On the formulation of the intensity method for determining sound reduction indices

The standard formulation of the sound intensity method for determining sound reduction indices is revised. In pursuance of the principle of Gaussian integration, the practical implications of the conditions relating to the shape and absorption contents of the measurement surface used in determining the transmitted power are investigated. An extended formula is presented, containing additional terms to account for the size of the measurement surface, boundary interference effects and calibration mismatch. It is shown that, since the intensity method employs both sound pressure and sound intensity measurement systems, it is more prone to calibration error than the classic two-room method involving the subtraction of two pressure levels. If the receiving room is reverberant, sound absorption by the test object will constitute a component of sound power in a direction opposite to that of the transmitted power. The net effect is a reduction in the apparent transmission, resulting in an overestimation of the sound reduction index. The absorption error increases with the flanking ratio and with the fraction of the total receiving room absorption located on the surface of the test object.     

Enhancing maximum measurable sound reduction index using sound intensity method and strong receiving room absorption

The sound intensity method is usually recommended instead of the pressure method in the presence of strong flanking transmission. Especially when small and/or heavy specimens are tested, the flanking often causes problems in laboratories practicing only the pressure method. The purpose of this study was to determine experimentally the difference between the maximum sound reduction indices obtained by the intensity method, RI,max, and by the pressure method, Rmax. In addition, the influence of adding room absorption to the receiving room was studied. The experiments were carried out in an ordinary two-room test laboratory. The exact value of RI,max was estimated by applying a fitting equation to the measured data points. The fitting equation involved the dependence of the pressure-intensity indicator on measured acoustical parameters. In an empty receiving room, the difference between RI,max and Rmax was 4-15 dB, depending on frequency. When the average reverberation time was reduced from 3.5 to 0.6 s, the values of RI,max increased by 2-10 dB compared to the results in the empty room. Thus, it is possible to measure wall structures having 9-22 dB better sound reduction index using the intensity method than with the pressure method. This facilitates the measurements of small and/or heavy specimens in the presence of flanking. Moreover, when new laboratories are designed, the intensity method is an alternative to the pressure method which presupposes expensive isolation structures between the rooms.     

Characterisation of valves as sound sources: Fluid-borne sound

The sound pressure level in receiving rooms, caused by taps at the ends of pipe systems, is considered. The structure-borne sound power, from the pipes to the supporting wall, was obtained from intensity measurement of the fluid-borne sound power of the tap. The fluid-borne sound power is combined with a ratio of structure-borne sound power to fluid-borne sound power, obtained from laboratory measurements of similar pipe assemblies. Alternatively, a reception plate method is proposed, which avoids the necessity for intensity measurements. The structure-borne power into walls, to which the pipe work is attached, provides input to the standard building propagation model, which yields the predicted sound pressure level in the adjacent room.     

An investigation of simple methods for assessing reverberation time

There is considerable interest in the development of a simple test for sound insulation between dwellings. The assessment of reverberation time is the most difficult part of the procedure to simplify. In this paper six alternative methods are described and evaluated. The first three require no electronic apparatus but are not accurate enough for general use. The fourth involves using sufficient absorbing material in the receiving room to effectively determine the RT. This approach appears worth further development. The fifth approach requires a source of known sound power and the final method employs a simple meter giving a direct reading of the decay time. The last two methods appear to be accurate enough for inclusion in a simplified test method but the simple meter seems to have some advantage.     

混响室内声源的声功率输出及声强分布

本文讨论了混响室内的声强分布,指出混响室内声强分布与自由场一样,对点声源服从平方反比律。对混响室及消声室的声压及声强随时间的起伏作了初步摸索,得到了几条实验规律,指出声强起伏比声压起伏更大。采用声强测量方法对同-声源在消声室及混响室内的声功率输出作了测量,说明声源的声功率输出是随环境变化的声学量,在混响室内声源的低频发射要比消声室内的发射要低。     

Possibilities and limits of sound power measurements with a real-time intensity analyzer

The sound power of a number of test objects was determined from spatially averaged intensity measurements. The results show that the influence of room acoustics is insignificant even for rooms of widely different room constants, if the measuring surfaces are exactly defined and if a good space-averaging technique is used. The intensity integrated over a closed surface defining a source-free space compared to the sound pressure integrated over the same surface gives a measure of the capability of a specific intensity measuring system to suppress external noise. For the test arrangements measured with broad band noise, this suppression was found to be 14–18 dB(A). A similar value of 15 dB was found from sound power measurements on a source with high external sound and an analysis of the results in one-third octave bands. From these measurements an analytical function was derived which describes the average error of the spatially averaged intensity as a function of the difference between the external sound level and the source sound level. For practical measurement situations a further analytical function was derived which gives this intensity error as a function of the difference between the measured (spatially averaged) pressure and intensity levels. Thus it is possible to estimate the error of intensity measurements directly from measured intensity and pressure data.     

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.     

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.     

线阵波束形成声强缩放方法估算声源的辐射声功率

为了解决波束形成声源识别过程中声源辐射声功率定量计算的问题,给出了阵型简洁、便于组合的线阵声强缩放模型。通过推导线阵的声强缩放系数,建立起线阵波束输出结果与声源辐射声功率之间的换算关系。无论是线阵还是平面阵的声强缩放方法,对于偏离阵列中心位置较远处的声源进行辐射声功率估算时都存在较为明显的误差。通过理论推导和仿真模拟计算,研究了同一单极子点声源在不同位置处的声功率估算偏差随频率、幅度的变化规律,发现该估算偏差只与声源偏离位置有关,而与声源自身的强度信息无关的结论,据此给出了相应的声功率估算修正方法。半消声室实验结果和声压法测量结果对比表明:修正后的线阵声强缩放方法用于中高频声源的辐射声功率计算时,单频声源的估算误差不超过1.0 dB,宽带声源的估算误差不超过1.8 dB。     

Enhancing low frequency sound transmission measurements using a synthesis method

The characterization of low frequency sound transmission between two rooms via a flexible panel is investigated experimentally in this work. Previously, the individual effects of the transmission suite on the measured sound reduction index have been studied analytically, and the results have been compared with the ideal case of having free field radiation conditions on both sides of the panel. A new approach is proposed using a near-field array of loudspeakers driven by a set of optimized signals such that a diffuse pressure field is reproduced on the surface of the partition to be tested. The practical effectiveness of this method is assessed when using a set of 16 acoustic sources located in the source reverberant room in close proximity to an aluminium panel. The experimental results obtained confirm the dependence of the characterized sound reduction index on the particular test chamber considered in the low frequency range. They also validate the proposed synthesis method for providing an estimate that only depends on the properties of the partition itself.     

Indirect measurement of acoustic power into a small room at low frequencies

An essential step towards improving sound insulation is a reliable means of quantifying the performance. However, for various reasons sound insulation measurements at low frequencies are associated with relatively high uncertainty and wide variance values. The objective of this research is to develop a method of sound insulation measurement which complements the standard ISO 140 measurement methods by providing improved accuracy at low frequencies. In this paper part of the problem is considered, namely the measurement of power radiated into the receiver room. The ‘peak envelope method’ is based on mode theory and the measurement employs a pair of microphones in the receiver room and a calibrated volume velocity source. No reverberation time measurements are required. The theory is outlined and computer simulations and trial measurements are carried out in order to validate the theory. Good agreement in numerical and experimental validation is demonstrated. We conclude that the peak envelope method is suitable for the measurement of radiated sound power at modal frequencies where ISO 140 methods are poorly adapted. In order to obtain transmission loss, a measure of incident power in the source room will also be required, which will be the subject of future works.     

Ensemble statistics of active and reactive sound intensity in reverberation rooms

This paper examines fundamental statistical properties of the active and reactive sound intensity in reverberant enclosures driven with pure tones. The existing theory for sound intensity in a diffuse sound field, which is based on Waterhouse's random wave model and therefore limited to the region of high modal overlap, is extended to the region of low modal overlap by taking account of the random fluctuations of the sound power emitted by the source that generates the sound field. The validity of the extended model is confirmed by experimental and numerical results.     

室内声场的声像法计算机模拟

本文根据声像模型提出一适宜的算法。原则上,利用该算法能模拟符合简化条件的任意形状房间内某点与声源间的脉冲响应。并由此得出其它声学参数。脉冲响应模拟图样与现场实测图样较为吻合。此外,还就吸声材料分布及房间形状等因素对房间内某点声场的影响作了模拟运算。     

The choice of frequency weightings and RT correction for measurements in a simple test for sound insulation

A simple form of test for airborne sound insulation could involve generating a broad band pink noise in the source room and measuring the source and receiving room levels with a meter containing frequency weighting networks. In practice, the actual source room spectrum shape will depend on room absorption characteristics and this could lead to an error. The effect of this has been examined by computer simulation. It is also necessary to make some form of RT correction to the receiving room level and two possible forms have been simulated. The results of the simulations have been compared with ISO 717 ratings.     

双声源位置和强度识别的三维声强方法

目前在远场识别声源空间位置和强度缺乏行之有效的方法。针对此问题,提出采用四传声器进行三维声强测量,从而构建出声强、声源坐标和声功率的非车线性方程组,求解方程得出声源空间坐标和强度的方法。以3个三维声强探头对两个同频率单极子声源的识别为例,分别利用数值仿真和半消声室内的实验进行方法验证,并对声源的识别空间分辨率做了测试,得出角度识别最大误差为3.83°,为真实值的8.5%,距离识别最大误差0.1 m,为真实距离的10%。结果表明采用该方法空间坐标和声功率识别均具有很高的准确度,双声源的空间位置分辨力也优于远场声全息方法。     

Verbal communication and noise in eating establishments

A new simple prediction model has been derived for the average A-weighted noise level due to many people speaking in a room with assumed diffuse sound field. Due to the feed-back influence of noise on the speech level (the Lombard effect), the speech level increases in noisy environments, and the suggested prediction model gives a 6 dB reduction of the noise level by doubling the equivalent absorption area of the room. This is in contrast to the lowering by 3 dB by doubling of the absorption area for a constant power sound source. The prediction model is verified by experimental data found in the literature. In order to achieve acceptable conditions for speech communication within a small group of people, a guide for the recommended minimum absorption area per person in eating establishments is provided.     

Simple sound power measurements

A simple method of measuring sound power is described. Noise is added from a second calibrated source until the room sound pressure level is increased by 3 dB. The noise output of the device is then equal to the added power. A simple loudspeaker, modified to reduce the effect of the environment on the sound power output, is used as the calibrated source of sound power. Checks show that the sound power readings are substantially independent of the room in which they are made.     

Characterisation of valves as sound sources: Structure-borne sound

Taps and other valves are major sound sources in piping systems and can cause unacceptable noise levels in buildings. The noise results from the fluid-, structure- and air-borne sound emission. At present the acoustic emission of water appliances is tested according to a European standard, the shortcomings of which are apparent as a result of a round robin test of different European laboratories. As a result, there are currently neither acceptable measurement methods for water appliances available nor input data for prediction models. This paper considers methods of characterizing water appliances as sources of structure-borne sound. The concepts of mobility and free velocity are employed for a source characterisation based on power. Taps are considered alone and also in combination with a basin, where again the mobility and free velocity are used. A reception plate method is assessed as an alternative. The two methods each provide an independent characterisation of a structure-borne sound source as a single value. The values are on a power basis and provide input data suitable for prediction of the installed structure-borne power and thence the resultant sound pressure in adjacent rooms. Measured and predicted values of sound pressure level, caused by a wash-basin installed in an adjacent room, are compared.     

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.