Influence of wall absorption on low-frequency dependence of reverberation time in room of irregular shape

In this paper, a modal analysis was used to describe a reverberation phenomenon in a room of complex shape. A theoretical model was limited to low sound frequencies, when eigenmodes are lightly damped, thus they may be approximated by uncoupled normal acoustic modes of a hard-walled room. A utility of this method was demonstrated in a numerical example where the enclosure in a form of two coupled rooms was considered. A reverberation time was evaluated from a time decay of spatial root mean square pressure, the overall measure of room pressure. The results of calculations, performed for three different distributions of absorbing materials on room walls, showed how various location of the material can effect a dependence of the reverberation time on a frequency of sound source.     

The reverberation time of furnished rooms in dwellings

The study gives the results of the measurements of the reverberation time in 11,687 rooms, of which 11,457 are furnished (8246 bedrooms, 3211 living rooms) and 230 unfurnished. All the rooms have heavy walls and ceilings, and a heavy floor covering. The reverberation times measured are quite similar in bedrooms and living rooms within the same size range, and decrease fairly uniformly as the frequency increases. Moreover, in each frequency band the greater the volume of the room, the greater the reverberation time.The results of this extensive fieldwork allow us to predict accurately the reverberation time in these kinds of spaces as a function of their size and the frequency. These data may be useful for improving the accuracy of calculations models to estimate the reverberation time of enclosed spaces. A comparison between the reverberation time measured in this work and that proposed in PrEN ISO 10052.2002(E) has been made.     

The Norwegian Façade Insulation Study: the efficacy of façade insulation in reducing noise annoyance due to road traffic

The ergodic propriety of a room has strong effects on its reverberation. If the room is ergodic, the reverberation can be broken up in two steps: a deterministic process followed by a stochastic one. The late reverberation can be then modeled by a reverberation algorithm instead of more computationally consuming methods. In this study, the free path temporal distribution obtained by ray-tracing is used as an indicator of the room's mixing: the energetic average of the path lengths is computed at each time step. Ergodic rooms are thus characterized by rapidly convergent distributions. The free path value becomes independent of time. On the other hand, path selection mechanism and orbits are observed in non-ergodic rooms. The transition time from the deterministic process to the stochastic one is also studied through the evaluation of the room's time constant. It is shown that its value depends only on the mean free path and the boundaries scattering value. An empirical expression is obtained which agrees well with simulations carried out in a concert hall. This transition time from a deterministic model to a stochastic one can be used to speed up the acoustical predictions and auralizations in ergodic rooms.     

Room acoustical model of external reverberation

A theory of external reverberation in urban built-up environments is developed, based on a classical room acoustical model. In the model, external reverberation is analyzed as a special limiting case of internal reverberation in rooms. Explicit formulae are deduced for the statistical value of the external reverberation time, and the spatial distribution of the external sound field amplitude with distance from a fixed, constant power, sound source, for which comparison with published experimental results is possible. Predictions of the theory compare reasonably well with the experimental values. It is found that the external reverberation time in a built-up area depends chiefly on the average building height, and to a lesser extent on the packing fraction, and the ratio of surface area to cross-sectional area of buildings which make up the built-up environment, apart from the absorptive properties of the building and ground plane surfaces. For the spatial distribution of the steady state sound field amplitude in a built-up environment, it is found that the diffuse field amplitude attenuates with distance from a fixed, constant power source exponentially faster than the inverse square law.     

The average absorption coefficient for enclosed spaces with non uniformly distributed absorption

This study concerns the determination of an equivalent acoustic absorption model of the flat heterogeneous walls present in industrial rooms. Numerous measurements of the reverberation time in reverberant room were carried out for several facings with different distributed spatial absorption. Experimental results were compared to classical reverberation time models. The measurements showed that the change in average acoustic absorption depends on the relative distance between the sound source and the absorbent panels, as it is this which creates heterogeneity. Therefore, taking into consideration, in the theoretical models of average acoustic absorption studied, the solid angles representing the equivalent area of the panels as viewed by the source, improved the accuracy of the calculated reverberation time compared to the measurements. This equivalent acoustic absorption model, based on Sabine's absorption coefficient and employing the solid angle ratio, was used to calculate the reverberation time of several industrial rooms. The results obtained are better than those obtained with the standard formula.     

Walk away method versus reverberation time method in determining the room constant

The room constant is a quantity relating to the sound absorption in an enclosure. It is necessary to room acoustics engineering calculations based on the perfectly diffuse sound field theory, both in noise control work and sound system design. Normally, in the validation of the above-mentioned theory, in situ evaluations of the room constant are performed on the basis of reverberation time measurements in the relevant space. Such methods fail to give accurate results when uncertainties regarding the volumes and areas effectively involved in the reverberation process arise. In this case the walk away method is more advantageous in that it only needs steady state sound pressure level measurements at various distances from a small sound source whose directivity factor is known.In this paper both methods are discussed and experimental results relative to four different rooms are compared.     

On site validation of sound absorption measurements of occupied pews

Laboratory measurements of sound absorption by audiences are known to be scarcely reliable when applied to actual rooms as a consequence of several problems, among which the different area of the “sample” and the different distribution of the reflected sound may play important roles. When dealing with worship places, characterized by a variable degree of occupation and much lower absorption due to unoccupied seats, things become more complicated as absorption seems to be proportional to the number of occupants rather than to the area they cover (as normally accepted in performing spaces). The combination of these variables has been investigated by taking advantage of laboratory measurements and analysing their application to six churches, where on site measurements of reverberation time were carried out with and without occupation. The results are discussed both in terms of simple prediction formulae (Sabine, Eyring, and Arau-Purchades) and of computer simulations, showing that laboratory measurements may be reliably used in computer simulations (at least in the frequency range from 500 Hz on). At low frequencies greater attention must be paid as the absorption coefficients need to be corrected as a function of the actual distribution of the sound field in the room.     

An improved broad-spectrum room acoustics model including diffuse reflections

More and more literature has paid attention to the diffuse reflections in enclosed space during the past few years. In this paper, the current computer models including diffuse reflections have been reviewed briefly at first. Then, to realize the broad-spectrum simulation for enclosed sound fields including diffuse reflections, an improved ray-tracing algorithm, which combines the splitting coefficient diffusion model and a dynamic sound ray receiving method, has been given. The algorithm can deal with broad frequency bands simultaneously by using the frequency independent splitting coefficient. To test the algorithm and also to investigate the significance of the diffuse reflections in enclosed sound fields, experiments have been made in three spaces including a virtual room and two real rooms. The results and discussions have validated the applicability of the improved algorithm and they have also shown that diffuse reflections can improve room acoustic prediction, although it not always promote a sound field to be more diffused.     

Decay rates and wall absorption at low frequencies

This paper is concerned with evaluating the error of conventional estimates of the boundary absorption of rectangular enclosures, with particular reference to reverberation room sound power measurements. The reverberation process is examined theoretically; the relative contributions to the decay rate from different modes in a rectangular room are calculated from an ensemble average over rooms with nearly the same dimensions. It is shown that the traditional method of determining the absorption of the walls of reverberation rooms systematically underestimates the absorption at low frequencies; the error is computed from the ensemble average. Finally, an unbiased estimate of the sound power radiated by a source in a reverberation room is derived. This estimate involves measurement of the initial decay rates of the room and is, unlike the usual reverberation room sound power estimate, neither based on statistical diffuse field considerations nor on the normal mode theory.     

Ray-tracing prediction of optimal conditions for speech in realistic classrooms

Recent papers have discussed the optimal reverberation times in classrooms for speech intelligibility, based on the assumption of a diffuse sound field. Here this question was investigated for more ‘typical’ classrooms with non-diffuse sound fields. A ray-tracing model was modified to predict speech-intelligibility metric U₅₀. It was used to predict U₅₀ in various classroom configurations for various values of the room absorption, allowing the optimal absorption (that predicting the highest U₅₀)—and the corresponding optimal reverberation time—to be identified in each case. The range of absorptions and reverberation times corresponding to high speech intelligibility were also predicted in each case. Optimal reverberation times were also predicted from the optimal surface-absorption coefficients using Sabine and Eyring versions of diffuse-field theory, and using the diffuse-field expression of Hodgson and Nosal. In order to validate the ray-tracing model, predictions were made for three classrooms with highly diffuse sound fields; these were compared to values obtained by the diffuse-field models, with good agreement. The methods were then applied to three ‘typical’ classrooms with non-diffuse fields. Optimal reverberation times increased with room volume and noise level to over 1 s. The accuracy of the Hodgson and Nosal expression varied with classroom size and noise level. The optimal average surface-absorption coefficients varied from 0.19 to 0.83 in the different classroom configurations tested. High speech intelligibility was, in general, predicted for a wide range of coefficients, but could not be obtained in a large, noisy classroom.     

耦合空间中不同衰变类型声场的边界研究

在耦合空间中,除了传统的指数衰变形式,通常还存在其它两种不同类型的声衰变方式,一种开始时衰变很快后来衰变速度降低,另一种相反,每一种衰变的形式在耦合空间中都占据一定的区域。目前的一般观点是:系统的物理边界,如两子空间之间的隔墙和耦合口,是空间中两类不同声场的边界。通过数值模拟和实验验证,发现这种物理边界划分并不能准确区分具有不同特性的声场。本文中给出了这两种不同特性声场之间的边界,发现这边界并不处在通常人们认为的隔墙之类的物理边界上,而是越过物理分隔进入相对更为活跃的子空间中。这种侵入的程度取决于两子空间自身混响之间的差异,差异愈大侵入得愈多。      

Speech segregation in rooms: effects of reverberation on both target and interferer

Speech reception thresholds were measured to investigate the influence of a room on speech segregation between a spatially separated target and interferer. The listening tests were realized under headphones. A room simulation allowed selected positioning of the interferer and target, as well as varying the absorption coefficient of the room internal surfaces. The measurements involved target sentences and speech-shaped noise or 2-voice interferers. Four experiments revealed that speech segregation in rooms was not only dependent on the azimuth separation of sound sources, but also on their direct-to-reverberant energy ratio at the listening position. This parameter was varied for interferer and target independently. Speech intelligibility decreased as the direct-to-reverberant ratio of sources was degraded by sound reflections in the room. The influence of the direct-to-reverberant ratio of the interferer was in agreement with binaural unmasking theories, through its effect on interaural coherence. The effect on the target occurred at higher levels of reverberation and was explained by the intrinsic degradation of speech intelligibility in reverberation.     

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’.     

On the accuracy of models for predicting sound propagation in fitted rooms

The objective of this article is to make a contribution to the evaluation of the accuracy and applicability of models for predicting the sound propagation in fitted rooms such as factories, classrooms, and offices. The models studied are 1:50 scale models; the method-of-image models of Jovicic, Lindqvist, Hodgson, Kurze, and of Lemire and Nicolas; the emprical formula of Friberg; and Ondet and Barbry's ray-tracing model. Sound propagation predictions by the analytic models are compared with the results of sound propagation measurements in a 1:50 scale model and in a warehouse, both containing various densities of approximately isotropically distributed, rectangular-parallelepipedic fittings. The results indicate that the models of Friberg and of Lemire and Nicolas are fundamentally incorrect. While more generally applicable versions exist, the versions of the models of Jovicic and Kurze studied here are found to be of limited applicability since they ignore vertical-wall reflections. The Hodgson and Lindqvist models appear to be accurate in certain limited cases. This preliminary study found the ray-tracing model of Ondet and Barbry to be the most accurate of all the cases studied. Furthermore, it has the necessary flexibility with respect to room geometry, surface-absorption distribution, and fitting distribution. It appears to be the model with the greatest applicability to fitted-room sound propagation prediction.     

An image source computer model for room acoustics analysis and electroacoustic simulation

A computer program for the determination of the impulse response of rooms, using the mirror image method, has been developed. It is intended for acoustical planning of auditoria and the simulation of sound fields. The computer also generates the necessary information for the automatic mixer, delay unit and reverberation unit, which makes it possible to change sound field simulations in a short time.The program allows arbitrary room shapes to be analyzed by representing curved surfaces with plane approximations. The program also calculates energy time gaps, radiation angles and angles of incidence for the investigated source and receiver positions. The absorption coefficients of the reflecting surfaces are also taken into account. The source and the receiver may be positioned anywhere in the room. Compared with other programs described in the literature, this program yields more information, necessary particularly for electroacoustic room acoustics simulation and acoustical planning.     

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.     

耦合空间的声学研究进展

近年来由于混响室在演艺厅堂工程上的应用,唤发了对耦合空间声场研究的广泛兴趣。本文简短地回顾了马大猷先生对室内声学简正模态理论的贡献,对近代特别是最近15年来室内声学研究在耦合空间方面的发展,做了综述性讨论,包括简正模态分析和模态分解法、统计声学方法、几何声学方法、扩散方程、以及高分辨率实验技术和贝叶斯概率分析在耦合空间研究的应用。值马大猷先生诞辰一百周年之际,以此文缅怀德高望重的声学前辈。      

A STUDY OF VARIOUS BARRIERS IN ENCLOSED SOUND FIELD BY USING THE COMPUTER PROGRAM—SOFIS

In the study of the behaviors of barriers in an enclosed field, one should take into account such phenomena as sound energy reflection, absorption, scattering and diffraction. Therefore, the study is much more difficult than that in free field. In this paper, sound barriers are classified into four kinds according to their size, number and shape. Each kind of barriers is modelled by a corresponding method based on a computer program—SOFIS. The program combines the ray-tracing technique and statistical method. The impulse response and some acoustical parameters such as sound pressure level at different positions can be calculated by the program, no matter there are a certain kind of barriers in the field or the field is empty. The ray-tracing program and the algorithms for various barriers are validated by the comparison between measurement and prediction of the reverberation room and the anechoic room of the Northwestern Polytechnic University.     

Acoustic method for measuring air temperature and humidity in rooms

A method is proposed to determine air temperature and humidity in rooms with a system of sound sources and receivers, making it possible to find the sound velocity and reverberation time. Nomograms for determining the air temperature and relative air humidity are constructed from the found sound velocity and time reverberation values. The required accuracy of measuring these parameters is estimated.     

Localization of sound in rooms

This paper is concerned with the localization of sources of sounds by human listeners in rooms. It presents the results of source-identification experiments designed to determine whether the ability to localize sound in a room depends upon the room acoustics, and how it depends upon the nature of the source signal. The experiments indicate that the localization of impulsive sounds, with strong attack transients, is independent of the room reverberation time, though it may depend upon the room geometry. For sounds without attack transients, localization improves monotonically with the spectral density of the source. Localization of continuous broadband noise does depend upon room reverberation time, and we propose the concept of direct signal to reverberant noise ratio to study that effect. Source identification experiments reveal certain localization biases, invisible to minimum-audible-angle experiments, and of uncertain origin. Appendices to this paper develop the statistics of the source-identification paradigm and show how they relate to the minimum audible angle.