Relationship between listening difficulty rating and objective measures in reverberant and noisy sound fields for young adults and elderly persons

Listening difficulty ratings [Morimoto et al., J. Acoust. Soc. Am. 116, 1607-1613 (2004)] were obtained for 20 young adult listeners and 34 elderly listeners in reverberant and noisy sound fields simulated in an anechoic room. The listening difficulty ratings were compared with acoustical objective measures. The results and analyses showed the following: (i) The correlation between listening difficulty ratings and the revised speech transmission index (STI(r)), and that for the useful-detrimental ratio (U(50)) were high, regardless of the age of the listeners. (ii) STI(r) and U(50) need to be increased by 0.12 and 4.2 dB, respectively, to equalize the listening difficulty ratings for the elderly listeners with those for the young listeners. (iii) The estimation accuracies for STI(r) and U(50) can be improved by calculating them with the L(eq) of background noise linearly increased by 4 to 10 dB, which depends on the age of the listeners and the objective measures. However, the improvement was not statistically significant for the elderly listeners.     

The coherence of reverberant sound fields

A new method of measuring spatial correlation functions in reverberant sound fields is presented. It is shown that coherence functions determined with appropriate spectral resolution contain the same information as the corresponding correlation functions, and that measuring such coherence functions is a far more efficient way of obtaining this information. The technique is then used to verify theoretical predictions of the spatial correlation between various components of the particle velocity in a diffuse sound field. Other possible applications of the technique are discussed and illustrated with experimental results obtained in an ordinary room.     

Spatial cross-correlation of acoustic pressures in steady and decaying reverberant sound fields

From the equation for the steady state sound pressure distribution produced in a rectangular reverberation chamber by a point source, and by using the usual high frequency approximations, it is shown that, for a random source position, the cross-correlation function for two points not too far apart approaches that of Cook et al. in the reverberant field of the chamber. When the same approach is used on the equation for sound pressure decay when the point source excitation is cut off, the cross-correlation function obtained for the initial portion of the decay corresponds with that determined experimentally by Balachandran and Robinson.     

The use of cross-spectral density measurements in partially reverberant sound fields

The cross-spectral densities of sound at two points in a partially reverberant space are shown to be sensitive to the power flow out of the space. A nearly homogeneous sound field is found to possess a co-spectral density which is nearly insensitive to the nature of the field. This spectrum is traditionally obtained as a cross-correlation and its analytical form has been well established. Perhaps less well known is the imaginary part of the cross-spectrum which is shown in this paper to be sensitive to the direction of propagation from a sound scattering enclosure. Specific analytical forms of the spectrum are derived for incidence included within certain cones by using a wavefunction model of the scattered sound field. In some cases the co- and quadrature-spectra are related to each other by the Hilbert transform.     

Acceptable range of speech level for both young and aged listeners in reverberant and quiet sound fields

The speech level of verbal information in public spaces should be determined to make it acceptable to as many listeners as possible, while simultaneously maintaining maximum intelligibility and considering the variation in the hearing levels of listeners. In the present study, the universally acceptable range of speech level in reverberant and quiet sound fields for both young listeners with normal hearing and aged listeners with hearing loss due to aging was investigated. Word intelligibility scores and listening difficulty ratings as a function of speech level were obtained by listening tests. The results of the listening tests clarified that (1) the universally acceptable ranges of speech level are from 60 to 70 dBA, from 56 to 61 dBA, from 52 to 67 dBA and from 58 to 63 dBA for the test sound fields with the reverberation times of 0.0, 0.5, 1.0 and 2.0 s, respectively, and (2) there is a speech level that falls within all of the universally acceptable ranges of speech level obtained in the present study; that speech level is around 60 dBA.     

On the angular error of intensity vector based direction of arrival estimation in reverberant sound fields

An acoustic vector sensor provides measurements of both the pressure and particle velocity of a sound field in which it is placed. These measurements are vectorial in nature and can be used for the purpose of source localization. A straightforward approach towards determining the direction of arrival (DOA) utilizes the acoustic intensity vector, which is the product of pressure and particle velocity. The accuracy of an intensity vector based DOA estimator in the presence of noise has been analyzed previously. In this paper, the effects of reverberation upon the accuracy of such a DOA estimator are examined. It is shown that particular realizations of reverberation differ from an ideal isotropically diffuse field, and induce an estimation bias which is dependent upon the room impulse responses (RIRs). The limited knowledge available pertaining the RIRs is expressed statistically by employing the diffuse qualities of reverberation to extend Polack's statistical RIR model. Expressions for evaluating the typical bias magnitude as well as its probability distribution are derived.     

Similarities of vowels in nonreverberant and reverberant fields

Perceptual distances among single tokens of American English vowels were established for nonreverberant and reverberant conditions. Fifteen vowels in the phonetic context (b-t), embedded in the sentence "Mark the (b-t) again" were recorded by a male talker. For the reverberant condition, the sentences were played through a room with a reverberation time of 1.2 s. The CVC syllables were removed from the sentences and presented in pairs to ten subjects with audiometrically normal hearing, who judged the similarity of the syllable pairs separately for the nonreverberant and reverberant conditions. The results were analyzed by multidimensional scaling procedures, which showed that the perceptual data were accounted for by a three-dimensional vowel space. Correlations were obtained between the coordinates of the vowels along each dimension and selected acoustic parameters. For both conditions, dimensions 1 and 2 were highly correlated with formant frequencies F2 and F1, respectively, and dimension 3 was correlated with the product of the duration of the vowels and the difference between F3 and F1 expressed on the Bark scale. These observations are discussed in terms of the influence of reverberation on speech perception.     

Relaxation of sound fields in rooms of diffusely reflecting boundaries and its application in acoustical radiosity simulation

The acoustical radiosity method is a computationally expensive acoustical simulation algorithm that assumes an enclosure with ideal diffuse reflecting boundaries. Miles observed that for such an enclosure, the sound energy decay of every point on the boundaries will gradually converge to exponential manner with a uniform decay rate. Therefore, the ratio of radiosity between every pair of points on the boundaries will converge to a constant, and the radiosity across the boundaries will approach a fixed distribution during the sound decay process, where radiosity is defined as the acoustic power per unit area leaving (or being received by) a point on a boundary. We call this phenomenon the "relaxation" of the sound field. In this paper, we study the relaxation in rooms of different shapes with different boundary absorptions. Criteria based on the relaxation of the sound field are proposed to terminate the costly and unnecessary radiosity computation in the later phase, which can then be replaced by a fast regression step to speed up the acoustical radiosity simulation.     

Approximate reconstruction of sound fields close to the source surface using spherical nearfield acoustical holography

This paper presents an investigation of the reconstruction of sound field parameters close to the surface of arbitrarily shaped sound sources. The field is reconstructed using nearfield acoustical holography (NAH) in spherical coordinates. Of particular interest are source shapes where the Rayleigh hypothesis is violated. To overcome the limitation of the minimal sphere given by the validity restriction of the Rayleigh hypothesis an algorithm is proposed for extracting local information from the nonconvergent NAH solution. For the assessment of the results an appropriate virtual test rig is developed employing the Kirchhoff-Helmholtz integral theorem.     

Low latency localization of multiple sound sources in reverberant environments

Sound source localization algorithms determine the physical position of a sound source in respect to a listener. For practical applications, a localization algorithm design has to take into account real world conditions like multiple active sources, reverberation, and noise. The application can impose additional constraints on the algorithm, e.g., a requirement for low latency. This work defines the most important constraints for practical applications, introduces an algorithm, which tries to fulfill all requirements as good as possible, and compares it to state-of-the-art sound source localization approaches.     

Computational studies of steady-state sound field and reverberant sound decay in a system of two coupled rooms

The acoustical properties of an irregularly shaped room consisting of two connected rectangular subrooms were studied. An eigenmode method supported by a numerical implementation has been used to predict acoustic characteristics of the coupled system, such as the distribution of the sound pressure in steady-state and the reverberation time. In the theoretical model a low-frequency limit was considered. In this case the eigenmodes are lightly damped, thusthey were approximated by normal acoustic modes of a hard-walled room. The eigenfunctions and eigenfrequencies were computed numerically via application of a forced oscillator method with a finite difference algorithm. The influence of coupling between subrooms on acoustic parameters of the enclosure was demonstrated in numerical simulations where different distributions of absorbing materials on the walls of the subrooms and various positions of the sound source were assumed. Calculation results have shown that for large differences in the absorption coefficient in the subrooms the effect of modal localization contributes to peaks of RMS pressure in steady-state and a large increase in the reverberation time.      

混响声场条件下水声互易传递函数的测量及应用

对不规则的混响声场情况的互易测量方法进行了理论研究,对相关测量误差提出了修正方法。通过测量结果的空间平均和混响环境对声能密度影响的评估,修正了声源体积速度的计算值,并在水中进行了试验验证。经修正后的互易测量结果与正向实测结果基本一致。表明混响声场不影响互易原理有效性的成立,但影响了声源体积速度的计算,进而影响了应用互易原理对传递函数的测量。提出的修正方法在不规则的混响声场情况下简单有效,结论为今后基于互易原理的工程应用提供了参考和依据。      

Evaluation of sound absorbing coefficients in a reverberant room by computer-ray simulation

Standardized methods for the measurement of the sound absorption coefficient, α, of materials in a reverberant room use well known relations between α and the reverberation time of the room. Actually, these relations are correct only in rooms with high diffusion at low α values and for uniform distribution of the absorption on the surfaces. The introduction of acoustic materials with high α values concentrated on only one surface, as recommended by standards, causes a derandomization of the sound field, which may overcome the ‘randomization strength’ of the enclosure and therefore make the use of traditional formulae incorrect. This paper proposes a computer-ray tracing model, which enables these phenomena to be described, and operatively defines the ‘randomization strength’ of room design. Lastly, an alternative approach is suggested for evaluating the sound absorption coefficient, based on the characterization of the reverberant room by its own ‘calibration curve’.     

Application and measurement of underwater acoustic reciprocity transfer functions in reverberant sound field

The reciprocity measurement theory in anomalous reverberant sound fields was investigated.An improved method Was proposed due to the interrelated errors.The source volume velocity Was corrected by spatial average of measurement results and evaluation of the reverberant sound field influence on acoustic energy density.The result was validated in underwater experiment,corrected reciprocity measurement results were almost the same as direct measurement results.It indicates that reverberant sound field does not affect the validitv of the principle,but influences the obtainment of source volume velocity,then influences the measurement of transfer functions with the principle.The proposed method is simple and effective in anomalous reverberant sound fields.The study mav be valuable for the applications which are based on the principle.     

Relationship between impulse response and other types of room acoustical responses

This paper presents a method of calculating sound build up, steady state and sound reduction phenomena from the impulse response of rooms. The noise components of both the testing signal and the room response are omitted and wave phenomena occurring in the room are also neglected. A situation corresponding to the geometrical propagation of sound is thus simulated. The resulting formulae are an extension of corresponding methods for the numerical modelling of acoustical fields in rooms. In this way, as well as the impulse response, sound build up and reverberation curves may also be obtained. An example using the ray tracing technique is presented.     

Effect of source spectrum on sound localization in an everyday reverberant room

Two experiments explored how frequency content impacts sound localization for sounds containing reverberant energy. Virtual sound sources from thirteen lateral angles and four distances were simulated in the frontal horizontal plane using binaural room impulse responses measured in an everyday office. Experiment 1 compared localization judgments for one-octave-wide noise centered at either 750 Hz (low) or 6000 Hz (high). For both band-limited noises, perceived lateral angle varied monotonically with source angle. For frontal sources, perceived locations were similar for low- and high-frequency noise; however, for lateral sources, localization was less accurate for low-frequency noise than for high-frequency noise. With increasing source distance, judgments of both noises became more biased toward the median plane, an effect that was greater for low-frequency noise than for high-frequency noise. In Experiment 2, simultaneous presentation of low- and high-frequency noises yielded performance that was less accurate than that for high-frequency noise, but equal to or better than for low-frequency noise. Results suggest that listeners perceptually weight low-frequency information heavily, even in reverberant conditions where high-frequency stimuli are localized more accurately. These findings show that listeners do not always optimally adjust how localization cues are integrated over frequency in reverberant settings.     

Optimum speech level to minimize listening difficulty in public spaces

For ideal speech communication in public spaces, it is important to determine the optimum speech level for various background noise levels. However, speech intelligibility scores, which is conventionally used as the subjective listening test to measure the quality of speech communication, is near perfect in most everyday situations. For this reason, it is proposed to determine optimum speech levels for speech communication in public spaces by using listening difficulty ratings. Two kinds of listening test were carried out in this work. The results of the tests and our previous work [M. Morimoto, H. Sato, and M. Kobayashi, J. Acoust. Soc. Am. 116, 1607-1613 (2004)] are jointly discussed for suggesting the relation between the optimum speech level and background noise level. The results demonstrate that: (1) optimum speech level is constant when background noise level is lower than 40 dBA, (2) optimum speech level appears to be the level, which maintains around 15 dBA of SN ratio when the background noise level is more than 40 dBA, and (3) listening difficulty increases as speech level increases under the condition where SN ratio is good enough to keep intelligibility near perfect.