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.     

Acoustic energy relations in Mudejar-Gothic churches

Extensive objective energy-based parameters have been measured in 12 Mudejar-Gothic churches in the south of Spain. Measurements took place in unoccupied churches according to the ISO-3382 standard. Monoaural objective measures in the 125-4000 Hz frequency range and in their spatial distributions were obtained. Acoustic parameters: clarity C80, definition D50, sound strength G and center time Ts have been deduced using impulse response analysis through a maximum length sequence measurement system in each church. These parameters spectrally averaged according to the most extended criteria in auditoria in order to consider acoustic quality were studied as a function of source-receiver distance. The experimental results were compared with predictions given by classical and other existing theoretical models proposed for concert halls and churches. An analytical semi-empirical model based on the measured values of the C80 parameter is proposed in this work for these spaces. The good agreement between predicted values and experimental data for definition, sound strength, and center time in the churches analyzed shows that the model can be used for design predictions and other purposes with reasonable accuracy.     

The acoustics of the caves

This work shows the acoustic measurements realized at three underground places: the caves of Pertosa or of the Angel, the caves of Castelcivita and the caves of Castellana. These places are used for some years to represent music and theater performances, whose success is also due to the mystery and the charm that distinguish them, based on the natural setting of the caves. The places of the caverns where the shows are played were characterized analyzing the monaural acoustic parameters (T₃₀, EDT, C₈₀, D₅₀ and Rasti) obtained with the impulse response resulting from blowing up some balloons inside them. The values of monaural acoustic parameters measured in the caves can be compared with the parameters of the concert halls currently in use.     

室内声学时域有限差分模拟中的边界条件

给出了时域有限差分法用于室内声学问题模拟中的边界条件,结合声波方程的基本差分格式,模拟并分析了高斯脉冲在4m×4m房间中的波动过程和脉冲响应;模拟了一9m×6m×4m房间的简正频率,并与经典理论计算值进行了对比;模拟了一12m×5m×4m水平地面房间中的坐席吸声低谷效应,并与Joe LoVtri的模拟结果进行了对比;模拟并实际测量了一10.6m×5.8m×3.4m房间在几个受声点的脉冲响应和早期衰变时间EDT,将模拟结果与实际测量结果进行了对比分析,计算程序是用Metlab语言编写的。模拟与经典理论、相关研究、实际测量几方面的对比分析,验证了本边界条件的可靠性。     

Room acoustical parameters: A factor analysis approach

In this study, we determined the most representative acoustical parameters for halls intended for verbal or music audition. Our study was carried out in nine halls of different shapes and designed for different uses. We measured the impulse response at a great number of points (many more than the minimum required by the ISO 3382 norm). From a physical viewpoint, all halls are enclosed three-dimensional areas. Our work hypothesis is that objective (measurable) acoustical parameters, or a combination of such parameters, must provide the acoustical information specific to each hall and must make it possible to grade each hall. Factor analysis was used to obtain these grading parameters and the considerable number of measurements we determined guaranteed the application of this type of analysis. The convergence provides corroboration of the main correlations between parameters. A group of orthogonal parameters was thus obtained, made up of three factors that group the parameters used by different outstanding researchers. These factors provide a clear acoustical interpretation. We have termed the first of these “intelligibility” as it contains intelligibility parameters; the second is associated with spaciousness; the third and last parameter has been termed “strength” as it is a linear combination of the parameters that measure the amplification (G) and the bass ratio (BR). The optimal scores of these factors for different uses of halls make it possible to grade any hall, independently of its shape, for its corresponding use.     

Study on measurement of refractive index profile of GI-POF by light scattering

This paper is devoted to the study on measurement of refractive index profile of graded-index polymer optical fiber (GI-POF) by light scattering. Using Generalized Airy theory and Debye series of an inhomogeneous cylinder, the scattering intensity distributions are obtained of Airy structure of rainbows for different refractive index profile. The results show that positions of Airy peaks depend closely on refractive index profile of GI-POF. Since each order of rainbow penetrates it to different depths, such methods could be used to provide information of the refractive index profile of GI-POF. For GI-POF with given diameter, positions of Airy peaks of rainbows are simulated as a function of refractive index profile, which can be used to inverse unknown parameters of refractive index profile. The least square method is used in inversion of refractive index profile with the given refractive index of the cladding. The results obtained agree with theoretical values with high precision. The method has the advantages of non-instructive and on-line measurement, and can be used for the measurement of other inhomogeneous droplets.     

Predicted and experimental results of acoustic parameters in the new Symphony Hall in Pamplona, Spain

Two room acoustical simulation software have been used to predict the main acoustic parameters of a Symphony Hall in the planning stage, when only drawings were available. The modelled room is the Symphony Hall of the Conference Hall of Navarre, in Pamplona, Spain. Although the values of the calculation parameters (number of rays, reflection order, etc.) recommended by each software are slightly different, in this work the same values were used for both programs. Once the Hall was built, experimental results were obtained using the MLS-measurement technique. The values predicted and measured for several parameters defined in ISO 3382 at 9 receiver positions are compared. Even though the values predicted by both software are very similar for most of the acoustic parameters, there are notable differences at particular values, mainly when evaluating energy ratios. Different statistical corrections for late reflections between both programs seem to be the main reason for these differences. A more exhaustive knowledge of scattering coefficients is required to improve predictive accuracy. Important differences at 250 Hz frequency band were found between calculated and measured values probably due to the yet to be implemented seat dip effect in room simulation software. The comparison of calculated and measured impulse responses seems to be the first choice for the assessment of room simulation software. However, it should be kept in mind that its usability is also determined by many additional features. This work is not only a comparison of software dealing with the same object as well as equal input data but also shows the power of this kind of tool to predict the acoustic parameters of a room before its construction.     

Monaural and binaural parameters of Rudolfinum concert halls in Prague

Objective parameters for the evaluation of the Rudolfinum concert hall in Prague, Czech Republic are the focus of the present article. The measured results for Reverberation parameters, Energy parameters, Intelligibility parameters, and Spatial parameters of the building’s two halls are presented and discussed including a comparison with recommended values or theory, as well as several unique architectural and acoustical qualities of the halls. The early lateral energy fraction parameter is measured by the intensity probe method discussed in the supplement. The performance is verified by tests in anechoic and reverberant rooms.     

Measuring the sound absorption properties of noise barriers with inverse filtered maximum length sequences

Loudspeaker characteristics can have an appreciable influence on the sound absorption properties obtained with CEN/TS 1793-5 [1]. Although sound sources with omnidirectional radiation properties are favored, these often hold the problem of long impulse responses due to their design potentially incorporating ports [2]. In this paper, the inverse filtering approach is applied to two different sound sources. It is shown that it is a valuable measure to reduce the influence of the loudspeaker characteristics on the obtained sound absorption values.Furthermore, an attempt is made to measure the absorption properties according to CEN/TS 1793-5 without the need for the subtraction procedure of a free-field measurement. Thereby, as long as only high frequencies are considered, reasonable values can be obtained.     

Measurement of relevant elastic and damping material properties in sandwich thick plates

An easy-to-implement method to measure relevant elastic and damping properties of the constituents of a sandwich structure, possibly with a heterogeneous core, is proposed. The method makes use of a one-point dynamical measurement on a thick-plate. The hysteretic model for each (possibly orthotropic) constituent is written generically as “E(1+jη)” for all mechanical parameters. The estimation method of the parameters relies on a mixed experimental/numerical procedure. The frequencies and dampings of the natural modes of the plate are obtained from experimental impulse responses by means of a high-resolution modal analysis technique. This allows for considerably more experimental data to be used. Numerical modes (frequencies, dampings, and modal shapes) are computed by means of an extended Rayleigh-Ritz procedure under the “light damping” hypothesis, for given values of the mechanical parameters. Minimising the differences between the modal characteristics yields an estimation of the values of the mechanical parameters describing the hysteretic behaviour. A sensitivity analysis assesses the reliability of the method for each parameter. Validations of the method are proposed by (a) applying it to virtual plates on which a finite-element model replaces the experimental modal analysis, (b) some comparisons with results obtained by static mechanical measurements, and (c) by comparing the results on different plates made of the same sandwich material.     

Estimating the direct-to-reverberant energy ratio from the coherence between coincident pressure and particle velocity

An analytical expression for the relationship between the direct-to-reverberant energy ratio (DRR) and the coherence estimation function between coincident pressure and particle velocity component is derived. The analytical solution is first validated with simulated room impulse responses and then used to estimate the DRR in five octave bands for several receiver positions measured in a total of 11 rooms of vastly different sizes and acoustic characteristics. The accuracy is evaluated by comparison with the DRR estimated directly from the room impulse response. The difference is typically 5 dB. For two rooms, the variation of the DRR estimate with source-to-receiver position is also shown. The method is blind in the sense that it is virtually independent of the signal generated by a single sound source.     

Feasibility of subjective speech intelligibility assessment based on auralization

Subjective speech intelligibility can be assessed by speech recorded in an anechoic chamber and then convolved with room impulse responses that can be created by acoustic simulation. The speech intelligibility (SI) assessment based on auralization was validated in three rooms. The articulation scores obtained from simulated sound field were compared with the ones from measured sound field and from direct listening in rooms. Results show that the speech intelligibility prediction based on auralization technique with simulated binaural room impulse responses (BRIRs) is in agreement with reality and results from measured BRIRs. When this technique is used with simulated and measured monaural room impulse responses (MRIRs), the predicted results underestimate the reality. It has been shown that auralization technique with simulated BRIRs is capable of assessing subjective speech intelligibility of listening positions in the room.     

A simple impulse sound source for measurements in room acoustics

This paper deals with the construction and characteristics of a specially designed wooden clapper intended to operate as an impulse sound source for measurements in rooms. It is to be used as an alternative impulse sound source for experimental estimations of impulse responses of rooms located at remote and specific places where none of the standard sound sources - an omnidirectional loudspeaker system, etc. - can be used. The paper describes the spectral characteristics and directivity of the clapper impulse. Its features are compared with other impulse sources.     

A simple method to detect audible echoes in room acoustical design

A simple method to detect audible echoes is proposed as an objective criterion for room acoustics. This method evaluates the perceptibility of sound reflections that are generated by an impulsive sound source and identifies from reflectograms harmful reflections perceived as echoes. Particularly with this method, the masking effect of reverberation is taken into consideration, which cannot be treated sufficiently by the existing objective criteria. The applicability to room acoustical design is verified by evaluating the impulse responses measured in real halls where audible echoes occurred. It is shown that the proposed method detects audible echoes at an accuracy of more than 90% and would be suitable for practical use.     

Dynamic measurement of room impulse responses using a moving microphone

A technique for the recording of large sets of room impulse responses or head-related transfer functions is presented. The technique uses a microphone moving with constant speed. Given a setup (e.g., length of the room impulse response), a careful choice of the recording parameters (excitation signal, speed of movement) leads to the reconstruction of all impulse responses along the trajectory. In the case of a moving microphone along a circle, the maximal angular speed is given as a function of the length of the impulse response, its maximal temporal frequency, the speed of sound propagation, and the radius of the circle. As a result of the presented algorithm, head-related transfer functions sampled at 44.1 kHz can be measured at all angular positions along the horizontal plane in less than 1 s. The presented theory is compared with a real system implementation using a precision moving microphone holder. The practical setup is discussed together with its limitations.     

An approximate treatment of reflection coefficient in the phased beam tracing method for the simulation of enclosed sound fields at medium frequencies

The phased beam tracing method (PBTM) was suggested as a medium-frequency simulation technique for the calculation of impulse response, although main assumptions of geometric acoustics still hold. The phased method needs the reflection coefficient for characterizing the acoustic property of a surface and the complex wave number for describing the propagation characteristics. In this study, two types of approximate real reflection coefficients derived from the measured absorption coefficient were tested for a practical applicability. As a test example, pressure impulse responses and energy impulse responses computed from the PBTM were compared with those from the measurement and the ordinary beam tracing method. The PBTM employing the approximate reflection coefficients greatly increased the accuracy of the prediction compared to the ordinary beam tracing method, in particular at the medium-frequency range in octave bands above the Schroeder cutoff frequency. A comparison was made between angle-dependent and angle-independent reflection coefficients in the calculation of acoustic measures. Although the angle-dependent reflection coefficient yielded best matched results with measured data, but the simple angle-independent reflection coefficient can be also used with a reasonably good precision.     

The evaluation of binaural playback systems for virtual sound fields

By means of virtual acoustics technologies the impulse response in a location of a real enclosure can be used to create a virtual room. Also in this virtual space, typically implemented in a dead room, an impulse response can be measured. From both “real” and respective “virtual” impulse response a group of room-acoustical parameters can be calculated. The match of the two groups of indicators mostly depends on the layout of the playback system and on the implemented processing. In this paper a procedure of virtual measurement in a dead room is developed and some typical layouts of playback systems employing two loudspeakers are compared. Based on the consistency of the room-acoustical indicators derived from “real” and “virtual” impulse responses can be investigated. It is shown how the performance depends on the angular separation of loudspeakers and the efficiency of the cross-talk is also tested and discussed. Finally, a set-up which is suitable for psychoacoustics tests is indicated.     

Evaluation of decay times in coupled spaces: reliability analysis of Bayeisan decay time estimation

This paper discusses quantitative tools to evaluate the reliability of "decay time estimates" and inter-relationships between multiple decay times for estimates made within a Bayesian framework. Previous works [Xiang and Goggans, J. Acoust. Soc. Am. 110, 1415-1424 (2001); 113, 2685-2697 (2003)] have applied Bayesian framework to cope with the demanding tasks in estimating multiple decay times from Schroeder decay functions measured in acoustically coupled spaces. A parametric model of Schroeder decay function [Xiang, J. Acoust. Soc. Am. 98, 2112-2121 (1995)] has been used for the Bayesian model-based analysis. The relevance of this work is that architectural acousticians need to know how well determined are the estimated decay times calculated within Bayesian framework using Schroeder decay function data. This paper will first address the estimation of global variance of the residual errors between the Schroeder function data and its model. Moreover, this paper discusses how the "landscape" shape of the posterior probability density function over the decay parameter space influences the individual decay time estimates, their associated variances, and their inter-relationships. This paper uses experimental results from measured room impulse responses in real halls to describe a model-based sampling method for an efficient estimation of decay times, and their individual variances. These parameters along with decay times are relevant decay parameters for evaluation and understanding of acoustically coupled spaces.     

Coherence three-dimensional imaging spectrometry based on measurement of rotated-hyperbolic volume interferograms

A new interferometric technique to obtain three-dimensional multispectral images of spatially incoherent polychromatic-source distributions is investigated. This technique is based on the measurement of a new type of volume interferogram, herein called the rotated-hyperbolic volume interferogram, which can be obtained by existing interferometer together with a novel aperture synthesis. This paper reports the first demonstration of the method: measurement of a monochromatic point source. The results obtained from this measurement included three-dimensional spatial information and spectral information corresponding to the impulse response function defined over a four-dimensional space. These experimental results confirmed that the imaging characteristics, particularly depth resolution, of the new method are superior to those of previous methods.     

应用声有限元素法模拟坐席吸声低谷效应

本文给出了应用声有限元素法模拟坐席吸声低谷效应的计算模型。该模型首先在时间域计算房间脉冲响应,再通过傅立叶变换得到声压频谱特性。将有限元分析结果与已有的模型实验及大厅实测数据相比较,证明该方法是研究坐席吸声低谷效应的有效手段。