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The microwave auditory effect has been widely recognized as one of the most interesting and significant biological phenomena from microwave exposure. The hearing of pulsed microwaves is a unique exception to the airborne or bone-conducted sound energy, normally encountered in human auditory perception. This chapter describes the research studies leading to scientific documentation that absorption of a single microwave pulse impinging on the head may be perceived as an acoustic zip, click, or knocking sound, depending on the incident microwave power. A train of microwave pulses to the head may be sensed as an audible buzz, chirp, or tone by humans. It discusses the neurophysiological, psychophysical, and behavioral observations from laboratory studies involving humans and animals as subjects. The objective is to present what is scientifically known about the microwave auditory effect.

Various sources of noise in cities and ways to combat them are considered. The effi ciency of two types of acoustic screens is analyzed: single-layer and two-and three-layer. The positive and negative characteristics of various designs of existing acoustic screens are listed. A fundamentally new design of the acoustic screen is described: the panels have a ridge profi le, they are located along the vertical axis of the frame with a step at which the panels above the located ridge cover the top of the lower ridge, the outer surfaces of the panels and their end parts are made of a material that has sound-refl ecting properties, and the inner surfaces of the panels are made of sound-absorbing material. Such an acoustic screen is devoid of the disadvantages inherent in acoustic screens of other structures and is more effi cient. The considered invention allows due to the use of the ridge shape of acoustic panels: prevent the refl ection of sound energy in the direction of noise sources (a small part will be directed upwards); provide almost complete absorption of sound energy as it passes through the channels formed by the ridge panels; provide the same effective protection against noise sources located on both sides of the screen; prevent the formation of snow and ice crust on sound-absorbing surfaces, which ensures the constant eff ectiveness of the screen in diff erent seasons of the year; reduce metal consumption, which will reduce the weight of the structure without reducing its effi ciency and reduce the cost of acoustic screens.

The study addresses the benefit of tuning single harmonics with vocal tract resonances to increase vocal loudness. The loudness of theoretically constructed vocal sounds with variable levels of sound energy in the first, second, and third harmonics is computed on the basis of ISO standard 226:2003. In comparison to increased loudness with changes in overall spectral slope, it is shown that single harmonic tuning requires a greater range of SPL to produce a similar range of loudness. For example, a 10-40 dB increase in the level of a single harmonic produces less than two doublings of loudness, whereas a spectral slope change from -12 dB/octave to -3 dB/octave can produce a similar doubling of loudness with only a 5 dB SPL increase.

To support purchasing low noise products, sound power and sound energy measurements of sufficient quality need to be routinely made by consumers on a global scale. Sound power measurements using ISO 3744, 3745, and 3746 are conducted in a free field using an acoustic far-field approximation of the intensity integrated over an enveloping measurement surface. Sound power and sound energy measurements generally use a hemispherical, parallelepiped, or cylindrical measurement surface. Those measurement surfaces have limitations and assume that the measurement points lie on the measurement surface often in preferred positions. An alternative approach is to choose microphone positions that optimally satisfy the assumptions of the measurement. The measurement surface should then be fit to the chosen microphone positions. Regression methodologies are available for fitting ellipsoids. The number of microphone positions can be as few as three to fit an ellipsoid. An ellipsoidal measurement surfaces can abut zero, one, two, or three orthogonal reflecting planes. Correction equations for the microphone locations and the angle errors for the microphone orientation and wave propagation direction are shown. This paper will present simulations of sound power, sound energy, and corrections for environmental reflections for ISO 3745 and other measurement surfaces.

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.

Polyacrylonitrile (PAN) nanofibers show piezoelectric properties and the capability to harvest sound energy for power generation purposes. However, their application as an acoustic sensor to detect sound was not studied. In this paper, we report the novel amazing ability of electrospun PAN nanofibers to precisely detect low-to-middle frequency sound (100–600 Hz) at the middle sound pressure level (60–95 dB), which covers the main sound spectrum in our daily activities. The nanofiber sensor device shows high sensitivity, with a signal-to-noise ratio as high as 57.2 dB and fidelity as high as 0.995. Compared to the acoustic sensor made of electrospun poly(vinylidene fluoride) nanofibers under the same condition, our PAN device has a much wider response bandwidth, larger sensitivity, and higher fidelity, indicating the important role of polymer type in the acoustic detection of electrospun nanofibers. The brilliant detection accuracy originates from the high piezoelectricity and rigidity of PAN nanofibers, facilitating deformation recovery and reduction of the interference from harmonics. To demonstrate the remarkable sound detection ability, we used the PAN device to convert a piece of music into electrical signals and then converted the electrical signals into sound. The playback sound was almost identical to the original sound waves. Electrospun polyacrylonitrile nanofibers may form a novel acoustoelectric transducer for accurate sound detection and other high-tech applications.

Compared with scalar sound field, vector sound field explained the spatial structure of sound field better since it not only presents the sound energy distribution but also describes the sound energy flow characteristics. Particularly, with more complicated interaction among different wavefronts, the vector sound field characteristics of an elastic structure in a shallow water waveguide are worthy of studying. However, there is no reliable prediction method for the vector sound field of an elastic structure with a high efficiency in a shallow water waveguide. To solve the problem, transfer functions in the waveguide have been modified with some approximations to apply for the vector sound field prediction of elastic structures in shallow water waveguides. The method is based on the combined wave superposition method (CWSM), which has been proved to be efficient for predicting scalar sound field. The rationality of the approximations is validated with simulations. Characteristics of the complex acoustic intensity, especially the vertical components are observed. The results show that, with constructive and destructive interferences in the depth direction, there could be quantities of crests and vortices in the spatial structure of time-dependent complex intensity, which manifest a unique dynamic characteristic of sound energy. With more complicated interactions among the wavefronts, a structure source could not be equivalent to a point source in most instances. The vector sound field characteristics of the two sources could be entirely different, even though the scalar sound field characteristics are similar. Meanwhile, source types, source parameters, ocean environment parameters, and geo parameters may have influence on the vector sound field characteristics, which could be explained with the normal mode theory.

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.

This study investigates and compares the acoustic signatures of a traditional wire-cable-pulled lure system and two alternative battery-operated lure systems jointly developed by Covey Associates Pty. Ltd. and Steriline Pty. Ltd. to eliminate the hazardous steel-wire cable and make the sport of greyhound racing safer for greyhounds, participants and spectators. The acoustical measurements of these three lure systems were conducted at the Murray Bridge greyhound racing track. The lure sounds were measured by the high-frequency Brüel &amp, Kjær (B&amp, K) Type 4191 microphones for the 395 m and 455 m starts at two positions: within the starting box and on the track adjacent to the starting boxes. The measurements capture the sounds that the greyhounds hear before and after the opening of the starting box gate. The frequency-domain analysis and sound quality analysis were conducted to compare the lure sounds. It was found when the battery-lure was installed with all nylon rollers, it presented less sound energy and lower frequency than the traditional wire-cable-pulled lure. When two of the nylon rollers were replaced with steel rollers, the battery-operated lure emitted a louder and higher frequency sound than the traditional wire-cable-pulled lure. The different acoustic characteristics of these lure systems suggest future research is warranted on the reaction of greyhounds to different lure sounds, particularly their excitement level within the starting box as the lure approaches. This initial research also suggests some greyhounds may not clearly hear the battery-operated lure with all nylon rollers approaching the starting boxes and the timing of these greyhounds to jump may be delayed, particularly during high wind conditions.

Efficient and rapid particle enrichment at the submicron scale is essential for research in biomedicine and biochemistry. Here, we demonstrate an acoustofluidic method for submicron particle enrichment within a spinning droplet driven by a unidirectional transducer. The unidirectional transducer generates intense sound energy with relatively low attenuation. Droplets placed offset in the wave propagation path on a polydimethylsiloxane film undergo strong pressure gradients, deforming into an ellipsoid shape and spinning at high speed. Benefitting from the drag force induced by the droplet spin and acoustic streaming and the radial force induced by the droplet compression and expansion, the submicron particles in the liquid droplet quickly enrich toward the central area following a spiral trajectory. Through numerical calculations and experimental processes, we have demonstrated the possible mechanism responsible for particle enrichment. The application of biological sample processing has also been exploited. This study anticipates that the strategy based on the spinning droplet and particle enrichment method will be highly desirable for many applications.

The theoretical basis of noise reduction by noise barrier is Huygens Fresnel's wave theory. When a noise barrier is added in the propagation path between the sound source and the receiving point, the reflection, transmission and diffraction of the noise sound wave will occur when it meets the noise barrier in the propagation process. A part of the sound energy is emitted from the sound source and transmitted to the receiving point through the diffraction of the top structure of the sound barrier; A part of the sound energy can reach the sound receiving point directly through the sound barrier; When a part of the energy reaches the sound barrier, it will be reflected. If the two sound barriers are close to each other, it will be reflected many times. The ability of sound insulation and noise reduction of sound barrier is to reduce the energy of direct and transmission to the sound receiving point by blocking, and reduce the energy of diffraction to the sound receiving point through the upper structure, so as to achieve the effect of sound energy attenuation, The influence of firewall geometry on audible noise distribution of transformer is obtained [1].

Noise annoyance depends not only on sound energy, but also on other features, such as those in its spectrum (e.g., low frequency and/or tonal components), and, over time, amplitude fluctuations, such as those observed in road, rail, or aircraft noise passages. The larger these fluctuations, the more annoying a sound is generally perceived. Many algorithms have been implemented to quantify these fluctuations and identify noise events, either by looking at transients in the sound level time history, such as exceedances above a fixed or time adaptive threshold, or focusing on the hearing perception process of such events. In this paper, four criteria to detect sound were applied to the acoustic monitoring data collected in two urban areas, namely Andorra la Vella, Principality of Andorra, and Milan, Italy. At each site, the 1 s A-weighted short LAeq,1s time history, 10 min long, was available for each hour from 8:00 a.m. to 7:00 p.m. The resulting 92-time histories cover a reasonable range of urban environmental noise time patterns. The considered criteria to detect noise events are based on: (i) noise levels exceeding by +3 dB the continuous equivalent level LAeqT referred to the measurement time (T), criteria used in the definition of the Intermittency Ratio (IR) to detect noise events, (ii) noise levels exceeding by +3 dB the running continuous equivalent noise level, (iii) noise levels exceeding by +10 dB the 50th noise level percentile, (iv) progressive positive increments of noise levels greater than 10 dB from the event start time. Algorithms (iii) and (iv) appear suitable for notice-event detection, that is, those that (for their features) are clearly perceived and potentially annoy exposed people. The noise events detected by the above four algorithms were also evaluated by the available anomalous noise event detection (ANED) procedure to classify them as produced by road traffic noise or something else. Moreover, the assessment of the sonic environment by the Harmonica index was correlated with the single event level (SEL) of each event detected by the four algorithms. The threshold value of 8 for the Harmonica index, separating the “noisy” from the “very noisy” environments, corresponds to lower SEL levels for notice-events as identified by (iii) and (iv) algorithms (about 88–89 dB(A)) against those identified by (i) and (ii) criteria (92 dB(A)).

The ability of sound energy confinement with high-quality factor resonance is of vital importance for acoustic devices requiring high intensity and hypersensitivity in biological ultrasonics, enhanced collimated sound emission (i.e. sound laser) and high-resolution sensing. However, structures reported so far have been experimentally demonstrated with a limited quality factor of acoustic resonances, up to several tens in an open resonator. The emergence of bound states in the continuum makes it possible to realize high quality factor acoustic modes. Here, we report the theoretical design and experimental demonstration of acoustic bound states in the continuum supported by a single open resonator. We predicted that such an open acoustic resonator could simultaneously support three types of bound states in the continuum, including symmetry protected bound states in the continuum, Friedrich-Wintgen bound states in the continuum induced by mode interference, as well as a new type-mirror symmetry induced bound states in the continuum. We also experimentally demonstrated their existence with quality factor up to one order of magnitude greater than the highest quality factor reported in an open resonator., Though bound states in the continuum (BICs) in acoustic systems are attractive for acoustic resonators design, acoustic BICs typically show low Q-factor. Here, the authors report a high performance open acoustic resonator that supports symmetry-protected, Friedrich-Wintgen and mirror symmetry-induced BICs.

The effective elimination of sound energy traveling in open ducts is critical in acoustics, with considerable potential in diverse applications ranging from duct noise control to exhaust system design. However, previous absorber mechanisms are bulky or have limited ventilation or unsatisfactory absorption capability. We propose and experimentally demonstrate the concept of meta-mufflers capable of quasi-perfectly absorbing airborne sound propagating in an open duct while minimizing the influence on airflow because of its vanishing thickness. The mechanism uses coupling among asymmetric metasurfaces to achieve impedance match and subsequent high absorption in a subwavelength scale, which is explained analytically and validated experimentally. The proposed meta-muffler features an uncomplicated design, high attenuation efficiency, and a thickness of nearly two orders of magnitude of wavelength, ensuring the free pass of other entities. Furthermore, our design offers the flexibility of tuning and extending working bandwidth by hybridization of weak resonances. The performance of the proposed muffler is verified via experiments, which correlate excellently with theoretical predictions. The realization of meta-mufflers opens a route to novel acoustic absorbers and could have far-reaching implications in a plethora of important scenarios calling for both perfect sound absorption and open acoustic paths.

[EN] Sound diffusion refers to the ability of a surface to evenly scatter sound energy in both time and space. However, omni-directional radiation of sound, or perfect diffusion, can be impractical or difficult to reach under traditional means. This is due to the considerable size required, and the lack of tunability, of typical quarter-wavelength scattering strategies necessary for producing the required complexity of the surface acoustic impedance. As such, it can be a challenge to design sound diffusing structures that can display near perfect diffusion performance within slim dimensions. In this work, we propose a method for obtaining quasi-perfect and broadband sound diffusion coefficients using deep-subwavelength acoustic diffusers, i.e., metadiffusers. The relation among the geometry of the metasurface, the bandwidth, and the diffusion performance is analytically and numerically studied. For moderate bandwidths, around 1/3 of an octave, the method results in nearly perfect sound diffusion, while for a bandwidth of 2.5 octaves, a normalized diffusion coefficient of 0.8 was obtained using panels 1/30th thinner than traditional phase-grating designs. The ratio between the wavelength and the size of the unit cell was identified as a limitation of the performance. This work demonstrates the versatility and effectiveness of metadiffusers to generate diffuse reflections outperforming those of classical sound diffusers., The collaboration between the authors of this article was made possible thanks to the European Cooperation in Science and Technology (COST) Action CA15125-Design in Noise Reducing Materials (DENORMS). This study was financed by the Royal Opera House, Covent Garden, and the United Kingdom Acoustics Network (UKAN). The authors gratefully acknowledge the ANR-RGC METARoom (No. ANR-18-CE08-0021) project and the project HYPERMETA funded under the program Etoiles Montantes of the Region Pays de la Loire. N.J. acknowledges financial support from Generalitat Valenciana through Grant No. APOSTD/2017/042.

The traditional sound-absorbing structure converts sound energy into heat energy through internal friction of the damping material to be absorbed in order to achieve the purpose of reducing sound energy. This paper proposes a piezoelectric shunt damping sound absorption control scheme based on the positive piezoelectric effect of piezoelectric materials. The external sound is connected to the composite material formed by piezoelectric ceramics and rubber by an external circuit system formed by resistors, inductances, and capacitors. It can be converted into electric energy, and the sound wave energy is consumed by resistance heating. This solution can effectively improve the low-frequency sound absorption performance, thereby realizing wide-band reflected sound control.

An ultralight graphene oxide (GO)/polyvinyl alcohol (PVA) aerogel (GPA) is proposed as a new class of acoustic materials with tuneable and broadband sound absorption and sound transmission losses. The interaction between GO sheets and PVA molecules is exploited in our environmentally friendly manufacturing process to fabricate aerogels with hierarchical and tuneable porosity embedded in a honeycomb scaffold. The aerogels possess an enhanced ability to dissipate sound energy, with an extremely low density of 2.10 kg m−3, one of the lowest values ever reported for acoustic materials. We have first experimentally evaluated and optimised the effects of composition and thickness on the acoustic properties, namely sound absorption and sound transmission losses. Subsequently, we have employed a semi-analytical approach to evaluate the effect of different processing times on acoustic properties and assessed the relationships between the acoustic and non-acoustic properties of the materials. Over the 400–2500 Hz range, the reported average sound absorption coefficients are as high as 0.79, while the average sound transmission losses can reach 15.8 dB. We envisage that our subwavelength thin and light aerogel-based materials will possess other functional properties such as fire resistance and EMI shielding, and will prove to be novel acoustic materials for advanced engineering applications.

Sound energy is a ubiquitous renewable energy source. How to comprehensively harvest and utilize this energy has become a hot research topic. Triboelectric Nanogenerator(TENG) can convert different forms of mechanical energy into kinetic energy, and has significant advantages in micro-nano power generation, self-driving sensing, and device performance control. This work designs an ultra-high power density Helmholtz resonant cavity based on triboelectric nanogenerator with a printed electrode(H-TENG) for harvesting acoustic energy efficiently. We proposed an optimized scheme for harvesting sound energy effectively by the special treatment of the electrode flexible film. Then we researched the structure of the resonant cavity, acoustic conditions, and the tension of flexible film, they can affect the output characteristics of the H-TENG acoustic energy converter. We find H-TENG follows the best output frequency deviation law, that is, introducing appropriate tension according to a specific sound wave range can improve the best frequency response. Compared with the best results in the early literature, the H-TENG has a high power density of 1.47 W/m2 sound pressure. The study showed that the high power output of H-TENG can light up 457 LEDs at the same time. Finally, this research also proposes the application potential of H-TENG in energy conversion and acoustic sensing.

Underwater noise produced by rainfall is an important component of underwater ambient noise. For example, the existence of rainfall noise causes strong disturbances to sonar performance. The underwater noise produced by a single raindrop is the basis of rainfall noise. Therefore, it is necessary to study the associated underwater noise when drops strike the water surface. Previous research focused primarily on the sound pressure and frequency spectrum of underwater noise from single raindrops, but the study on its sound energy is insufficient. The purpose of this paper is to propose a method for predicting the acoustic energy generated by raindrops of any diameter. Here, a formula was derived to calculate the underwater sound energy radiated by single raindrops based on a dipole radiation pattern. A series of experiments were conducted to measure the underwater sound energy in a 15 m × 9 m × 6 m reverberation tank filled with tap water. The analysis of the acoustic energy characteristics and conversion efficiency from kinetic to acoustic energy helped develop the model to predict the average underwater sound energy radiated by single raindrops. Using this model, the total underwater sound energy of all raindrops during a rainfall event can be predicted based on the drop size distribution.

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.

Low noise asphalt pavement mitigates the impact of traffic noise on the ecological environment and surrounding population to a certain extent. To explore the noise reduction characteristics of different asphalt pavements, an indoor simulation device for pavement noise reduction is developed in this paper, based on the principle of pavement noise reduction. The device simulates the process of noise reflection and interference on the surface and inside of the pavement, which consumes the sound energy to achieve the noise reduction effect. The mechanical performance of six different types of asphalt mixtures is first tested. The noise reduction performance of the asphalt mixtures is then studied using the self-made noise testing device and Kundt’s tube method to verify the accuracy of the device. The influence of surface texture on pavement noise is also analyzed, and test results show that the six types of asphalt mixtures used in this study all meet mechanical performance requirements. A strong correlation is also observed between the noise testing device and the traditional Kundt’s tube method. The noise testing device is found to accurately measure the noise reduction characteristics of asphalt mixtures. For noise of different sound pressure levels, the same type of asphalt mixture displays different noise reduction capabilities. The better the noise reduction performance, the lower the sensitivity to sound wave changes, and the more stable the noise reduction effect. Excessive air void content does not significantly improve the pavement noise reduction performance, and it is recommended that the pavement air void content is maintained within the range of 17%–24%. The density of the asphalt mixture is found to be negatively correlated with the noise reduction performance, while the air void content and the mean texture depth is positively correlated with noise reduction. The results of grey relational analysis show that air void content is the most important factor affecting the noise reduction characteristics of asphalt pavement, followed by mean texture depth. Aggregate particle size is also found to exert a significant influence on noise reduction characteristics, and the influence degree of 9.5 mm particles is found to be the greatest, with 1.18 mm and 2.36 mm particles also showing a high degree of influence. Reasonable control of aggregate particle size should therefore be carried out in the design of low noise asphalt pavement.

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.

This study handles a situation in which an underwater robot approaches a moving emitter while measuring the sound made by the emitter. Since radio waves are easily dissipated in underwater environments, sound is the main energy transferred from the emitter to the underwater robot. In order to chase the emitter while measuring the emitter's sound, it is desirable that the robot can measure the sound energy made by the emitter as much as possible. In this study, the robot's path is designed to maximise the emitter's sound measured by the robot as well as to minimise the time spent until encountering the emitter. The effectiveness of the proposed path plan approach is demonstrated utilising extensive simulations.

Sound energy harvesting is one of the promising technologies due to the abundant and clean sound sources. It can be the semi-permanent alternative power supplies for wireless sensor networks (WSNs), which is significant in the Internet of Things (IoT). However, sound waves have the low energy density, so there are many kinds of research in recent years to overcome this problem. This paper provides a comprehensive review of sound energy harvesting, focusing on presenting principles, examples and enhancement methods of sound energy harvesters. In this paper, various approaches are introduced which are classified as sound pressure amplification and transduction mechanism. For sound pressure amplification, two typical types of energy harvesters are presented that one is using a resonator, another one is using an acoustic metamaterial, and these are based on piezoelectric, electromagnetic, and triboelectric mechanisms to convert sound energy to electrical energy.

The discovery of quasi-periodic atomic order in the crystalline state has uncovered an exciting new class of symmetries that has never been explored before. Because of their non-periodic translational order and self-similar properties, quasi-periodic structures offer unique opportunities for investigating questions related to their acoustical behavior. Their unique long-range non-periodic formations have the ability to diffuse and orchestrate the flow of sound energy in many unique ways; offering intriguing potential for innovating a new wave of optimized sound diffusers. One key limitation with available periodic diffusers is that their repeating logic creates repetitive energy loops, which significantly reduce their ability to uniformly disperse sound energy. Quasi-periodic geometry can mitigate such limitation. By encapsulating an infinite variety of distinct profiles in all directions, quasi-periodic surfaces can eliminate the formation of bundled or looped reflections; considerably enhancing the ability of the diffuser to uniformly disperse sound energy. To investigate this hypothesis, an experimental approach is used to simulate sound reflection patterns of the quasi-periodic surface profiles using a ray tracing method. Both qualitative and quantitative analyses are used to interpret the simulated results. The international Standards (ISO) metrics are used to validate the proposed approach and verify the results. Results show that the diffusion quality of the tested quasi-periodic surface is superior to the diffusion performance of the tested periodic surface.

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.

Helmholtz resonators offer an ideal platform for advanced sound absorbers, but their utility has been impeded by inherent frequency range limitations and the lack of function reconfiguration. Here, we introduce a multi-order Helmholtz resonator (MHR) that allows multiple monopolar resonant modes theoretically and experimentally. The combination of these modularized MHRs further creates reconfigurable multi-band anomalous absorbers in a two-port transparent waveguide while maintaining undisturbed air ventilation. In asymmetric absorption state through coupling of artificial sound soft boundary with preposed MHR, sound energy is almost totally absorbed in multiple frequency ranges when sound waves are incident from one side while it is largely reflected back from the opposite side. Interestingly, the original asymmetric absorber would turn into symmetric bidirectional absorber if one post MHR concatenates after the soft boundary. Using combination of identical MHRs, we demonstrate function selective asymmetric/symmetric absorber in multi-bands, highlighting the potential to use MHRs in the design of diverse devices for more versatile applications.

The design of a compact low-frequency acoustic absorber whose thickness is less than the operating frequency wavelength is a difficult challenge. However, recent developments in metamaterial engineering have enabled significant advances towards this goal. This paper presents an investigation of a low-frequency absorber (i.e., operating between 100 and 600 Hz) structured around a new concept of a thin cavity of coiled spaces coupled with a panel and micro-slit. The ability of this acoustic metamaterial absorber to control sound energy with increased relative bandwidth is examined analytically and numerically considering the viscous-thermal losses in both the micro-slit and coiled spaces. A prototype absorber was manufactured using advanced additive manufacturing technique and evaluated in an impedance tube apparatus. The experimental results showed the absorber capable of controlling 95% of the sound energy at 365 Hz with a relative bandwidth of 60.3%. These results compare well to those obtained from the analytical analysis and finite element method. Thus, the proposed acoustic absorber is shown to be capable of operating in the deep-subwavelength scale.

Previous work has explored novel binaural combinations of reverberation and the resulting perceived reverberation strength (reverberance). The present study examines the perceptual effects of additional binaural combinations of reverberation with the goal of explaining reverberance in terms of basic psychoacoustic principles. Stimuli were generated using virtual space techniques simulating a speech source 3 m to the listener's right in a moderately reverberant environment. Reverberant energy at the ears was varied systematically relative to the natural level for the environment (0-dB gain). The method of magnitude estimation was used to estimate reverberance. Four experiments were conducted. Experiment 1 tested monaural listening conditions for both left and right ears at reverberation gains from -21 dB to 0 dB. Experiment 2 tested a binaural listening condition where only reverberant energy at the ear farther from the source was manipulated (-21 dB to 0 dB). Experiment 3 tested two binaural conditions over a wider range of reverberation gains (-18 dB to +24 dB). In one condition, reverberant energy was manipulated for both ears equally. In the other condition, reverberant energy was manipulated only for the ear nearer the source. In Experiment 4, reverberant tails of the stimuli were removed to test whether listeners were able to use ongoing reverberant information to judge reverberance. The results from all experiments were found to be well predicted by a model of time-varying binaural loudness that focused on “glimpses” in time with relatively high reverberant sound energy and low direct sound energy. These findings suggest that the mechanisms underlying reverberance and loudness may be similar.

The objective of this study is to explore the harvest maximum output changes occurs in piezoelectric transducer when connected in different configuration to produce highest electricity consumption and generate sound energy harvesting system. Acoustic energy is a type of environmental energy source that can be extracted and converted into electrical energy for small-scale energy applications. In this study, the corresponding load resistance for single piezoelectric transducer is 4.5 k$\Omega$, together with a constant vibration source at a frequency of 68 Hz and a 1-g acceleration. The performance of output voltage and power of piezoelectric are evaluated and the optimum output is measured by depending on the connection of the piezoelectric transducer arrangement into series, parallel and series, and parallel which stimulated using Proteus software. The experiment result presented that, when have single piezo, 5 piezo connected in series, and 5 piezo connected in parallel, the output powers are 1.664 mW, 1.671mW and 7.676 mW, respectively. During the combination series and parallel connections, the output power of 3S1P piezo increases to 5.05mW. In a parallel configuration, the output voltage that produced is much higher than a piezoelectric transducer arrangement connected in series connection. The piezoelectric transducer that connected in parallel configurations increases its voltage output from 2.83 V to 13.05 V with the same polarity. The arrangement of piezoelectric transducer in parallel configuration is affordable, with its higher production of a higher power output compared to the arrangement of piezoelectric transducer in series connection. Whereas in terms of power output, the maximum power remains constant in the range of 1.665 to 1.671 mW when three transducers are in series configuration connection with the same polarity. In conclusion, the proper implementation of the piezoelectric array configuration is needed in order to operate the minimum energy for low load devices and promise to accomplish generated optimum power output in harvester system.