Comparison of methods for measurement of reverberation time

Five methods for automatic measurement of reverberation time, T, are studied with respect to the mean value and the variance of T. The measurements were performed in two reverberation chambers. For each $\text{1}\text{3}\text{-}\text{octave}$ band and microphone position one hundred decay signals were evaluated. It may be concluded that the methods, which are all simple to implement in analog or digital techniques, give small differences in the estimates of the mean value of T. The variance of T obtained is however greatly affected by the method used. It is felt that an automatic method should be recommended to supplement the “straight line” method outlined in existing standards.     

An automatic reverberation time measuring system

The standard laboratory method of reverberation time measurement and its inherent disadvantages are discussed and an automatic method of carrying out the measurement procedures is described. A simplified circuit of the automatic system is given and its action explained. The two methods are compared quantitatively and it is demonstrated that they both produce results which are equally valid estimates of reverberation times.     

On the steady-state and the transient decay methods for the estimation of reverberation time

The discrepancy between reverberation times of an enclosed sound field measured by the steady-state method and by the transient decay method is well-known. So far, no clear explanation has been obtained. In this paper, the steady-state bandlimited energy in an enclosure and bandlimited power flow into modally reactive boundaries are derived to describe the energy balance relationship and thus the reverberation time in a frequency band. This reverberation time is then compared to that obtained from the transient decay of the sound field based on the modal analysis. The comparison provides an understanding of the discrepancy mentioned above as well as the physical interpretations of the reverberation times estimated by both methods.     

A method for automatic reverberation time measurement

A method for the automatic measurement of reverberation time is described. In order to reduce the influence of flutter a curve fitting method, based on integration of the decay curve, is applied. The method allows the reverberation time to be measured without subjective interactions, both in the field and in the laboratory. A comparison between various methods shows that the described method exhibits a high degree of reliability.     

Holographic measurement of reverberation time

A system employing the radio frequency holographic method for the measurement of the reverberation time of an acoustic chamber is proposed. This system has the advantage of supplying visible information on the diminishing intensity of the acoustic wave. A simple mathematical model has been taken to discuss the basic principles involved in the system. A possible experimental set-up for the measurement of the reverberation time is described in detail.     

Blind estimation of reverberation time

The reverberation time (RT) is an important parameter for characterizing the quality of an auditory space. Sounds in reverberant environments are subject to coloration. This affects speech intelligibility and sound localization. Many state-of-the-art audio signal processing algorithms, for example in hearing-aids and telephony, are expected to have the ability to characterize the listening environment, and turn on an appropriate processing strategy accordingly. Thus, a method for characterization of room RT based on passively received microphone signals represents an important enabling technology. Current RT estimators, such as Schroeder's method, depend on a controlled sound source, and thus cannot produce an online, blind RT estimate. Here, a method for estimating RT without prior knowledge of sound sources or room geometry is presented. The diffusive tail of reverberation was modeled as an exponentially damped Gaussian white noise process. The time-constant of the decay, which provided a measure of the RT, was estimated using a maximum-likelihood procedure. The estimates were obtained continuously, and an order-statistics filter was used to extract the most likely RT from the accumulated estimates. The procedure was illustrated for connected speech. Results obtained for simulated and real room data are in good agreement with the real RT values.     

一种基于最大似然的混响时间盲估计方法*

混响是室内声学中的重要现象,在室内设计与音频信号处理中都需要测量或估计混响时间。本文改进了一种基于最大似然估计的混响时间盲估计方法,即采用说话人在房间中自然说话时发出的混响语音信号来估计混响时间的方法。该方法首先确定语音衰减段的最优边界,其次计算该衰减段的两个额外参数,据此筛选出符合条件的语音段,最后将满足条件的语音段采用最大似然估计得到混响时间估计值。在五个不同混响时间条件下的仿真表明,与已有方法相比,改进方法估计的混响时间同真实混响时间的偏差更小,方差更低,估计准确性较高。     

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.     

Effect of non-uniform distribution of absorption on reverberation time

An attempt is made to develop a geometric theory of reverberation without bringing in the simplifying assumptions that are required by statistical considerations. The reverberation phenomenon is simulated on a digital computer by the ray tracing method. Experimental investigations to check the validity of the computer model are described. Large disagreements are found between the computed and experimental results. Consequently, the computer model is re-examined and modified to incorporate the scattering of sound that occurs at the edges and corners of an enclosure. Good agreement is reported between the modified computer results and the experimental results in situations where the absorbing material covers the enclosure surface(s) completely.     

Prediction of reverberation time using the residual minimization method

In many practical situations the assumption of sound field dispersion needed for the application of the Sabine’s theory is not fulfilled. In general, sound field is sufficiently dispersed if there are no large differences in the dimensions of the room, limiting partitions are not parallel, or the sound absorbing material is uniformly distributed. In practice, very few of these requirements are satisfied. As a result, a number of other formulas describing reverberation time have been created, for example Fitzroy’s or Neubauer’s formulas. However, these methods in many cases differ significantly from the actual measurements. The paper presents a method used to estimate reverberation time as well as its applicability potential involving laboratory models and auditorium rooms. The proposed method can be classified into a group of learning methods and involves the use of statistical methods which allow for approximation with the use of the least squares method.     

混响室内声功率发射的实验研究

本文简要论述了马大猷教授用简正波理论分析混响室内声功率测定问题所提理论。对位置函数F（d）进行了实验研究。证明位置函数确实存在。理论与实验结果符合甚好。本文最后提出了进一步的实验设想。     

Reverberation time measurements in non-diffuse acoustic field by the modal reverberation time

The increasing presence of low frequency sources and the lack of acoustic standard measurement procedures make the extension of reverberation time measurements to frequencies below 100 Hz necessary. In typical ordinary rooms with volumes between 30 m³ and 200 m³ the sound field is non-diffuse at such low frequencies, entailing inhomogeneities in space and frequency domains. Presence of standing waves is also the main cause of bad quality of listening in terms of clarity and rumble effects. Since standard measurements according to ISO 3382 fail to achieve accurate and precise values in third octave bands due to non-linear decays caused by room modes, a new approach based on reverberation time measurements of single resonant frequencies (the modal reverberation time) has been introduced. From background theory, due to the intrinsic relation between modal decays and half bandwidth of resonant frequencies, two measurement methods have been proposed together with proper measurement procedures: a direct method based on interrupted source signal method, and an indirect method based on half bandwidth measurements. With microphones placed at corners of rectangular rooms in order to detect all modes and maximize SNRs, different source signals were tested. Anti-resonant sine waves and sweep signal turned out to be the most suitable for direct and indirect measurement methods respectively. From spatial measurements in an empty rectangular test room, comparison between direct and indirect methods showed good and significant agreements. This is the first experimental validation of the relation between resonant half bandwidth and modal reverberation time. Furthermore, comparisons between means and standard deviations of modal reverberation times and standard reverberation times in third octave bands confirm the inadequacy of standard procedure to get accurate and precise values at low frequencies with respect to the modal approach. Modal reverberation time measurements applied to furnished ordinary rooms confirm previous results in the limit of modal sound field: for highly damped modes due to furniture or acoustic treatment, the indirect method is not applicable due to strong suppression of modes and the consequent deviation of the acoustic field from a non-diffuse condition to a damped modal condition, while standard reverberation times align with direct method values. In the future, further investigations will be necessary in different rooms to improve uncertainty evaluation.     

Sound strength and reverberation time in small concert halls

Sound strength and reverberation time measurements have been carried out in six small concert halls in Cambridge, UK. The sound strength G is a measure of the physical sound level in a concert hall and is closely related to the subjective sensation of loudness. It compares integrated impulse responses at a point in the measured room with that measured at ten metres distance in the free field.The aim of the measurements is to investigate the acoustic characteristics of the halls concerning sound strength and reverberation time. Furthermore the effect of the variable acoustics in the halls on these parameters is discussed in this paper. Especially for bigger ensembles it is often desirable to reduce the sound level in a small concert hall. The measurement results show that for a fixed hall volume, this can primarily be achieved by decreasing the reverberation time in the hall. However, with regard to the sound quality of a hall and the recommended reverberation times for chamber music, reverberation time cannot be reduced by an arbitrary extent. Therefore reverberation time and strength have to be balanced very carefully in order to obtain sufficient reverberation whilst at the same time avoiding excessive loudness. Finally the measured strength levels are compared to values derived from traditional and revised theory [Barron M, Lee L-J. Energy relations in concert auditoriums. J Acoust Soc Am 1988;84(2):618-28] on strength calculations in order to assess the accuracy of the theory for small chamber music halls. Possible reasons for the low measured strength levels observed are discussed with reference to related design features and objective acoustic parameters.     

Effects of frequency characteristics of reverberation time on listener envelopment

Spatial impression perceived in a listening space comprises at least two components: one is auditory (apparent) source width (ASW) and the other is listener envelopment (LEV). Both ASW and LEV are affected not only by temporal but also by spatial structures of reflections. It has been clarified that ASW for symphony music is significantly affected by low-frequency components of source signals and reflections, but not by their high-frequency components. The objective of this work is to investigate whether LEV is affected by the frequency characteristics of source signals and reverberation sounds, which are known to contribute to the creation of LEV. In this study, three experiments were performed to clarify the effects of reverberation time (RT) and its frequency characteristics on LEV. In contrast to the case of ASW, the experimental results show that RTs both at high and low frequencies affect LEV.     

厅堂混响时间的声场控制方法及实验研究

本文首先论述厅堂声场控制的各种方法，然后利用混响反馈法和卷积法分别进行了实验研究，其中着重对卷积法的实验系统进行阐述，指出卷积法具有混响时间调节方便、控制范围大等优点，混响时间可提高三倍以上．     

Blind estimation of reverberation time in classrooms and hospital wards

This paper investigates blind Reverberation Time (RT) estimation in occupied classrooms and hospital wards. Measurements are usually made while these spaces are unoccupied for logistical reasons. However, occupancy can have a significant impact on the rate of reverberant decay. Recent work has developed a Maximum Likelihood Estimation (MLE) method which utilises only passively recorded speech and music signals, this enables measurements to be made while the room is in use. In this paper the MLE method is applied to recordings made in classrooms during lessons. Classroom occupancy levels differ for each lesson, therefore a model is developed using blind estimates to predict the RT for any occupancy level to within ±0.07 s for the mid-frequency octave bands. The model is also able to predict the effective room and per person absorption area.Ambient sound recordings were also carried out in a number of rooms in two hospitals for a week. Hospital measurements are more challenging as the occurrence of free reverberant decay is rarer than in schools and the acoustic conditions may be non-stationary. However, by gaining recordings over a period of a week, estimates can be gained within ±0.07 s. These estimates are representative of the times when the room contains the highest acoustic absorption. In other words when curtains are drawn, there are many visitors or perhaps a window may be open.     

Fast estimation of speech transmission index using the reverberation time

The paper presents the function of STI in the domain of reverberation time. Through the application of the said function, we can quickly estimate the speech transmission index, knowing only the time of room reverberation. For that purpose we applied a known method which consists in physical estimation of speech intelligibility basing on the modulation transfer function (MTF) determined in a room. Then, the STI was described using a logarithmic function whereof argument was the room reverberation time. To verify the model, reverberation times of six rooms were measured. The selected rooms were very different deliberately. They had different cubature and shape. The selection included a small cuboid, lecture halls and a church. Then, the same rooms were modeled in the ODEON version 11.23 and their reverberation times were determined. Furthermore, the STI was determined in the ODEON and then compared with the reverberation time obtained in effect of fast estimation. The statistical verification with the use of correlation index and regression equation has demonstrated that the fast estimation yields results close to those obtained in the computer simulation in ODEON. We obtained the correlation index at the level close to 1. Furthermore, the test probability at the level lower than 0.05 bespeaks of a statistically significant linear relation for the confidence level of 95%.     

民乐片段混响感主观偏爱度的初步实验

为了探讨不同受试人群、不同实验素材对民乐片段混响感主观偏爱度的影响，以民族器乐中典型的弹拨乐器、拉弦乐器、吹奏乐器独奏作品片段为实验素材，以有经验的音响技师和普通大学生为受试人，采用对偶比较法对实验素材混响处理的主观偏爱度进行了实验测量。实验得出了两组受试对不同实验素材所偏爱的混响时间，验证了同一受试组对不同素材偏爱的混响感不同；发现不同受试组对同一实验素材实验结果的差别不显著。     

Improved space time prewhitener for linear frequency modulation reverberation using fractional Fourier transform

This letter presents an improved space time prewhitening method for linear frequency modulation (LFM) reverberation. The proposed method transforms the reverberation to fractional Fourier domain to whiten using fractional Fourier transform. The linear varying frequency in LFM reverberation is focused on a stationary frequency, and the adjacent block signal is used as the reference signal of prewhitening. Finally, experiment results with real reverberation data verify that the proposed method improves the detection performance of active sonar in shallow sea significantly.