Performance Evaluation and Effectiveness of the Reverberation Room

This research presents a thorough evaluation of the reverberation room at Acoustics Laboratory in National Institute of Standards (NIS) according to the related international standards. The evaluation aims at examining the room performance and exploring its effectiveness in the frequency range from 125 Hz to 10000 Hz according to the international standard requirements. The room, which was designed and built several years ago, is an irregular rectangular shape free from diffusers. Its volume is about 158.84 m³, which meets the requirement of the ISO 354 standard L_max < 1.9V^1/3. Cut-off frequencies of one and one-third octave are 63 Hz and 100 Hz respectively; however Schroder frequency is 400 Hz. Calculations of cut-off frequency and modal density showed adequate modes that give acceptable uniformity starting comfortably from frequency of 125 Hz. The room has a reverberation time that is suitable for its size over the frequency range of interest. The room sound absorption surface area and its sound absorption coefficient satisfy the criteria given in ISO 3741 and ISO 354. There is an accepted diffuse sound field inside the room due to the standard deviation of measured sound level, which is less than 1.5 dB over all the frequency range. The only exception was 125 Hz which may be due to a lack of diffusivity of the sound field at this frequency. The evaluation proves that the NIS reverberation room is in full agreement with the international standards, which in turns qualifies the room to host measurements inside without concerns.     

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.     

“Assisted resonance” in the Royal Festival Hall,London: 1965–19691

In 1964 an electronic system (“assisted resonance”) to lengthen the reverberation time over a limited frequency range was installed experimentally in the Royal Festival Hall, London. Following the success of this experimental installation, the frequency range covered was extended (to 58–700 Hz) and the installation made permanent. This paper describes the installation and operation of the permanent system, and its objective and subjective effects. Typically, the reverberation time in the empty auditorium has been increased in the 125 Hz octave band from 1·4 to 2·5 seconds.     

Measurements of the influence of fittings and roof pitch on the sound field in panel-roof factories

Measurements are presented of the sound absorption of two factory machines. Their third octave absorption, between 160 Hz and 5 kHz, is in the range 0.5–2.6 m².Measurements of reverberation time (RT) and/or sound propagation (SP) in four factories with panel roofs, when empty and/or fitted, are also presented. The general characteristics of RT and SP in such factories, and the factors that influence them, are discussed in the light of the results. Fittings reduce the RT and large-distance SP levels in proportion to their volume density and to the absorption of the panel roof. Levels near a source can increase due to backscattering. Preferentially orientated fitting arrangements do not significantly affect the variation of SP with direction in a factory.The effect of roof pitch on factory SP is complicated. Large distance levels are generally lowest in the direction across the roof pitches.     

Fundamental accuracy of time domain finite element method for sound-field analysis of rooms

This paper presents an assessment of the accuracy and applicability of a time domain finite element method (TDFEM) for sound-field analysis in architectural space. This TDFEM incorporates several techniques: (1) a hexahedral 27-node isoparametric acoustic element using a spline function; (2) a lumped acoustic dissipation matrix; and (3) Newmark time integration method with an absolute diagonal scaled COCG iterative solver. Sound fields in an irregularly shaped reverberation room of 166 m³ are computed using TDFEM. The computed values and measured values for 125-500 Hz are compared, revealing that the fine structure of the computed band-limited impulse responses agree with measured ones up to 0.1 s, with a cross-correlation coefficient greater than 0.93. The cross-correlation coefficient decreases gradually over time, and more rapidly for higher frequencies. Moreover, the computed decay curves, and the reverberation times, agree well with the respective measured ones, and with a better fit the higher the frequency (up to 500 Hz).     

Über den optimalen nachhall in kleinen bis mittelgrossen kirchen

The reverberation times of churches should be longer, for example, than those of theatres or concert halls but they must not be too long. By means of an examination of small and medium sized churches, a relation between measured reverberation times and judgments of acoustical qualities is established in this paper. From this relation, an optimum range of average reverberation times, $\text{T}{}_{\mathrm{m}}$, of unoccupied churches at 500 Hz and 1000 Hz octave bands can be derived as a function of volumes extending from 1000 m³ to 12 000 m³.     

混响强度及其衰减规律与脉宽的关系

本文由短脉宽下混响强度的普遍表达式I_r(t,τ)=Kτt^-me^-βt出发,导出对应的可用于长脉宽的混响强度精确表达式,并给出了忽略脉宽尺度范围的指数衰减损失后相应的近似表达式。当指数衰减系数β足够小和脉宽大于有效脉宽τ_m=a_mt(a_m随m增大而减小,m是大于2的实数)时,近程混响强度趋向饱和且随时间t的(m-1)次幂衰减。实验结果与理论符合较好。     

Experimental studies of low-frequency reverberation on the continental slope in the northwestern Pacific Ocean

Experimental data obtained on the continental slope near the Kamchatka peninsula for the reverberation at the frequencies 230, 600, and 850 Hz in the cases of coincident and spaced source and receiver of sound are presented. The data include the dependences of the reverberation level on time for both directional and omnidirectional receiving systems, as well as the dependences of the reverberation level on the duration of the probing pulses and on the sea depth at the source site. It is shown that, at the frequency 230 Hz, a substantial contribution to the reverberation is made by the reflection and scattering on the shelf near the coastline and in the region of the “depth drop.” At the frequencies 600 and 850 Hz, the predominant mechanism is bottom and surface scattering in the region of the continental slope.     

Aspects of the acoustical design and properties of Christchurch Town Hall,New Zealand

The Christchurch Town Hall (2650 seats) is the major hall in a complex which includes a 986 seat theatre. It was opened on 30 September 1972 and was designed primarily as a choral symphony hall. Reverberation time (125 Hz to 2 kHz) in excess of 2·2 seconds with the hall full is provided by a volume of 20 500 m³ and reflective surfaces. The basic design aim, provision of relatively strong early lateral reflections, is achieved by large interior oblique reflectors which hang free from the boundaries of the reverberant volume. The resulting early lateral energy and long R.T. gives the hall a combination of reverberance, envelopment and clarity. The paper gives an account of design aims and studies including a ray tracing computer program with an interactive graphics output. Measurements of T₆₀, T₁₅ and integrated early energy have been carried out in the hall. Echograms and other measured data are presented. Clarity has been measured by a speech intelligibility test with the Hall empty except for 40 participants. With T₆₀ at 500 Hz 2·7 seconds, the intelligibility was better than the Peutz criterion for public address systems.     

Reverberation times and speech transmission indices in classrooms

Octave band reverberation times, background noise levels and speech transmission indices measurements were carried out in eighteen government subsidized primary and secondary schools in Hong Kong. Various normal classroom operation conditions were considered. Results illustrate that strong correlation exists between the reverberation times and the speech transmission indices regardless of the background noise levels and their NC values in the present study. The arithmetic average of the reverberation times in the 250 Hz to 4 Hz octave bands and the 1 kHz octave band reverberation time are found to be more important in the correlation in general. These findings provide a convenient mean for speech transmission design in classrooms.     

Ray-tracing evaluation of empirical models for predicting noise in industrial workshops

To evaluate the accuracy of empirical models for predicting steady-state noise levels and reverberation times in typical industrial workshops, predictions by these models were compared with predictions by a ray-tracing model, nominally using the same input data. Comparisons were made for three workshops—‘long’, ‘flat’ and ‘quasi-cubic’ in shape—with reflective and absorbent ceilings, when empty and with four densities of fittings. In the case of the ‘long’ room, noise levels were predicted along both the long and short major horizontal axes. In ‘long’ and ‘flat’ workshops, steady-state prediction by the empirical models often agreed with ray-tracing prediction within 2 dB. This result suggests that the empirical models are fundamentally valid. However, agreement was worse at large source/receiver distances, and at 500 Hz. Empirical reverberation-time prediction generally agreed less well with ray tracing, possibly indicating that the empirical reverberation-time prediction models are less valid. Strong disagreement occurred between the models in the case of steady-state prediction in the ‘quasi-cubic’ workshop, indicating that the empirical steady-state models are invalid in this case. Agreement for reverberation time was good with a non-absorbent ceiling, but poor with an absorbent ceiling.     

Reverberation time and maximum background-noise level for classrooms from a comparative study of speech intelligibility metrics

Speech intelligibility metrics that take into account sound reflections in the room and the background noise have been compared, assuming diffuse sound field. Under this assumption, sound decays exponentially with a decay constant inversely proportional to reverberation time. Analytical formulas were obtained for each speech intelligibility metric providing a common basis for comparison. These formulas were applied to three sizes of rectangular classrooms. The sound source was the human voice without amplification, and background noise was taken into account by a noise-to-signal ratio. Correlations between the metrics and speech intelligibility are presented and applied to the classrooms under study. Relationships between some speech intelligibility metrics were also established. For each noise-to-signal ratio, the value of each speech intelligibility metric is maximized for a specific reverberation time. For quiet classrooms, the reverberation time that maximizes these speech intelligibility metrics is between 0.1 and 0.3 s. Speech intelligibility of 100% is possible with reverberation times up to 0.4-0.5 s and this is the recommended range. The study suggests "ideal" and "acceptable" maximum background-noise level for classrooms of 25 and 20 dB, respectively, below the voice level at 1 m in front of the talker.     

Investigating the absorption characteristics of open ceilings in multi-purpose halls using a 1:25 scale model

This paper investigated the absorption coefficients of acoustically transparent ceilings using 1:25 scale models. From a field survey of 18 existing halls, it was found that the open ceilings were equipped with steel truss structures, ducts, catwalks and ceiling surfaces. In order to investigate the absorption characteristics of the equipped ceilings, measurements were made using a 1:25 reverberation chamber based on ISO 354. Results showed that for an empty ceiling with a depth of 6 m, the absorption coefficient with a 50%-perforated ceiling surface is 0.2-0.3 above 500 Hz. If there were steel structures inside the ceiling, the absorption coefficient increased by 0.1 at 125 Hz to 2 kHz. Adding ducts and catwalks increased the absorption by 1-2 kHz. The absorption coefficients of the equipped ceilings ranged from 0.19 to 0.61 and the absorption characteristics were mainly found at high frequencies. Maximum absorption was observed in heavily equipped ceiling structures.     

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.     

Time reversal of noise sources in a reverberation room

Usually, time reversal is studied with pulsed emissions. Here, the properties of time reversal of the acoustic field emitted by noise sources in a reverberation room are studied numerically, theoretically, and experimentally. A time domain numerical simulation of a two-dimensional enclosure shows that the intensity of a time-reversed noise is strongly enhanced right on the initial source position. A theory based on the link that exists between time reversal of noise and the "well-known" time reversal of short pulse is developed. One infers that the focal spot size equals half a wavelength and the signal to noise ratio only depends on the number of transceivers in the time reversal mirror. This last property is characteristic of the time reversal of noise. Experimental results are obtained in a 5 X 3 X 3 m3 reverberation room. The working frequency range varies from 300 Hz to 2 kHz. The ability of the time reversal process to physically reconstruct the image of two noise sources is studied. To this end, care is given to the technique to separate two close random sources, and also to the influence of temperature fluctuations on the focusing quality.     

Field measurement of airborne sound insulation between rooms with non-diffuse sound fields at low frequencies

Procedures for the field measurement of airborne sound insulation between rooms with diffuse fields are described in International Standard ISO 140-4. However, many dwellings contain rooms with volumes less than 50 m³, where low frequency measurements are less reliable; hence there is a need for a measurement procedure to improve the reliability of field measurements in rooms with non-diffuse fields. Procedures are proposed for sound pressure level and reverberation time measurements for the 50, 63 and 80 Hz third octave bands. The sound pressure level measurement combines corner microphone positions with positions in the central region of each room. This provides a good estimate of the room average sound pressure level with significantly improved repeatability.     

Study of the mechanism for broadening of the spectrum of a low-frequency reverberation signal for sound scattering by near-surface inhomogeneities under conditions of intense wind waves

The paper considers the problem of backscattering of sound waves by near-surface volumetric inhomogeneities under conditions of intense wind waves. We calculate the expected share of the scattered signal spectrum based on the given wind-wave intensity and the depth distribution of volumetric inhomogeneities. For deep ocean conditions in the frequency range of 500–1000 Hz for a pulse duration of 10 s, we measure the levels and shape of the reverberation spectrum for time delays from 20 to 100 s. Comparison of the measured and calculated reverberation spectra has shown their good coincidence.     

强混响背景下悬浮目标的时反成像

提出了一种加权宽带全信号子空间时反成像方法,将扩展性目标建模为有限数量个、相互独立的点散射体构成,实现了时反成像在可疑目标区域的全信号子空间和整个宽带的累加。在实验室波导水池中,底部放置圆柱筒制造混响背景,共进行了两个实验,一是扩展性的悬浮目标位于圆柱筒附近,其回波与圆柱筒反射的混响不在同一个时间窗内,二是弱点目标回波与圆柱筒反射的混响重叠于同一个时间窗内,对所提出的方法进行强混响背景下弱悬浮目标的成像能力验证。结果表明,此方法对强混响背景下的弱悬浮目标的成像质量明显优于传统的时反成像方法。      

Coherent active methods for applications in room acoustics

An adjustment of reverberation time in rooms is often desired, even for low frequencies where passive absorbers fail. Among the active (electroacoustic) systems, incoherent ones permit lengthening of reverberation time only, whereas coherent active methods will allow sound absorption as well. A coherent-active wall lining consists of loudspeakers with microphones in front and adjustable control electronics. The microphones pick up the incident sound and drive the speakers in such a way that the reflection coefficient takes on prescribed values. An experimental device for the one-dimensional case allows reflection coefficients between almost zero and about 1.5 to be realized below 1000 Hz. The extension to three dimensions presents problems, especially by nearfield effects. Experiments with a 3 X 3 loudspeaker array and computer simulations proved that the amplitude reflection coefficient can be adjusted between 10% and 200% for sinusoidal waves at normal and oblique incidence. Future developments have to make the system work with broadband excitation and in more diffuse sound fields. It is also planned to combine the active reverberation control with active diffusion control.     

Comparison between measured and calculated parameters for the acoustical characterization of small classrooms

This paper presents a comparison between measured and calculated acoustical parameters in eight high school classrooms. The mid frequency unoccupied and occupied reverberation times and the 1 kHz sound propagation (SP) of the reverberant and total speech levels in occupied classrooms were compared with analytical and numerical predictions. The ODEON 6.5 code and the Sabine formula gave the most accurate results for reverberation time in the empty classrooms with overall relative differences of 8.1% and 9.7%, respectively. With students present, the Eyring and Sabine formulas and Hodgson’s empirical model resulted to be the most accurate with relative differences of 11.1%, 13.2% and 13.6%, respectively. The reverberant speech levels decrease with increasing distance from the source at rates varying from −1.21 to −2.62 dB/distance doubling, and the Hodgson model fits the slope values quite well. The best predictions of the SP of the reverberant and total speech levels are shown, in order of accuracy, for the ODEON code, the Barron and Lee theory and the classical diffuse field theory. Lower rms errors were found when the measured total acoustic absorptions were used. The lowest rms error of 1.4 dB for the SP of the total speech level were found for both the ODEON code and the Barron and Lee theory.