Sensitivity of the predicted acoustic pressure field to the wind and temperature profiles in a conventionally neutral boundary layer.

Simulations are widely used to predict noise emissions from traffic, railroad, aircraft, and wind farms and for sound field control. The latter employs multiple sources interacting and it requires accurate phase information. Acoustic models require precise characterization of the medium properties. The logarithmic profile is one of the most commonly used forms to model the wind speed. However, this profile is accurate only in neutral conditions, i.e., when there is not heat flux at the surface. The conventionally neutral boundary layer (CNBL) is the most frequently occurring neutral regime. In this case, the logarithmic profile underestimates the wind speed. This paper analyses the effect that this modelling error has on the sound field close to the ground, for near-ground sources. The first section introduces an approximation of the wind and temperature profiles in such a regime. Afterwards, the sound fields corresponding to the logarithmic profile, a representative CNBL profile, and three more test cases are simulated using the Crank-Nicholson parabolic equation; these are compared employing different metrics. The difference in wind speed introduces a phase error that increases with distance. Moreover, wind speed underestimations also lead to underpredictions of the energy refracted downward.

A controlled chamber study of effects of exposure to diesel exhaust particles and noise on heart rate variability and endothelial function.

BACKGROUND: Adverse cardiovascular effects are associated with both diesel exhaust and road traffic noise, but these exposures are hard to disentangle epidemiologically. We used an experimental setup to evaluate the impact of diesel exhaust particles and traffic noise, alone and combined, on intermediary outcomes related to the autonomic nervous system and increased cardiovascular risk. METHODS: In a controlled chamber 18 healthy adults were exposed to four scenarios in a randomized cross-over fashion. Each exposure scenario consisted of either filtered (clean) air or diesel engine exhaust (particle mass concentrations around 300 µg/m3), and either low (46 dB(A)) or high (75 dB(A)) levels of traffic noise for 3 h at rest. ECG was recorded for 10-min periods before and during each exposure type, and frequency-domain heart rate variability (HRV) computed. Endothelial dysfunction and arterial stiffness were assessed after each exposure using EndoPAT 2000. RESULTS: Compared to control exposure, HRV in the high frequency band decreased during exposure to diesel exhaust, both alone and combined with noise, but not during noise exposure only. These differences were more pronounced in women. We observed no synergistic effects of combined exposure, and no significant differences between exposure scenarios for other HRV indices, endothelial function or arterial stiffness. CONCLUSION: Three-hour exposure to diesel exhaust, but not noise, was associated with decreased HRV in the high frequency band. This indicates activation of irritant receptor-mediated autonomic reflexes, a possible mechanism for the cardiovascular risks of diesel exposure. There was no effect on endothelial dysfunction or arterial stiffness after exposure.

Absorption and scattering by perforated facings with periodic narrow slits.

This paper develops a theory for the sound absorption and scattering of perforated slit absorbers. A rigid plane, perforated periodically in one dimension with absorbing slits, scatters incoming sound waves as discrete wave components in different directions. The absorbing slits are assumed to be line-like in the sense that their width is much shorter than the wavelengths. The equation for the sound field is solved in the wavenumber domain. The slits are described with an impedance description, assuming local reaction of the slits (typically a Helmholtz resonator). The solution is found by means of an inverse transform, back to the spatial domain. This results in an explicit formulation of the sound field, including a sum consisting of components that either radiate energy in discrete directions or are surface waves. A similar sum is also included in a term that can be interpreted as radiation impedance. The explicit expressions for the absorption and scattering coefficients are found with the aid of the radiating part of the scattered and reflected field. Numerical results of the absorption and scattering coefficients are presented. The result is verified with finite element method and compared with the result from an alternative general formulation of the problem.

Wave transmission and vibration response in periodically stiffened plates using a free wave approach.

There are various structures constructed with periodically stiffened thin plates. Vibration prediction of such structures is not easy compared to the structures comprised of uniform plates only due to the mathematical complexity stemmed from the periodic nature. This study provides the analytic method to predict the wave transmission at junctions connecting two semi-infinite periodic structures and the response of a finite periodic structure to an external harmonic point force. The same theoretical framework is employed for dealing with both phenomena. First, free wave solutions are obtained by solving the governing equation for the bending motion of a periodically stiffened, infinite plate using the spatial Fourier Transform and the Floquet's theorem. Then, the free wave solutions are linearly superposed, and the linear coefficients are calculated by applying the appropriate boundary conditions. Numerical simulation is conducted. In dealing with the periodic finite structure, the result is compared with that by the finite element analysis. It is revealed that the periodic nature of the structures affects both the energy transmission and the vibration response of the periodically stiffened plates.

Hybrid analytical-numerical optimization design methodology of acoustic metamaterials for sound insulation.

Acoustic metamaterials are becoming promising solutions for many industry applications, but the gap between theory and practice is still difficult to close. This research proposes an optimization methodology of acoustic metamaterial designs for sound insulation that aims to start bridging this gap. The proposed methodology takes advantage of a hybrid analytical-numerical approach for computing the sound transmission loss of the designs efficiently. As a result, the implementation of optimization techniques on numerical model designs becomes practically possible. This is exemplified with two test cases: (i) optimization of the sound transmission loss of a single gypsum board panel and (ii) optimization of the noise reduction of outdoor HVAC units. Two resonator designs, one used previously for sound radiation in flat panel speakers and the other for enhancing the sound transmission loss at the mass-air-mass resonance of double panels, are here optimized for the two test cases. This shows how an existing resonator can be adapted for new purposes, thus making the design of acoustic metamaterials efficient. The optimized metamaterials outperform the original designs as well as traditional approaches to sound insulation.

Gaussian processes for sound field reconstruction.

This study examines the use of Gaussian process (GP) regression for sound field reconstruction. GPs enable the reconstruction of a sound field from a limited set of observations based on the use of a covariance function (a kernel) that models the spatial correlation between points in the sound field. Significantly, the approach makes it possible to quantify the uncertainty on the reconstruction in a closed form. In this study, the relation between reconstruction based on GPs and classical reconstruction methods based on linear regression is examined from an acoustical perspective. Several kernels are analyzed for their potential in sound field reconstruction, and a hierarchical Bayesian parameterization is introduced, which enables the construction of a plane wave kernel of variable sparsity. The performance of the kernels is numerically studied and compared to classical reconstruction methods based on linear regression. The results demonstrate the benefits of using GPs in sound field analysis. The hierarchical parameterization shows the overall best performance, adequately reconstructing fundamentally different sound fields. The approach appears to be particularly powerful when prior knowledge of the sound field would not be available.

An analytical model for broadband sound transmission loss of a finite single leaf wall using a metamaterial.

Acoustic metamaterials (AM) have emerged as an academic discipline within the last decade. When used for sound insulation, metamaterials can show high transmission loss at low frequencies, despite having low mass per unit area. This paper investigates the possibility of using AMs to increase the sound insulation of finite single leaf walls (SLWs), focusing on the coincidence effect problem. Formulas are derived using a variational technique for the forced sound transmission of finite SLWs with a coupled array of single degree of freedom resonators. An analytical model is presented for this simple case, and the effects of the band gap in sound transmission and radiation are analyzed. Moreover, the influence of each parameter is studied, especially the presence of losses, giving way to an optimized way of designing this type of structure using constrained parameter optimization. Numerical validations are performed and discussed. Finally, some conclusions are drawn regarding the effectiveness of the proposed model, including possible applications.

Large-scale outdoor sound field control.

The feasibility and the performance of controlling low frequency sound of loudspeaker systems under varying atmospheric conditions is examined experimentally. In the experiment, a control subwoofer array is canceling the sound of a primary subwoofer array over long distances (∼100 m) and in large areas (∼320 m2) using the pressure-matching method. To avoid the measurement of the sound field over the entire control area, a sound propagation model is introduced that is fitted in situ to model the radiation properties of the loudspeakers and the variation of the speed of sound. The results show that the control system reduces the sound pressure levels by up to 15-20 dB over the subwoofers' frequency range. However, the reduction can vary considerably depending on the specific atmospheric condition. The model-based approach reduces the number of required measurements and achieves similar reduction performance to the control based on direct measurements with considerably fewer microphone locations while also being more robust. Additionally, the sound propagation model enables the reduction of acoustic energy in virtual control zones that are far away from the microphone location. The investigated methodology has a direct application in the mitigation of sound from outdoor concerts.

Experimental characterization of the sound field in a reverberation room.

Measured values of acoustic absorption obtained from standardized reverberation-chamber measurements often differ across laboratories. These discrepancies arise due to non-isotropic sound incidence on the absorbing specimen, diffraction at the sample edges, and differences in the chambers' shapes and dimensions. The present study examines an experimental method for characterizing the distribution of sound incidence on the specimen in the steady state. The methodology relies on a plane wave decomposition (i.e., estimation of the wavenumber spectrum) to determine the magnitude of the sound waves arriving from definite directions onto the absorbing sample. Based on this decomposition, the sound pressure, particle velocity, and sound intensity can be reconstructed in the vicinity of the absorbing specimen. One can distinguish between the incident and reflected components of the sound field, making it possible to characterize the incident energy flows. Measurements with a programmable robotic arm are conducted in a reverberation chamber in two damping conditions (empty and with absorption on the floor). The quantitative accuracy of the method is examined via an estimation of the sample's angle-dependent absorption coefficient, showing good agreement with theoretical predictions. It is anticipated that the proposed method will be of value in explaining the deviations encountered across standardized laboratories.

Characterization of sound scattering using near-field pressure and particle velocity measurements.

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.

A Bayesian spherical harmonics source radiation model for sound field control.

In sound field reproduction and sound field control systems, the acoustic transfer functions between a set of sources and an extended reproduction area need to be accurately estimated in order to achieve good performance. This implies that large amounts of measurements should be performed if the area is large compared to the wavelengths of interest. In this paper, a method for reconstructing these transfer functions in highly damped conditions is proposed by using only a small number of measurements in the reproduction area. The source radiation is modeled with the spherical harmonics basis and its amplitude coefficients are fitted with Bayesian inference. This approach is validated in a sound field control experiment where a set of 12 control loudspeakers attenuate the sound pressure level generated by a set of six primary loudspeakers in a quiet zone while minimizing their radiation into a listening zone. The performance of the approach is studied by analyzing the sound field reconstruction and the sound field control performance. It is shown that it is possible to get-with few measurements and the source radiation model-results similar to those achieved using a dense grid of transfer function measurements.

A wavenumber approach to quantifying the isotropy of the sound field in reverberant spaces.

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.

An optical flow-based state-space model of the vocal folds.

High-speed movies of the vocal fold vibration are valuable data to reveal vocal fold features for voice pathology diagnosis. This work presents a suitable Bayesian model and a purely theoretical discussion for further development of a framework for continuum biomechanical features estimation. A linear and Gaussian nonstationary state-space model is proposed and thoroughly discussed. The evolution model is based on a self-sustained three-dimensional finite element model of the vocal folds, and the observation model involves a dense optical flow algorithm. The results show that the method is able to capture different deformation patterns between the computed optical flow and the finite element deformation, controlled by the choice of the model tissue parameters.

Estimation of surface impedance at oblique incidence based on sparse array processing.

A method is proposed to estimate the surface impedance of a large absorptive panel from free-field measurements with a spherical microphone array. The method relies on the reconstruction of the pressure and the particle velocity on the studied surface using an equivalent source method based on spherical array measurements. The sound field measured by the array is mainly composed of an incident and a reflected wave, so it can be represented as a spatially sparse problem. This makes it possible to use compressive sensing in order to enhance the resolution and the quality of the estimation. The results indicate an accurate reconstruction for angles of incidence between 0° and 60°, and between approximately 200 and 4000 Hz. Additionally, experimental challenges are discussed, such as the sample's finiteness at low frequencies and the estimation of the background noise.

In situ measurements of the oblique incidence sound absorption coefficient for finite sized absorbers.

Absorption coefficients are mostly measured in reverberation rooms or with impedance tubes. Since these methods are only suitable for measuring the random incidence and the normal incidence absorption coefficient, there exists an increasing need for absorption coefficient measurement of finite absorbers at oblique incidence in situ. Due to the edge diffraction effect, oblique incidence methods considering an infinite sample fail to measure the absorption coefficient at large incidence angles of finite samples. This paper aims for the development of a measurement method that accounts for the finiteness of the absorber. A sound field model, which accounts for scattering from the finite absorber edges, assuming plane wave incidence is derived. A significant influence of the finiteness on the radiation impedance and the corresponding absorption coefficient is found. A finite surface method, which combines microphone array measurements over a finite sample with the sound field model in an inverse manner, is proposed. Besides, a temporal subtraction method, a microphone array method, impedance tube measurements, and an equivalent fluid model are used for validation. The finite surface method gives promising agreement with theory, especially at near grazing incidence. Thus, the finite surface method is proposed for further measurements at large incidence angles.

Development and validation of a combined phased acoustical radiosity and image source model for predicting sound fields in rooms.

A model, combining acoustical radiosity and the image source method, including phase shifts on reflection, has been developed. The model is denoted Phased Acoustical Radiosity and Image Source Method (PARISM), and it has been developed in order to be able to model both specular and diffuse reflections with complex-valued and angle-dependent boundary conditions. This paper mainly describes the combination of the two models and the implementation of the angle-dependent boundary conditions. It furthermore describes how a pressure impulse response is obtained from the energy-based acoustical radiosity by regarding the model as being stochastic. Three methods of implementation are proposed and investigated, and finally, recommendations are made for their use. Validation of the image source method is done by comparison with finite element simulations of a rectangular room with a porous absorber ceiling. Results from the full model are compared with results from other simulation tools and with measurements. The comparisons of the full model are done for real-valued and angle-independent surface properties. The proposed model agrees well with both the measured results and the alternative theories, and furthermore shows a more realistic spatial variation than energy-based methods due to the fact that interference is considered.

Investigation of model based beamforming and Bayesian inversion signal processing methods for seismic localization of underground sources.

Techniques have been studied for the localization of an underground source with seismic interrogation signals. Much of the work has involved defining either a P-wave acoustic model or a dispersive surface wave model to the received signal and applying the time-delay processing technique and frequency-wavenumber processing to determine the location of the underground tunnel. Considering the case of determining the location of an underground tunnel, this paper proposed two physical models, the acoustic approximation ray tracing model and the finite difference time domain three-dimensional (3D) elastic wave model to represent the received seismic signal. Two localization algorithms, beamforming and Bayesian inversion, are developed for each physical model. The beam-forming algorithms implemented are the modified time-and-delay beamformer and the F-K beamformer. Inversion is posed as an optimization problem to estimate the unknown position variable using the described physical forward models. The proposed four methodologies are demonstrated and compared using seismic signals recorded by geophones set up on ground surface generated by a surface seismic excitation. The examples show that for field data, inversion for localization is most advantageous when the forward model completely describe all the elastic wave components as is the case of the FDTD 3D elastic model.

An objective measure for the sensitivity of room impulse response and its link to a diffuse sound field.

This study is relevant to acoustic measurements in reverberation rooms such as measurements of sound transmission, sound absorption, and sound power levels of noise sources. The study presents a quantitative measure for the diffuseness in a room, which is first introduced theoretically and subsequently examined experimentally. The sensitivity of a room due to changes in the initial conditions is quantified by measuring a pair of impulse responses in a room differing only in the sound source position. Such changes are linked to mixing and the diffuse sound field. The measure is based on the maximum of the absolute value of the cross-correlation between the time windowed sections of the two impulse responses. By integrating this quantity normalized by the energy of the impulse response of the room, a single number rating is obtained. Results based on three sets of experiments indicate that the diffusers and absorbers in the room influence the proposed sensitivity measures systematically.

Sex differences in vocal behavior in virtual rooms compared to real rooms.

This study investigates speech production under various room acoustic conditions in virtual environments, by comparing vocal behavior and the subjective experience of speaking in four real rooms and their audio-visual virtual replicas. Sex differences were explored. Males and females (N = 13) adjusted their voice levels similarly to room acoustic changes in the real rooms, but only males did so in the virtual rooms. Females, however, rated the visual virtual environment as more realistic compared to males. This suggests a discrepancy between sexes regarding the experience of realism in a virtual environment and changes in objective behavioral measures such as voice level.

The equivalent incidence angle for porous absorbers backed by a hard surface.

An equivalent incidence angle is defined as the incidence angle at which the oblique incidence absorption coefficient best approximates the random incidence absorption coefficient. Once the equivalent angle is known, the random incidence absorption coefficient can be estimated by a single experiment using a free-field absorption measurement technique with a source at the equivalent angle. This study investigates the equivalent angle for locally and extendedly reacting porous media mainly by a numerical approach: Numerical minimizations of a cost function that is the difference between the oblique incidence absorption coefficient at a specific incidence angle and the random incidence absorption coefficient. The equivalent angle is found to be around 55° under local reaction conditions, and 45° for extendedly reacting porous absorbers. As practical guidelines for measuring absorption coefficients by free-field techniques, a broad incidence angle range can be suggested: 20° ≤ θi ≤ 65° for extended reaction and θi ≤ 65° for locally reacting porous absorbers, if an average difference of 0.05 is allowed.