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Room acoustic simulations can be performed by means of numerical methods, which typically solve the wave equation in an enclosure using discretization techniques. These methods provide high-fidelity solvers that include all the wave phenomena but are computationally costly. This paper presents the potential of a reduced basis method for simulating room acoustics when the boundary properties are changed iteratively, e.g., design changes in rooms where different boundary properties are simulated, which increase the computational cost when using traditional high-fidelity solvers. The presented framework allows reducing the computational burden by applying reduced basis methods and solving the problem in a reduced low-dimensional subspace where the absorption properties of complex boundary conditions are parameterized, e.g., the thickness of a porous material. The potential of the proposed framework is analyzed in terms of computational efficiency, accuracy and storage requirements. We show a computational reduction of two orders of magnitude for a 2D case with an upper frequency of 2 kHz and three orders of magnitude for a 3D simple case with an upper frequency of 1 kHz showing a potential of having an increase in terms of speedup when increasing the size of the domain. The storage requirement for a 2D case with an upper frequency of 8 kHz is 10.8GB while for a 3D case with an upper frequency of 1 kHz is 8GB.

For Sabine absorption coefficient measurements in reverberation rooms, it is preferable to excite as many room modes as possible. But the number of excited room modes is significantly affected by many factors, among which one dominating factor is the loudspeaker location particularly at frequencies below the Schroeder frequency. In turn, this could significantly influence the statistical absorption coefficient obtained according to ISO 354. Therefore, this study aims to investigate the impact of loudspeaker configurations on the excitation of room modes. Frequency response functions are measured using a broad band steady state noise signal using four sound sources at various locations, and the numbers of excited room modes in the one-third octave bands are quantified and compared against those by Green's function simulations. A procedure for determining favorable loudspeaker positions based on the excited room modes is proposed, which can be a useful input to the working group of the ISO 354 standard.

The Reflections series takes a look back on historical articles from The Journal of the Acoustical Society of America that have had a significant impact on the science and practice of acoustics.

Hearing-impaired people need more stringent acoustic and noise requirements than normal-hearing people in terms of speech intelligibility and listening effort. Multiple guidelines recommend a maximum reverberation time of 0.4 s in classrooms, signal-to-noise ratios (SNRs) greater than 15 dB, and ambient noise levels lower than 35 dBA. We measured noise levels and room acoustic parameters of 12 classrooms in two schools for hearing-impaired pupils, a dormitory apartment for the hearing-impaired, and a church mainly for the hearing-impaired in the Republic of Korea. Additionally, subjective speech clarity and quality of verbal communication were evaluated through questionnaires and interviews with hearing-impaired students in one school. Large differences in subjective speech perception were found between younger primary school pupils and older pupils. Subjective data from the questionnaire and interview were inconsistent; major challenges in obtaining reliable subjective speech perception and limitations of the results are discussed.

Litter and compost were obtained at a cowshed of a livestock farm in Andong city in Korea. The morphological change of nitrogen and phosphorous from these samples were examined and suggested a more useful and realistic way for reducing them. Constituents and their content of sample were identified by XRF. The nitrite ion (NO2 −), nitrate ion (NO2 −) and phosphate ion (PO4 3−) and ammonium ion (NH4 +), total phosphorous (T-P) and total nitrogen (T-N) released from sample were analyzed using ion chromatograph and UV/Vis spectrometry. As the results of this study, the ammonia in the early stage of cow excretion is a need to make an ammonia gas state that can be immediately volatile by increasing the pH. Nitrogen and phosphorous, the main source of nutrition in green algal bloom can be reduced by transforming insoluble salts such as calcium phosphate (CaHPO4·3H2O) and struvite (NH4MgPO4·6H2O), respectively, with addition of Ca and Mg after stimulating fermentation of compost.

Acoustic simulation and auralization are important tools for predicting the acoustic environment before construction. Today we use two main approaches for simulating room acoustics, namely geometrical acoustics (GA) and wave-based methods (WBM). The often-favored GA methods show inaccuracy in small spaces at low frequencies where the wave-phenomena play a larger role. Solving the wave-equations using numerical WBM can provide more accurate results, but due to expensive computations it is often deselected. In an attempt to combine GA and WBM, we investigated the perceptual crossover frequency that has little to no audible difference between the two auralizations. We used ODEON for a GA method, and discontinuous Galerkin finite element method for WBM. A listening test was performed on 9 subjects whose task was to evaluate octave-band filtered speech and music stimuli in two rooms via a 3-alternative forced choice test: a small rectangular room and an L-shaped room. The results reveal that the monaural auralizations from thetwo simulation schemes are perceived differently although the reverberation time and clarity indices are within the just noticeable differences.

Vehicle driving simulators allow to recreate the experience of driving a real vehicle. The implementation of such simulators requires the use of sound reproduction methods, which may influence the sound perception differently inside the vehicles. This study investigated sound localization in a virtual-reality-based driving simulator for three sound reproduction methods: higher-order Ambisonics (HOA) reproduced with a spherical array of 64 loudspeakers, the matrix inversion method reproduced with a circular array including eight loudspeakers and the binaural reproduction with headphones. Ten experienced drivers provided sound localization judgments in terms of perceived distance and horizontal angle. Sound recordings were made inside a real vehicle and presented at five source positions inside the car in the virtual sound environment. The simulated scenarios included the operating conditions of idling and constant speed at 100 km/h, as well as a warning and a speech signal. The binaural reproduction over headphones outperformed the reproduction methods with loudspeakers in terms of perceived horizontal angle, whereas perceived distance was more accurate for the methods including loudspeakers. Moreover, distance perception was more affected for the 100 k/m than for the idling condition. The results highlight the need to consider the sound reproduction method when developing and testing vehicle features.

The sound-field auditory steady-state response (ASSR) is a promising measure for the objective validation of hearing-aid fitting in patients who are unable to respond to behavioral testing reliably. To record the sound-field ASSR, the stimulus is reproduced through a loudspeaker placed in front of the patient. However, the reverberation and background noise of the measurement room could reduce the stimulus modulation used for eliciting the ASSR. As the ASSR level is heavily dependent on the stimulus modulation, any reduction due to room acoustics could affect the clinical viability of sound-field ASSR testing. This study investigated the effect of room acoustics on the level and detection rate of sound-field ASSR. The study also analyzed whether early decay time and an auditory-inspired relative modulation power model could be used to predict the changes in the recorded ASSR in rooms. A monaural auralization approach was used to measure sound-field ASSR via insert earphones. ASSR was measured for 15 normal-hearing adult subjects using narrow-band CE-Chirps® centered at the octave bands of 500, 1000, 2000, and 4000 Hz. These stimuli were convolved with simulated impulse responses of three rooms inspired by audiological testing rooms. The results showed a significant reduction of the ASSR level for the room conditions compared with the reference anechoic condition. Despite this reduction, the detection rates for the first harmonics of the ASSR were unaffected when sufficiently long recordings (up to 6 min) were made. Furthermore, the early decay time and relative modulation power appear to be useful predictors of the ASSR level in the measurement rooms.

The development of efficient and accurate numerical methods for simulating realistic sound in virtual environments—such as computer games and VR/AR—has been an active research area for the last decades. However, handling dynamic scenes with many moving sources is still challenging due to intractable storage requirements and extensive computation time. A recently proposed physics-informed neural network (PINN) approach learns a compact and efficient surrogate model with parameterized moving sources and impedance boundaries on a grid-less 1-D domain. Contrary to traditional “black-box” deep learning, PINNs minimize the residuals of the governing equations through the loss function. We will extend this work using flow maps implemented as Residual Networks (ResNets). ResNets are interpreted from a dynamic systems perspective as ordinary differential equations that can be used as building blocks to approximate the governing equations in time. We will examine the pros and cons of ResNets in acoustics and compare them with state-of-the-art numerical methods and vanilla feed-forward neural networks in terms of accuracy and efficiency.

The use of model-based numerical simulation of wave propagation in rooms for engineering applications requires that acoustic conditions for multiple parameters are evaluated iteratively and this is computationally expensive. We present a reduced basis methods (RBM) to achieve a computational cost reduction relative to a traditional full order model (FOM), for wave-based room acoustic simulations with parametrized boundary conditions. In this study, the FOM solver is based on the spectral element method, however other numerical methods could be applied. The RBM reduces the computational burden by solving the problem in a low-dimensional subspace for parametrized frequency-independent and frequency-dependent boundary conditions. The problem is formulated and solved in the Laplace domain, which ensures the stability of the reduced order model based on the RBM approach. We study the potential of the proposed RBM framework in terms of computational efficiency, accuracy and storage requirements and we show that the RBM leads to 100-fold speed-ups for a 2D case with an upper frequency of 2kHz and around 1000-fold speed-ups for an analogous 3D case with an upper frequency of 1kHz. While the FOM simulations needed to construct the ROM are expensive, we demonstrate that despite this cost, the ROM has a potential of three orders of magnitude faster than the FOM when four different boundary conditions are simulated per room surface. Moreover, results show that the storage model for the ROM is relatively high but affordable for the presented 2D and 3D cases., 10 figures and 2 tables

It is well known that input data uncertainty has a major influence on the correctness of room acoustic simulations. This paper proposes a comprehensive framework for experimental validation and uncertainty quantification in room acoustic simulations. The sources of uncertainty in room acoustic simulations are many, but especially the boundary conditions present significant uncertainty. The input data uncertainty is propagated through the simulation model, which in this work is a wave-based time-domain discontinuous Galerkin finite element method (DGFEM) simulation scheme, and is then presented as a ‘probability box’ (p-box), expressing both aleatory and epistemic input data uncertainties and model form uncertainties. The p-box can then be compared against the perceptual limits of the human auditory system to gauge the severity of the uncertainty. The framework is applied on a small room with an absorbing wall for the experimental validation of the simulation results and the uncertainty quantification of three boundary parameters: the flow resistivity of the porous absorber, the thickness of the porous absorber, and the absorption characteristics of the remaining hard walls. It is found that the input data uncertainty and the model form uncertainty are too high for the model to be able to predict several room acoustic parameters within just noticeable thresholds.

The image source method (ISM) is inherently difficult to parallelize efficiently due to the data dependency of the image source tree. This paper presents and compares two different high-performance computing implementation strategies for the image source method. One strategy is designed such that the number of operations needed and the memory usage are reduced as much as possible using validity checks. However, this implies a data dependency that makes efficient parallelization impossible when scaling to massively parallel computing architectures, e.g., graphics processing units (GPUs). Thus, this method is mainly suited for multi-core CPUs, where the core count is low. The other strategy is designed for massively parallel computing architectures by eliminating data dependencies and conditional flow statements. However, in order to preserve data independence, considerable amount of unnecessary calculations must be carried out, some of which are ultimately discarded due to the image sources being invalid or invisible. The performance of the implementations is assessed when running the former method on a multi-core CPU and the latter on a GPU. Both implementations are found to be able to simulate within real-time constraints in fairly complex rooms up to a third reflection order, however, in most cases the GPU calculation is faster.

Realistic sound is essential in virtual environments, such as computer games and mixed reality. Efficient and accurate numerical methods for pre-calculating acoustics have been developed over the last decade; however, pre-calculating acoustics makes handling dynamic scenes with moving sources challenging, requiring intractable memory storage. A physics-informed neural network (PINN) method in 1D is presented, which learns a compact and efficient surrogate model with parameterized moving Gaussian sources and impedance boundaries, and satisfies a system of coupled equations. The model shows relative mean errors below 2\%/0.2 dB and proposes a first step in developing PINNs for realistic 3D scenes., Comment: 19 pages (double line spacing), 3 figures, 2 tables

In this study, simulations of a scale model of an unfurnished rectangular room with an absorbing ceiling using an energy-based geometrical acoustic model and a phased geometrical acoustic model are compared to measurements. This room can represent a typical classroom or office. This comparison concluded that the phased model can capture more details of the sound field, particularly at low frequencies. For accurate predictions, precise surface descriptions are needed for the phased GA method, which are not always available. The main drawback of the phased model is a longer calculation time.

This study proposes an experimental method for evaluating isotropy in enclosures, based on an analysis of the wavenumber spectrum in the spherical harmonics domain. The wavenumber spectrum, which results from expanding an arbitrary sound field into a plane-wave basis, is used to characterize the spatial properties of the observed sound field. Subsequently, the obtained wavenumber spectrum is expanded into a series of spherical harmonics, and the moments from this spherical expansion are used to characterize the isotropy of the wave field. The analytical framework is presented. The method is examined numerically and experimentally, based on array measurements in four chambers: two anechoic chambers (one with a single source and another with an array of 52 sources), a reverberation chamber, and the same reverberation chamber with a sample of absorbing material on the floor. The results indicate that the proposed methodology is suitable for assessing the isotropy of a sound field.

Subjective ratings from 25 professional musicians and sound engineers were obtained to assess two Danish rock venues of similar size and similar low frequency reverberation times, but different high frequency reverberation times. The musicians judged one hall significantly better than the other, confirming a hypothesis that rock venues can have a longer reverberation time at mid to high frequencies at least in the empty condition. A fairly long reverberation time in the 63 Hz octave band is found to be acceptable, so the 125 Hz octave band is probably the single most important band to control for amplified music.

Architects require the insight of acoustic engineers to understand how to improve and/or optimize the acoustic performance of their buildings. Normally this is supported by the architect providing digital models of the design to the acoustic engineer for analysis in the acoustician’s disciplinary software, for instance Odeon. This current workflow suffers from the following challenges: (1) architects typically require feedback on architectural disciplinary models that have too much geometric information unnecessarily complicating the acoustic analysis process; (2) the acoustician then has to waste time simplifying that geometry, (3) finally, this extra work wastes money which could otherwise be spent on faster design iterations supported by frequent feedback between architects and acousticians early in the design process. This paper focuses on the architect / acoustician workflow, however similar challenges can be found in other disciplines. OpenBIM workflows provide opportunities to increase the standardization of processes and interfaces between disciplines by reducing the reliance on the proprietary discipline specific file formats and tools. This paper lays the foundation for an OpenBIM workflow to enable the acoustic engineer to provide near real time feedback on the acoustic performance of the architectural design. The proposed workflow investigates the use of the international standard IFC as a design format rather than simply an exchange format. The workflow is presented here with the intention that this will be further explored and developed by other researchers, architects and acousticians.

This study aims to evaluate the effects of architectural layouts of emergency departments on activity patterns/work routines and consequent noise levels. Three Danish hospitals' emergency departments that had different layouts were investigated via on-site noise measurements over three days and observations of noisy activities. The time-averaged noise levels in the three emergency departments turned out to be significantly different, ranging from 50 to 59 dBA. During the observation of noisy activities, the noise levels and occurrences of individual activities were noted and correlated with the long-term measurements. Major noise sources that have high correlations with the three-day noise levels are identified as loud staff communication and noise from alarms/electronic devices. Potential remedies are suggested in connection to the emergency departments’ architectural layouts. Especially unnecessarily loud communication between medical staff and loud and frequent equipment/patient transportation noise need to be improved for more comfortable acoustic environments.

In recent years, the potential of sound absorbing metamaterials as an alternative to traditional acoustic absorbers has been demonstrated in several publications. A common way to design these absorbing metamaterials, is through periodic arrangements of resonators. By combining acoustic resonators tuned to different resonance frequencies in each unit cell, the frequency range for which the material achieves high absorption can be extended. Nevertheless, there is limited research on their behavior when they are exposed to waves incident at oblique directions, and this information is highly relevant for a large amount of practical applications; particularly in room acoustics. In this work, the finite element method (FEM) is used, to study the angle-dependent absorption properties of infinitely large 2D surfaces consisting of a periodic arrangement of Helmholtz resonators. It is found that the incidence angle influences not only the magnitude of the (maximum) absorption coefficient, but also the frequencies at which the maximum absorption coefficient is observed, as well as the absorption bandwidth.

There are useful conversion methods from Sabine absorption coefficients according to ISO 354 to other acoustic properties for room boundaries, e.g., surface impedance or flow resistivity. However, most available sound absorption coefficients are practical absorption coefficients, which are simplified according to ISO 11654 from the Sabine absorption coefficients and widely used by absorber manufacturers as their absorbers’ performance indicators. In this study, practical absorption coefficients are used to inversely characterize the flow resistivity via reliable models. 15 absorber samples with varying mounting conditions are used for validating the flow resistivity estimation, having a wide flow resistivity range of 10–110 kNsm−4. As expected, the practical absorption coefficients are found to be less reliable input parameters for estimating the flow resistivity, but with more datasets, the degradation becomes insignificant compared to the Sabine absorption coefficients. The estimated flow resistivity will be a valuable parameter to predict the absorption characteristics of different thickness and mounting conditions.

The acoustic properties of surfaces are commonly evaluated using samples of finite size, which generate edge diffraction effects that are often disregarded. This study makes use of sound scattering theory to characterize such finite samples. In a given sound field, the samples can be described by a unique complex directivity function called the far-field pattern. Numerical results show that the far-field pattern contains extensive information on the tested samples, including sound absorption and surface scattering, as well as scattering due to finiteness. In this paper, a method is introduced to estimate the far-field pattern of a finite sample. The method relies on measurements of the sound pressure and acoustic particle velocity in the near-field of the sample, and it makes use of the Helmholtz integral equation. The proposed technique is examined in an anechoic room where the sound field near the test sample is scanned with a three-dimensional sound intensity probe. The estimated far-field pattern is compared with numerical predictions up to 1 kHz.

This work investigates the use of the initial decay time to obtain the Sabine absorption coefficient from measurements conducted in a reverberation chamber. Due to non-uniform distribution of sound absorption in the test chamber, measured energy decay functions exhibit multiple slopes, which cannot be evaluated unambiguously using linear regression as prescribed in the current standard (ISO 354, International Organization for Standardization, Geneva, Switzerland, 2003). As an alternative, this study proposes a Bayesian framework that allows estimating multiple decay parameters, hence capturing more accurately the energy decay features. Measurements are carried out in a reverberation chamber with and without diffusing elements to investigate the influence of diffusers on the absorption coefficient and on the decay process. Measured absorption coefficients of a porous sample are compared to theoretical values estimated with a transfer matrix model. The results show that the Sabine absorption coefficient calculated using the shortest decay time agrees well with the size-corrected theoretical absorption coefficient.

This work reports on the use of bandgaps to increase the efficiency of sound radiation employing defect modes on a phononic crystal (PnC). A PnC consisting of a 2D array of studs on an aluminum plate is considered, and a defect is created by removing four studs. Numerical simulations predict 8 dB higher radiation efficiency and significantly more uniform directivity of sound due to the piston-like defect modes that suppress interference between acoustic waves. An experimental study of the vibrational response is carried out in order to validate the numerical result. Comparisons of the radiation efficiency and the directivity index between the numerical and experimental results show good agreement. These findings may pave the way to use bandgap structures as effective acoustic radiators.

This paper presents a wave-based numerical scheme based on a spectral element method, coupled with an implicit-explicit Runge-Kutta time stepping method, for simulating room acoustics in the time domain. The scheme has certain features which make it highly attractive for room acoustic simulations, namely (a) its low dispersion and dissipation properties due to a high-order spatio-temporal discretization; (b) a high degree of geometric flexibility, where adaptive, unstructured meshes with curvilinear mesh elements are supported; and (c) its suitability for parallel implementation on modern many-core computer hardware. A method for modelling locally reacting, frequency dependent impedance boundary conditions within the scheme is developed, in which the boundary impedance is mapped to a multipole rational function and formulated in differential form. Various numerical experiments are presented, which reveal the accuracy and cost-efficiency of the proposed numerical scheme.

Background and Objectives: Noise levels and room acoustic parameters at a tertiary referral hospital, Seoul National University Hospital (SNUH) in Korea, are investigated. Materials and Methods: Through a questionnaire, acoustically problematic rooms are identified. Noise levels in emergency rooms (ERs) and intensive care units (ICUs) are measured over about three days. Acoustically critical and problematic rooms in the otolaryngology department are measured including examination rooms, operating rooms, nurse stations, receptions, and patient rooms. Results: The A-weighted equivalent noise level, L Aeq , ranges from 54 to 56 dBA, which is at least 10 dB lower than the noise levels of 65 to 73 dBA measured in American ERs. In an ICU, the noise level for the first night was 66 dBA, which came down to 56 dBA for the next day. The noise levels during three different ear surgeries vary from 57 to 62 dBA, depending on the use of surgical drills and suctions. The noise levels in a patient room is found to be 47 dBA, while the nurse stations and the receptions have high noise levels up to 64 dBA. The reverberation times in an operation room, examination room, and single patient room are found to be below 0.6 s. Conclusions: At SNUH, the nurse stations and receptions were found to be quite noisy. The ERs were quieter than in the previous studies. The measured reverberation times seemed low enough but some other nurse stations and examination rooms were not satisfactory according to the questionnaire.

The absorption properties of room surfaces have a major influence on the acoustics of rooms. In wave-based room acoustic simulations it is common practice to model room surfaces using a local-reaction approximation, instead of modeling the full extended-reaction behavior. However, previous research has indicated that the local-reaction assumption is not appropriate for surfaces that have elastic properties or fluid layers, such as soft porous materials, porous materials backed by an air cavity and airtight membranes. In this talk we present a method for incorporating the extended-reaction behavior of porous absorbers into a time-domain discontinuous Galerkin (dG) based room acoustic simulation scheme. The dG method is attractive for room acoustic simulations due to its flexibility and cost-efficiency. The porous material is modeled using an equivalent fluid model (EFM). The EFM formulation is validated analytically and experimentally, and it is shown that using the EFM leads to significant and perceptually noticeable improvements in simulation accuracy, when modeling single reflections and full room simulations with various types of porous absorbers.

This study presents a new model order reduction technique applied to room acoustic simulations using a high-order numerical scheme based on the spectral element method. The goal is to efficiently simulate iterative design processes in room acoustics, where the room acoustics with different boundary absorption properties are evaluated much quickly without solving the full wave equation. The wave-based methods are highly accurate but expensive when simulating high-frequencies and long simulation times in large and complex geometries, which are needed for industrial applications. Hence, it remains a key challenge to improve the computational performance of room acoustics simulations with different boundary material possibilities. With model order reduction, the boundary condition can be parametrized in the subsequent modelling. This allows to reduce the dimensionality of a wave-based simulator without compromising the transient wave propagation accuracy while enabling significant reductions of computational costs after initial training using proper orthogonal decomposition. We provide evidence that the combination of the high-order numerical scheme with model order reduction has significant potential value for building acoustics in terms of computational efficiency and accuracy.

The periodic distribution of mechanical resonators on a thin plate has been studied to improve panel loudspeakers by exploiting the band gap phenomenon (BG), effectively forbidding vibrational modes in certain frequency ranges. Here, we present an optimization procedure to determine the distribution with a minimized number of resonators while achieving the desired sound radiation. A thin aluminum plate with periodic resonators is considered to open a BG from 355 Hz to 710 Hz, which covers an octave band. The structure is modeled using the finite element method. Each resonator is parameterized with a topological design variable, which represents the absence and presence of the resonator. Numerical examples are presented to demonstrate the optimization for the design of a defect and removing ineffective resonators to obtain the desired sound radiation. Experimental validations show good agreement with the numerical results in terms of spectral smoothness and spatial uniformity of sound radiation. In this case, the overall SPL is amplified by 10 dB, and the standard deviation is reduced by 5 dB by comparison with a bare plate.