Reverberant Field Research Articles

Pengukuran Sound Pressure Level (SPL) dalam kabin mobil dengan menggunakan SEA Model

Statistical energy analysis (SEA) is a well-known method to analyze the flow of acoustic and vibration energy in a complex structure. For an acoustic space where significant absorptive materials are present, direct field component from the sound source dominates the total sound field rather than a reverberant field, where the latter becomes the basis in constructing the conventional SEA model. Such environment can be found in a car interior and thus a corrected SEA model is proposed here to counter this situation. The model is developed by eliminating the direct field component from the total sound field and only the power after the first reflection is considered. A test car cabin was divided into two subsystems and by using a loudspeaker as a sound source, the power injection method in SEA was employed to obtain the corrected coupling loss factor and the damping loss factor from the corrected SEA model. These parameters were then used to predict the sound pressure level in the interior cabin using the injected input power from the engine. The results show satisfactory agreement with the directly measured SPL.Keywords : Statictical Energy Analysis (SEA), Sound Pressure level (SPL), DLF and CLF

A suggested modification to ANSI S12.51 (ISO 3741) Annex D

Annex D of ANSI S12.51 (ISO 3741) addresses the issue of qualifying reverberation rooms for determining sound power of fixed sources that emit sound at discrete frequencies. At low frequencies reverberation rooms have reduced modal density which causes a highly varying (spatially) sound field, resulting in a less accurate measure of the space-average sound pressure level in the reverberant field of the room compared to higher frequency bands. Annex D sets limits on the maximum allowable standard deviation in the 1/3-octave bands from 100 Hz to 2,500 Hz, and Annex E provides limits for 50 Hz, 63 Hz, and 80 Hz. This presentation discusses the required test procedure and presents examples of real room qualification test results and how rooms can be modified to meet this requirement. One fairly unique example will also be shown where the room fails to meet the qualification limits, when in fact it truly should. A recommended modification to the data analysis process is suggest to resolve this unique problem.

Isotropy in decaying reverberant sound fields.

A method of evaluating sound field isotropy in decaying reverberant sound fields is presented. The proposed method extends the experimental framework outlined in [J. Acoust. Soc. Am. 143(4), 2514-2526 (2018)] and analyzes the decaying sound field in a reverberation room. Spatio-temporal measurements of the sound field are obtained, and a wavenumber decomposition is performed as a function of time, which serves to examine the directional properties of the sound field and its angular symmetry. Experimental results are obtained in a reverberation room in four different configurations (the empty room, with an absorber on the floor, with panel diffusers, and without them). The results demonstrate how isotropy tends to increase or decrease as a function of time, depending on the disposition of the diffusing and absorbing elements. Diffusers are found to effectively redirect the energy in the room, although they do not succeed in generating a uniform incidence on the sample. The proposed approach makes it possible to analyze the specific processes occurring in a reverberation chamber and can provide valuable insights in the process of standardization to verify the directional properties found in each reverberation room.

Comparison of sound location variations in free and reverberant fields: An event-related potential study.

This study compares event-related potentials (ERPs) elicited by variations of sound location in free and reverberant fields. The virtual sound sources located at azimuths 0°-40° were synthesized with head-related transfer functions and binaural room impulse responses for free and reverberant fields, respectively. The sound stimulus at 0° was assigned as standard in the oddball paradigm. Results show that the P3 amplitude is larger in the free field and acoustical conditions have no significant effect on the amplitudes of N2 and mismatch negativity. Moreover, a linear relationship between sound angle and amplitude of ERP components is observed.

Denoising Directional Room Impulse Responses with Spatially Anisotropic Late Reverberation Tails

Directional room impulse responses (DRIR) measured with spherical microphone arrays (SMA) enable the reproduction of room reverberation effects on three-dimensional surround-sound systems (e.g., Higher-Order Ambisonics) through multichannel convolution. However, such measurements inevitably contain a nondecaying noise floor that may produce an audible “infinite reverberation effect” upon convolution. If the late reverberation tail can be considered a diffuse field before reaching the noise floor, the latter may be removed and replaced with an extension of the exponentially-decaying tail synthesized as a zero-mean Gaussian noise. This has previously been shown to preserve the diffuse-field properties of the late reverberation tail when performed in the spherical harmonic domain (SHD). In this paper, we show that in the case of highly anisotropic yet incoherent late fields, the spatial symmetry of the spherical harmonics is not conducive to preserving the energy distribution of the reverberation tail. To remedy this, we propose denoising in an optimized spatial domain obtained by plane-wave decomposition (PWD), and demonstrate that this method equally preserves the incoherence of the late reverberation field.

The Acoustic Environment of York Minster’s Chapter House

York Minster is the largest medieval Gothic cathedral in Northern Europe, renowned for its magnificent architecture and its stained glass windows. Both acoustic measurements and simulation techniques have been used to analyse the acoustic environment of its Chapter House, which dates from the 13th-century and features an octagonal geometry with Gothic Decorated stone walls replete of geometric patterns and enormous stained glass windows, covered by a decorated wooden vault. Measured and simulated room impulse responses served to better understand how their architectural features work together to create its highly reverberant acoustic field. The authors start by analysing its acoustic characteristics in relation to its original purpose as a meeting place of the cathedral’s Chapter, and end by reflecting on its modern use for a variety of cultural events, such as concerts and exhibitions. This work is part of the “Cathedral Acoustics” project, funded by the EC through the Marie-Sklodowska-Curie scheme.

Theoretical analysis on structure of sound energy field decay of acoustical radiosity model with finite initial excitation.

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

Source localization in reverberation environment based on improved equivalent sound source near-field acoustic holography algorithm

The compressive-equivalent source method near-field acoustic holography (C-ESM) is disturbed by reverberation in the enclosed space such as room and cabin, which leads to large reconstruction error and disturbs the judgment of sound source position. In order to solve this problem, an improved C-ESM algorithm based on room impulse response (RIR) is proposed to filter out reverberation interference in this paper. Different from the original equivalent source method, the improved algorithm constructs the transfer function through the room impulse response to establish the relationship between the equivalent source and the sound pressure on any plane in space, and the sparse signal reconstruction method of the compressive sensing technology is used to obtain the strength of the equivalent source. Then the transfer function to any plane of space is established according to the free field Green’s function, to eliminate the interference of reverberation and improve the effect of sound source location. The accuracy and effectiveness of the improved method are verified by preliminary numerical simulation. And the results show that compared with the original algorithm, this method has obvious advantages in sound source localization in a reverberant field.

Near Direct and Far Reverberant Field Response of Direct Radi ator Loudspeakers

Typical audience seating arrangements in rooms and auditoria warrant reinvestigation of the direct radiator speaker response in the near-direct and far-reverberant fields, as the response data provided by the manufacturer is always ideal and does not account for the effect of those fields. The speaker response characteristics of a variety of direct radiator loudspeakers ranging from the conventional squawker to the full range radiator have been investigated in these fields. The speaker response is investigated in the 50 -10 kHz frequency range, by measuring the A-weighted SPL (sound pressure level) in the near-direct and far-reverberant fields, using an acoustic analyzer. The field-specific characteristic for each of the radiators is determined by fitting the SPL data obtained to an appropriate polynomial. The coefficients obtained thereby, allow an objective field-specific study amongst radiators. When a set of direct radiator loudspeakers is available, it is necessary to configure their application, depending upon the optimum sound quality required for a given enclosure, in near-direct field and far-reverberant field. The outcome of this work assists one to configure the best radiator ensemble for a given enclosure, despite placement constraints.

Research on creation and evolution of artificial diffuse sound field

In a closed space, it is difficult to create a diffuse sound field by using a single source in the reverberation tank. In this paper, a methodology for construction of diffuse sound field using multiple source excitation is proposed. Proposed methodology is based on space processing technique for a single source sound field and also involves reciprocity of sound field transfer function. Firstly, the analytical method of acoustic excitation reverberation field is studied and subsequently the sound pressure equation for excitation reverberation sound field in case of multi sound is obtained. In addition, the numerical method to observe the influence of sound source distribution upon uniformity of sound field is studied by the acoustic analysis software ACTRAN. Finally, diffuse sound field in a glass tank of 1.5 m × 0.9 m × 0.6 m dimensions, is constructed by using 30 emission transducers of the same type. In the end, the diffuse sound field is scanned using scanning array and is then evaluated by measuring the standard difference of the space distribution of sound field. The results show that the standard deviation of the sound pressure distribution in the diffused sound field in comparison with 30 transmitting transducers simultaneously within the reverberation control area is not greater than 3.9 dB under the narrow band condition. Hence, it clearly illustrates that construction of diffused sound field by this method is definitely a viable option.

A hybrid Finite Element-Statistical Energy Analysis method for impulsive and transient loading

The hybrid approach coupling Statistical Energy Analysis (SEA) and the finite element method has become a prominent technique for analysing structures under steady-state loads in the ‘mid-frequency’ range where some components behave in a deterministic manner with low modal density and others in a statistical manner with high modal density and statistical overlap. In this paper, the method is extended from its current steady-state capability to provide calculation of structural responses under impulsive and time-varying loads. Similar to the steady-state method, a system is split into deterministic components that are modelled using the finite element approach and statistical components that are modelled as SEA subsystems.An evolutionary spectrum approach based on the Priestley description of random processes is applied to model the response of both the SEA and deterministic components which are coupled by considering a power balance between the SEA subsystems. The diffuse field reciprocity relationship that relates the reverberant forces generated by a subsystem to the energy within it is explored under transient conditions where it is found that it can be important to account for the build-up of a reverberant field following an impulse. Results from the method are compared against finite element simulations for a system of plates coupled by a beam and it is found to generate predictions with the accuracy expected of an SEA-based method, although deterministic oscillations of the deterministic system at early times are not accounted for.

An initial study of complete 2D shear wave dispersion images using a reverberant shear wave field

Within the field of elastography, a relatively new approach analyzes the limiting case of shear waves established as a reverberant field. In this framework, it is assumed that a distribution of shear waves exists, oriented across all directions in 3D and continuous in time. The simultaneous multi-frequency application of reverberant shear wave fields can be accomplished by applying an array of external sources that can be excited by multiple frequencies within a bandwidth, for example 50, 100, 150, …, 500 Hz, all contributing to the shear wave field produced in the liver or other target organ. This enables the analysis of the dispersion of shear wave speed as it increases with frequency, indicating the viscoelastic and lossy nature of the tissue under study. Furthermore, dispersion images can be created and displayed alongside the shear wave speed images. We report preliminary studies on breast and liver tissues using the multi-frequency reverberant shear wave technique, employing frequencies up to 700 Hz in breast tissue, and robust reverberant patterns of shear waves across the entire liver and kidney in obese patients. Dispersion images are shown to have contrast between tissue types and with quantitative values that align with previous studies.

Two definitions of the inner product of modes and their use in calculating non-diffuse reverberant sound fields.

There are two definitions of the inner product of modal spatial functions used in the literature. Both definitions integrate the product of the modal spatial functions over a line, area, or volume. The only difference is that one of the definitions takes the complex conjugate of one of the modal spatial functions before multiplying the modes together. The definitions are the same if the modal spatial functions are real. If the modal spatial functions are complex, only the definition which takes the complex conjugate is an inner product. If the specific acoustic impedance of the boundaries has a real part, then the modes are only orthogonal with the definition which does not take the complex conjugate, although this definition is not strictly an inner product because the modal spatial functions are complex in this situation. However, this definition of "inner product" can be used to calculate the coefficients in the modal expansion of the system response. On the other hand, when it comes to calculating the mean pressure squared and the mean sound intensity, the modal spatial functions cross-products cannot be ignored because the modes are not orthogonal for the definition which takes the complex conjugate.

Empirical prediction of traffic noise transmission loss across plenum windows

A parametric study on the traffic noise transmission loss across plenum windows was carried out experimentally in this investigation in an attempt to establish a simple empirical model for predicting this transmission loss. A total of fourteen full scale plenum windows were included in this study. The results of a site mockup measurement were used for model validation. The present model was developed based on the existing plenum chamber theory in which the sound fields inside the plenum window cavities were assumed to make up of a diffracted wave and a reverberant field. Results suggest that both the diffracted and reverberant field inside the plenum window cavities are weaker than those assumed in existing plenum chamber theory. It is found that a model, which assumes frequency-independent diffraction directivity and percentage reverberant field attenuation, gives the best prediction of traffic noise transmission loss. This prediction model is also able to predict site test results with good accuracy.

Strains/Stresses in Plates Subjected to Reverberant Acoustic Excitation Using Statistical Energy Analysis

Objective of the present work is to theoretically estimate the strains generated in a plate by a reverberant acoustic excitation, in SEA frame work. An expression for determining the strains due to the trace wave generated in the plate is derived, which is an essential component of vibration caused due to the acoustic excitation. Strains developed in a plate when subjected to reverberant acoustic field are obtained experimentally and the results are compared with the theoretically estimated results and they are in good agreement.

Sparse Representation of a Spatial Sound Field in a Reverberant Environment

This paper investigates sound-field modeling in a realistic reverberant setting. Starting from a few point-like microphone measurements, the goal is to estimate the direct source field within a whole three-dimensional (3-D) space around these microphones. Previous sparse sound field decompositions assumed only a spatial sparsity of the source distribution, but could generally not handle reverberation. We here add an explicit model of the reverberant sound field, that has two components: the first component sparse in the plane-wave domain, the other component low-rank as a multiplication of transfer functions and source signals. We derive the corresponding decomposition algorithm based on the alternating direction method of multipliers. We furthermore provide empirical rules for tuning the two parameters to be set in the algorithm. Numerical and experimental results indicate that the decomposition and reconstruction performances are significantly improved, in the case of reverberant environments.

Volumetric reconstruction of acoustic energy flows in a reverberation room.

This study examines the spatial and directional properties of net energy flows in a reverberation chamber. Based on measurements with a spherical array, a method is proposed to estimate the flows of acoustic energy in the volume surrounding the array. The proposed method is used to examine the steady state, early decay, and late decay of the sound field in a reverberation room (both empty and with an absorber on the floor). The results show that the approach is successful in characterizing the spatio-spectral and spatio-temporal properties of power flows in reverberant sound fields, constituting a valuable analysis tool.

Acoustic properties of sound-absorbing polyester fabrics woven with thick staple and thin draw textured yarn for use in interior decoration

Environmental pollution in our daily life because of noise is an increasing public health concern. We designed and woven with staple- and draw-textured yarns, for sound absorbance purposes. For wefts and warps, two different thicknesses of the thick staple-yarn and thin draw-textured filament yarn were used. The prepared fabrics were classified in porous, medium, and dense, on the basis of their densities and air permeabilities, and the sound absorption coefficients of the fabrics were found to be linearly related to the air permeability. The sound absorption coefficient determined by the impedance tube method was higher for porous fabrics compared with that for dense fabrics in the high frequency region. By the reverberant field method, the sound absorption coefficients for all fabrics were highest in the low frequency region. Porous fabrics exhibited a shorter reverberation time and a higher sound absorption coefficient than dense fabrics owing to their good sound absorption property.

Base-Case Model for Measurement Uncertainty in a Reverberation Chamber Including Frequency Stirring

We address the uncertainty of reverberation chamber (RC) measurements in presence of both mechanical and frequency stirring (FS). A base-case model is derived for reverberation fields affected by the measurement uncertainty due to the lack of a perfect statistical uniformity of fields in a RC. It is found that the measurement uncertainty associated with the FS depends on both the total uncorrelated samples and the local insertion loss (IL). The local IL depends on the frequency stirring bandwidth (FSB). The model allows us for obtaining separate measurement uncertainty contributions. Measurements support the achieved uncertainty model. In particular, results show that the dependence on the IL is normally rather weak also when very wide FSBs are used.

Correlation matrix methods to assess the stirring performance of electromagnetic reverberation chambers

The use of correlation matrices to evaluate the number of uncorrelated stirrer positions of electromagnetic reverberation chambers has widespread applications in electromagnetic compatibility. We present a review of recent methods based on multivariate correlation functions that relates statistical inhomogeneities in space (frequency) to the reduction of uncorrelated cavity configurations. Full wave finite-difference time domain simulations of an actual reverberation chamber are performed through an in-house parallel code. The efficiency of this code allows for capturing extensive inhomogeneous/anisotropic reverberation fields at frequencies close to the lowest usable frequency (LUF) of the chamber. The concept of effective independent position is revised in light of random sampling and a model-driven relation with the probability distribution of correlation matrix entries is used to take into account spatial (frequency) inhomogeneities. Driven by extensive simulation data, an empirical probability density function is found for the correlation matrix elements to be non-central t-student distributed with asymmetry increasing towards low frequencies.