points in an anechoic chamber. The results indicate that the sound intensity metrics LZ or LA as well as the subjective loudness metric N are distance-dependent. The value of N was approximately equal to 0.727 times the original loudness when doubling the distance from the source. Strong correlations were found between four sound energy content-related metrics (Prms, LZ, LA, and N). However, the fifth metric (S) was closely related to sound spectral content. Moreover, the interquartile range of N was found to be an option of representing sound temporal content. This is the first study to establish the predictability of psychoacoustic metrics with spatial variation. These findings provide a new insight into environmental sound quality assessment and prediction.

The middle ear is the part of peripheral auditory system that works to transmit the sound energy from the outer to the inner ear, matching their highly different impedances. Due to its importance for the human hearing, the middle ear has been studied through mathematical models over the past 70 years. The use of deterministic approaches has been the mainstream on the development of these models despite it is well recognized that middle ear has a natural variability with respect to its mechanical and dynamical properties, kwon as random uncertainties. In this work a lumped-element model of the middle ear is used as a baseline deterministic model for the development of a probabilistic model using a non-parametric approach. This probabilistic approach is characterized by adding the uncertainties directly into the global matrices of the modeled system. Furthermore, an optimization process using a single objective function was used for fitting both probabilistic and baseline deterministic models. The probabilistic model developed presents promising results, showing statistical responses in agreement with experimental evidences. In addition, advantages were identified in comparison to a model previously developed with a parametric probabilistic approach.

Objects that move in depth on an approaching trajectory (looming) are often encountered in both the real and virtual worlds. Examples include navigating oncoming traffic, and sporting and gaming activities where judgements are made to avoid or attack approaching objects. How people react to looming objects may impact on their survival and progression in the real, virtual, and gaming worlds, and relies on a person’s ability to precisely interpret movement and depth cues. Psychological studies investigating auditory looming often depict an object’s movement using simple audio cues (primarily amplitude increase) which are applied to tones (often sine or triangle waves) which are not normally encountered in the natural world. Whilst these studies provide valuable information about human perception and responses, technological advances allow us to present complex auditory stimuli with a range of audio cues and real-world sound sources, and to collect measurements on human perception and responses to ecologically valid stimuli. In this study we investigate human responses to audio cues for movement in depth, and how the cues affect people’s responses to the approaching object. We present an experiment on human perception where observers respond to auditory looming stimuli with real-world sounds that contain multiple audio cues, and introduce the direct-to-reverberant sound energy ratio as an audio cue. We measure the participants responses to the stimuli, asking them to indicate the approaching object’s perceived contact time (measuring their amount of over-/underestimation); to rate their emotional (valence and arousal) responses; and to rate the engagement quality of the stimuli. Our results show that listener’s responses to the audio cues differed, revealing a hierarchy across the individual audio cues, with the amplitude increase being the most dominant cue, followed by the direct-to-reverberant energy ratio. The results also demonstrated that conditions with multiple audio cues prompted earlier estimates of the contact times, greater arousal and engagement ratings, than single audio cues; and that real world sound sources prompted significantly greater engagement ratings than both the artificial sound sources.

This paper aims to explore the influence factors of speech intelligibility and the applicability of the speech transmission index (STI) in large spaces, where the sound energy is unevenly distributed and non-exponentially decays. The subjective speech intelligibility tests were conducted in Mandarin (China mainland) in two large spaces with volumes of 97,000 m3 and 246,000 m3. Objective indicators such as the Reverberation Time (RT), Early Decay Time (EDT), Definition (D50), and Speech Transmission Index (STI) under different signal-to-noise ratio (SNRs) were also measured in these two spaces. The results showed that both the SNR and room acoustics had significant effect on the speech intelligibility in these two spaces, but the effect of room acoustics on speech intelligibility was also affected by SNR. The speech intelligibility scores significantly increased with the increase in SNR when the SNR was less than 14.4 dB. In terms of room acoustics, D50 was more relevant to speech intelligibility than RT and EDT in these two large spaces when SNR ranged from −5dB to 15 dB. The STI value in large spaces should not be used as in ordinary spaces to evaluate the speech intelligibility. Based on the tests in this paper, the corresponding relation between STI and speech intelligibility in large spaces was modified, and a new rating threshold of STI was also proposed according to the revised relation, which indicated a necessity to modify the rating criteria of using STI to predict speech intelligibility in large spaces.

Although the visual and geomagnetic orientation cues used by sea turtle hatchlings during sea-finding have been well studied, the potential for auditory stimuli to act as an orientation cue has not been explored. We investigated the response of sea turtle hatchlings to natural and anthropogenic noises present on their nesting beaches during sea-finding. The responses of hatchling leatherback sea turtles, Dermochelys coriacea, collected from the Sandy Point National Wildlife Refuge, St. Croix, were measured in the presence of aerial acoustic sounds within hatchlings’ hearing range of 50 to 1600 Hz. The highest sound energy produced by beach waves occurs at frequencies 50–1000 Hz, which overlaps with the most sensitive hearing range of hatchling leatherbacks (50–400 Hz). Natural beach wave sounds, which have highest sound energy at frequencies of 50–1000 Hz, may be masked by human conversations (85–650 Hz) and vehicle traffic noise (60–8000 Hz). In the presence of three stimuli, a) beach wave sounds (72.0 dB re: 20 μPa), b) human conversation (72.4 dB re: 20 μPa), and c) vehicle traffic noise (71.1 dB re: 20 μPa), hatchlings exhibited no phonotaxic response (wave sounds: mean angle = 152.1°, p = 0.645; human conversation: mean angle = 67.4°, p = 0.554; traffic noise: mean angle = 125.7°, p = 0.887). These results may be due to the hatchlings being unable to localize sounds in the experimental arena. Visual and auditory cues may also converge to affect sea-finding orientation. Future studies should focus on the localization ability of sea turtles and on the role that sound may play in orientation when combined with other sensory and environmental cues.

Noise in marine ecosystems has increased significantly in the last decades. One of the most significant sources is vessel traffic. This affects animals that depend on sound to interact within their ecosystems, like interrupting communication which could lead to adaptive strategies to avoid the noise. We recorded the soundscape from 0 to 15 kHz in Morro Mico and Nuqui, Colombia, from 2018 and 2019—one of the most biodiverse locations in the world, and one that has never experienced high levels of vessel traffic and has no shipping lanes. Here, humpback whale Stock G migrates to their breeding ground. The species is well-known for its songs and social calls that facilitate biological and ecological processes. One process that could shape these vocalizations is noise generated by the artisanal fishing family boats. We explored correlations between the sound energy from the humpback whale song cycle, temporal features of the song structure, other biological acoustic signals, physical acoustic cycles, and the presence of small boats. These comparisons are important because there are no large vessels (requiring AIS) that transit closely to the study area. Therefore, we focused on soundscape cycles and acoustic responses of humpback whales from purely small vessel noise.

Although sound radiation from sound-induced vibration and from force-excited vibration of solid structures are similar phenomena in terms of radiating from vibrating structures, the general relationship between them has not been explicitly studied to date. In particular, airborne sound transmission through walls and sound radiation from structurally vibrating surfaces in buildings are treated as different issues in architectural acoustics. In this paper, a fundamental relationship is elucidated through the use of a simple model. The transmission coefficient for random-incidence sound and the radiated sound power under point force excitation of an infinite elastic plate are both analyzed. Exact and approximate solutions are derived for the two problems, and the relationship between them is theoretically discussed. A conversion function that relates the transmission coefficient and radiated sound power is obtained in a simple closed form through the approximate solutions. The exact solutions are also related by the same conversion function. It is composed of the specific impedance and the wavenumber, and is independent of any elastic plate parameters. The sound radiation due to random-incidence sound and point force excitation are similar phenomena, and the only difference is the gradient of those characteristics with respect to the frequency.

High intensity underwater sounds may cause temporary hearing threshold shifts (TTSs) in harbor porpoises, the magnitude of which may depend on the exposure duration. After exposure to playbacks of pile driving sounds, TTSs in two porpoises were quantified at 4 and 8 kHz with a psychophysical technique. At 8 kHz, the pile driving sounds caused the highest TTS. Pile driving sounds had the following: pulse duration 124 ms, rate 2760 strikes/h, inter-pulse interval 1.3 s, duty cycle ∼9.5%, average received single-strike unweighted broadband sound exposure level (SELss) 145 dB re 1 μPa(2)s, exposure duration range 15-360 min (cumulative SEL range: 173-187 dB re 1 μPa(2)s). Control sessions were also carried out. Mean TTS (1-4 min after sound exposure stopped in one porpoise, and 12-16 min in the other animal) increased from 0 dB after 15 min exposure to 5 dB after 360 min exposure. Recovery occurred within 60 min post-exposure. For the signal duration, sound pressure level (SPL), and duty cycle used, the TTS onset SELcum is estimated to be around 175 dB re 1 μPa(2)s. The small increase in TTS between 15 and 360 min exposures is due to the small amount of sound energy per unit of time to which the porpoises were exposed [average (over time) broadband SPL ∼144 dB re 1 μPa].

In acoustic and ultrasonic non-destructive testing techniques, it is sometimes beneficial to concentrate sound energy at a chosen location in space and at a specific instance in time, for example to improve the signal-to-noise ratio or activate the nonlinearity of damage features. Time Reversal (TR) techniques, taking advantage of the reversible character of the wave equation, are particularly suited to focus ultrasonic waves in time and space. The characteristics of the energy focusing in solid media using principles of time reversed acoustics are highly influenced by the nature and dimensions of the medium, the number of transducers and the length of the received signals. Usually, a large number of transducers enclosing the domain of interest is needed to improve the quality of the focusing. However, in the case of highly reverberant media, the number of transducers can be reduced to only one (single-channel TR). For focusing in a non-reverberant medium, which is impossible when using only one source, an adaptation of the single-channel reciprocal TR procedure has been recently suggested by means of a Chaotic Cavity Transducer (CCT), a single element transducer glued on a cavity of chaotic shape. In this paper, a CCT is used to focus elastic energy, at different times, in different points along a predefined line on the upper surface of a thick solid sample. Doing so, all focusing points can act as a virtual phased array transducer, allowing to focus in any point along the depth direction of the sample. This is impossible using conventional reciprocal TR, as you need to have access to all points in the bulk of the material for detecting signals to be used in the TR process. To asses and provide a better understanding of this concept, a numerical study has been developed, allowing to verify the basic concepts of the virtual phased array and to illustrate multicomponent time reversal focusing in the bulk of a solid material. publisher: Elsevier articletitle: Simulation study of a chaotic cavity transducer based virtual phased array used for focusing in the bulk of a solid material journaltitle: Ultrasonics articlelink: http://dx.doi.org/10.1016/j.ultras.2016.01.010 content_type: article copyright: Copyright © 2016 Elsevier B.V. All rights reserved. ispartof: Ultrasonics vol:67 pages:151-159 ispartof: location:Netherlands status: published

Studies to find alternative low environmental-impact materials for acoustic absorbers are still progressing, particularly those originated from natural materials. However, most of the established works are mainly focused on the fibrous-type absorbers. Discussion on the non-fibrous-type absorbers is still lacking and this therefore becomes the objective of this paper. Use of bamboo by utilizing its hollow structure to absorb sound energy is discussed here. The normal incidence absorption coefficient was measured based on the length and diameter of the bamboo, as well as different arrangement of the bamboo structure subjected to the incidence sound, namely, axial, transverse, and crossed-transverse arrangements. The trend of absorption coefficient appears in peaks and dips at equally spacing frequencies. For all arrangements the peak of absorption can reach above 0.8. Introducing an air gap behind the bamboo shifts the peak absorption to lower frequency. Covering the front surface of the absorber improves the sound absorption coefficient for axial arrangement by widening the frequency range of absorption also towards lower frequency range. The transverse arrangement is found to have average absorption coefficient peaks of 0.7 above 1.5 kHz. By arranging the bamboo structure with crossed-transverse arrangement, the suppressed absorption peaks in normal transverse arrangement can be recovered.