Effect of diffusive and nondiffusive surfaces combinations on sound diffusion

One of room acoustic goals, especially in small to medium rooms, is sound diffusion in low frequencies, which have been the subject of lots of researches. Sound diffusion is a very important consideration in acoustics because it minimizes the coherent reflections that cause problems. It also tends to make an enclosed space sound larger than it is. Diffusion is an excellent alternative or complement to sound absorption in acoustic treatment because it doesn’t really remove much energy, which means it can be used to effectively reduce reflections while still leaving an ambient or live sounding space. Distribution of diffusive and nondiffusive surfaces on room walls affect sound diffusion in room, but the amount, combination, and location of these surfaces are still the matter of question. This paper investigates effects of these issues on room acoustic frequency response in different parts of the room with different source-receiver locations. Room acoustic model based on wave method is used (implemented) which is very accurate and convenient for low frequencies in such rooms. Different distributions of acoustic surfaces on room walls have been introduced to the model and room frequency response results are calculated. For the purpose of comparison, some measurements results are presented. Finally for more smooth frequency response in small and medium rooms, some suggestions are made.     

Broad spectrum diffusion model for room acoustics ray-tracing algorithms

Geometrical acoustics methods have already been transformed to account for diffusely reflecting boundaries. In randomized ray-tracing algorithms, the sound rays are either specularly reflected or scattered, according to the value of a random number which is compared with the diffusion factor. However, this method becomes inefficient if this factor depends on frequency, since the process must then be repeated for each frequency band. A method is proposed in this paper to compute all frequency components simultaneously in a single pass. The diffuse reflection model is based on the definition of a new concept: the "splitting coefficient," which can differ from the diffusion factor. First, the randomized ray-tracing method is described and the diffusion model is applied to a single diffusing surface in free field. It is shown that the results computed by the program are in accordance with theoretical results. Then, the method is shown to work properly when it is applied to more realistic enclosures: This is proved in theory and illustrated by some examples. A particular problem is the increase of the statistical error which has been solved by an appropriate control of the splitting coefficient.     

Spatial correlation and coherence in reverberant acoustic fields: Extension to microphones with arbitrary first-order directivity

Spatial correlation and coherence functions in reverberant sound fields are relevant to the acoustics of enclosed spaces and related areas. Theoretical expressions for the spatial correlation and coherence functions between signals representing the pressure and/or the components of the particle velocity vector in a reverberant sound field are established in the literature and most of these have also been corroborated with measurements [F. Jacobsen, J. Acoust. Soc. Am. 108, 204-210 (2000)]. In the present paper, these expressions are generalized to microphones of first-order directivity, whereby the directivity can be expressed in terms of pressure and pressure gradient. It is shown that the resulting spatial correlation and coherence functions can be expressed in terms of the established spatial correlation and coherence functions. The derived theoretical expression for the spatial coherence function is validated with a modeled diffuse sound field. Further, it is compared with the experimental coherence obtained from the reverberant tails of room impulse responses measured with two common surround sound microphone setups in a concert and a lecture hall.     

Design of a multipolar weighting for acoustic antennae

This work describes the development of an acoustic antenna for which several kinds of weightings have been studied to obtain a narrow directivity with attenuated rear lobes. Later, this antenna will be used in a new device to measure the sound absorption coefficient of flat panels in industrial sites. It is often necessary, although quite difficult, to measure acoustic absorption coefficients in partially diffuse conditions, because predicting the sound pressure level at, for instance, a workplace requires in situ characterization of the facings. This antenna has been optimized so that the major part of the received acoustic energy would come from one portion only of an investigated facing present in an industrial room. The multipolar weighting is quite efficient because the associated directivity can be carried out with a limited number of microphones. The calculation technique is improved to achieve a constant sensitivity antenna within the desired frequency range with most of the secondary lobes rejected. The designed receiving system is composed of four antennae, each equipped with five sensors. The directivity, at 150-5000 Hz range, has been verified in an anechoic room.     

Spatial-temporal correlation of a diffuse sound field

The spatial correlation function of the sound in a diffuse field is a quantity widely used in many reverbrant room acoustic applications. Although results for the spatial and temporal correlation for pure-tone and band-limited diffuse fields have already been developed, these have not been generalized for other signal types. This work presents a generalized derivation of the diffuse field spatial-temporal correlation which can be used for stationary random signals with given power spectral density. It is shown that the spatial-temporal correlation depends entirely on the temporal correlation of the signal exiting the diffuse field, or alternatively on its power spectral density. A simulation using the plane wave model is presented for tonal and broadband diffuse sound fields.     

The influence of absorption factors on the sensitivity of a virtual room’s sound field to scattering coefficients

Currently there is limited information on what scattering coefficients (SCs) to assign materials in geometrical room acoustic computer models. As a result, room modelers rely on general guidelines and intuition when assigning SCs. How sensitive is the predicted sound field to the user’s choice of scattering coefficients? The sound field’s sensitivity depends on its diffusivity (without SCs); the more diffuse the room’s sound field is, the less sensitive the virtual room is to the selection of SCs. In rooms with no fittings to diffuse sound energy, the sound field diffusivity is influenced by (1) room shape, (2) volume, (3) amount and (4) location of absorption, and (5) the choice of SCs. This investigation focuses only on the latter three factors. A rectangular room is modeled in ODEON v6.5 with 10 absorption schemes. These schemes vary in terms of the area of mirrored reflective surfaces, average absorption coefficient, and standard deviation of absorption. The amount of diffuse reflections at each room boundary as dictated by the SC is increased uniformly in each room. Changes in the room sound field, in particular in the reverberation time (T30), are examined at each step. Sound field diffusivity, and consequently a virtual room model’s sensitivity to SCs, is found to depend most on the area of mirrored reflective surfaces. Also, a proposed quantity called the Scattering Sensitivity Index appears to predict sound field diffusivity.     

Generalized acoustic energy density

The properties of acoustic kinetic energy density and total energy density of sound fields in lightly damped enclosures have been explored thoroughly in the literature. Their increased spatial uniformity makes them more favorable measurement quantities for various applications than acoustic potential energy density (or squared pressure), which is most often used. In this paper, a generalized acoustic energy density (GED), will be introduced. It is defined by introducing weighting factors into the formulation of total acoustic energy density. With an additional degree of freedom, the GED can conform to the traditional acoustic energy density quantities, or it can be optimized for different applications. The properties of the GED will be explored in this paper for individual room modes, a diffuse sound field, and a sound field below the Schroeder frequency.     

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.     

Experimental evaluation of radiosity for room sound-field prediction

An acoustical radiosity model was evaluated for how it performs in predicting real room sound fields. This was done by comparing radiosity predictions with experimental results for three existing rooms--a squash court, a classroom, and an office. Radiosity predictions were also compared with those by ray tracing--a "reference" prediction model--for both specular and diffuse surface reflection. Comparisons were made for detailed and discretized echograms, sound-decay curves, sound-propagation curves, and the variations with frequency of four room-acoustical parameters--EDT, RT, D50, and C80. In general, radiosity and diffuse ray tracing gave very similar predictions. Predictions by specular ray tracing were often very different. Radiosity agreed well with experiment in some cases, less well in others. Definitive conclusions regarding the accuracy with which the rooms were modeled, or the accuracy of the radiosity approach, were difficult to draw. The results suggest that radiosity predicts room sound fields with some accuracy, at least as well as diffuse ray tracing and, in general, better than specular ray tracing. The predictions of detailed echograms are less accurate, those of derived room-acoustical parameters more accurate. The results underline the need to develop experimental methods for accurately characterizing the absorptive and reflective characteristics of room surfaces, possible including phase.     

Audible Threshold of Early Reflections with Different Orientations and Delays

Early reflection is an important component in an enclosed sound field. Due to the precedence effect, the early reflection may not be the dominant factor in sound source localization; however, it still has obvious influences on spatial position, loudness, timbre, and etc. Till now, there have sparse studies on evaluation of the audible threshold of early reflections with lacking of general and quantitative results. This work investigated the audible threshold of early reflection with a simplified sound field model under various experimental conditions including the combination of eight incidence angles and five time delays. Three-down-one-up adaptive strategy with three-interval three-alternative forced-choice (3I-3AFC) paradigm was used due to its efficiency and robustness. Results indicate that (1) the audible threshold of early reflections decreases monotonically with increasing time delay relative to the direct sound. Furthermore, a linear equation between early reflection threshold and time delay is established with correlation coefficient higher than 0.9; (2) When the direct sound and the reflection locate in the same half-plane, the audible threshold of early reflections decreases with increasing angle deviation between the direct sound and the reflection. Moreover, a front-back symmetry of early reflection threshold is observed for stimuli below 5 kHz; (3) Considerable variations in early reflection threshold are found among individuals, especially at large angle deviation and time delay of early reflections relative to the direct sound.     

In situ measurements of surface impedance and absorption coefficients of porous materials using two microphones and ambient noise

An in situ measurement method is proposed for obtaining the normal surface impedance and absorption coefficient of porous materials using two microphones located close to the material without a specific sound source such as a loudspeaker. Ambient environmental noise that does not excite distinct modes in the sound field is employed as the sound source. Measurements of the normal surface impedance of glass wool and rockwool have been made using this method in various sound fields. The repeatability and wide applicability of the method are demonstrated by comparing results of measurements in one room with different noise conditions and in three other environments (corridor, cafeteria and terrace). The assumed diffuse nature of the sound field on the material is validated by using absorption characteristics obtained experimentally at oblique incidence. This method allows simple and efficient in situ measurements of absorption characteristics of materials in a diffuse field.     

On the importance of early reflections for speech in rooms

This paper presents the results of new studies based on speech intelligibility tests in simulated sound fields and analyses of impulse response measurements in rooms used for speech communication. The speech intelligibility test results confirm the importance of early reflections for achieving good conditions for speech in rooms. The addition of early reflections increased the effective signal-to-noise ratio and related speech intelligibility scores for both impaired and nonimpaired listeners. The new results also show that for common conditions where the direct sound is reduced, it is only possible to understand speech because of the presence of early reflections. Analyses of measured impulse responses in rooms intended for speech show that early reflections can increase the effective signal-to-noise ratio by up to 9 dB. A room acoustics computer model is used to demonstrate that the relative importance of early reflections can be influenced by the room acoustics design.     

Reliability of estimating the room volume from a single room impulse response

The methods investigated for the room volume estimation are based on geometrical acoustics, eigenmode, and diffuse field models and no data other than the room impulse response are available. The measurements include several receiver positions in a total of 12 rooms of vastly different sizes and acoustic characteristics. The limitations in identifying the pivotal specular reflections of the geometrical acoustics model in measured room impulse responses are examined both theoretically and experimentally. The eigenmode method uses the theoretical expression for the Schroeder frequency and the difficulty of accurately estimating this frequency from the varying statistics of the room transfer function is highlighted. Reliable results are only obtained with the diffuse field model and a part of the observed variance in the experimental results is explained by theoretical expressions for the standard deviation of the reverberant sound pressure and the reverberation time. The limitations due to source and receiver directivity are discussed and a simple volume estimation method based on an approximate relationship with the reverberation time is also presented.     

SOUND FIELDS AND SUBJECTIVE EFFECTS OF SCATTERING BY PERIODIC-TYPE DIFFUSERS

Walls and ceilings with quasi-periodic unevenness have often been designed to provide diffuse reflections. As for the shape and the size of this unevenness, with the help of some statistical treatment, it is possible to design effective diffusers with an intended directional pattern; this has great merit in sound fields with respect to spatial aspects. In addition to this spatial property of diffusion, reflection from the surface should not have particular response characteristics that may have a serious effect on our subjective experience. A well-known problem is “colouration” caused by interference between direct and reflected waves, which might create an odd tonal distortion in the case of a surface with periodic unevenness. This may be the reason that creating a periodic series of reflections at the receiver is generally avoided. Two issues need to be addressed: (1) is a periodic structure of reflection a major obstacle in practical use? and (2) to what degree is structural repetition acceptable? Clarification of these matters is necessary in order to design effective diffusers for practical applications. This study examines the physical properties of, and the subjective effects of tonal response to, sound fields caused by scattering from periodic-type diffusers.     

采用振速测量的改进统计最优近场声全息方法

针对常规统计最优近场声全息在空间多源声场重建过程中所需波函数项数多、重建精度不理想的问题,本文提出了一种基于振速测量的改进统计最优近场声全息算法。与常规算法不同,改进算法主要根据声源特点选取合适的波函数组合来计算声场传递矩阵。通过数值仿真初步验证了该方法的准确性和有效性,并与常规算法进行了详细的对比分析。仿真结果表明,改进算法重建精度高,随频率的变化相对误差波动较小,且随着频率的增大相对误差有逐渐减小的趋势;此外,不同的波函数组合,重建效果差异很大,当选取的波函数与声源共形且数量一致时重建效果最好。     

Sound propagation in railway line cuttings

Sound propagation in and around railway line cuttings is in many cases dominated by multiple reflections between train body and cutting wall or between the cutting walls. Typical situations were examined by a large number of scale model experiments. Based on these results a simple empirical calculation scheme was deduced to estimate sound fields in the surroundings of railway line cuttings. The basic idea is to compose the sound field from three parts: the direct sound from the wheels, first order specular reflections and a diffuse sound portion that is emitted from the upper edges of the train body. The scheme can easily be incorporated into existing railway noise models.     

Early energy decays in two churches in Hong Kong

Acoustical measurements based on impulse responses have been made in a Protestant and a Catholic church in Hong Kong. Analysis has concentrated on reverberation time, early decay time and C₈₀, the early-to-late ratio, as well as integrated decays. The measured results have been compared with predictions according to two different theoretical room acoustic models: a model for a single diffuse space and one for a set of coupled-rooms. The comparison has shown that treating the churches as a series of coupled spaces is more valid. However the clarity, C₈₀, at different receiver positions was found to be either under- or over-estimated by the classical coupled-room model. Following this observation, a delayed coupled-room model was developed in the hope of achieving more accurate predictions. In this model, the initial condition under the impulse excitation was modified by introducing a time delay for sound propagated from the source to achieve diffuse sound fields in each subspace. The predicted results of the coupled-room model agree well with the measured results.     

Acoustics of courtyard theatres

The traditional Chinese theatre was often built with a courtyard. In such open-top space, the absence of a roof would mean little reverberation and non-diffused sound field. Acoustically the situation is quite different from that of any enclosed space. Therefore, the classic room acoustics, such as Sabine reverberation formula, would no longer be applicable due to the lack of sound reflections from the ceiling. As the parameter of reverberation time T30 shows the decay rate only, it would not properly characterize the prominent change in the fine structure of the echogram, particularly in case of a large reduction of reflections during the decay process. The sense of reverbrance in a courtyard space would differ noticeably from that of the equivalent 3D-T30 in an enclosed space. Based upon the characteristic analysis of the sound field in an open-top space, this paper presents a preliminary study on the acoustics of the courtyard theatres.     

庭院空间的音质

中国传统戏场有不少是庭院式的。在这种顶面敞开的空间中,混响很少,声场基本上是非扩散的,故其声学性能与全封闭房间有很大不同。经典的室内声学例如赛宾混响公式在此不再适用。且由于缺失了大量来自顶面的反射声,使反射声序列的精细结构有很大变化,在仅仅表征声音衰变速率的混响时间参量中却反应不出来。致使虽具有与封闭房间相同3D混响时间的庭院空间中,其混响感可能明显不同。本文分析了庭院空间内声场的若干特征,对庭院式戏场音质作了初步探讨。     

使用脉冲声和矢量传声器的现场测量吸声系数

使用不同声源利用矢量传声器对毛毡材料进行现场吸声系数测试,研究了不同背景下不同声源的抗噪能力。矢量传声器可以同时测得声压和质点振速信号,进而可计算得到阻抗,利用自由空间和材料表面的阻抗可计算得到材料的吸声系数。在此次实验中,使用不同声源分别在无干扰和有一白噪声干扰源的两种情况下进行测试。结果表明,使用对数扫频脉冲和巴特沃斯脉冲测试所得的吸声系数曲线更平滑,说明脉冲声可以有效降低环境反射的影响,在高噪声背景下使用对数扫频脉冲测试所得的结果基本没有受到背景噪声的影响,说明对数扫频脉冲的抗噪能力更强。因此,使用对数扫频脉冲作为声源进行测试可有效减弱环境反射和背景噪声的影响。