椰胡声功率级的半消声室测试

椰胡是具有地方特色的拉弦乐器之一,多用于演奏广东音乐和广东戏曲、曲艺的伴奏。但对其发声强度一直未进行过科学的测试。本文介绍在华南理工大学半消声室内参照ISO(GB)标准对椰胡声功率级的测量工作。由两位资深乐师分别用两把椰胡在pp,mp,f和ff力度下演奏单音、音阶和乐曲,对每一把椰胡的每一测试内容,由十通道测试设备同步测试中心频率为100～10000 Hz的1/3倍频带声压级谱,通过计算获得每把椰胡在演奏每项内容时的声功率级和动态范围。通过对两把椰胡的测试结果进行平均,获得该乐器在演奏上述内容时的典型声功率级数值及频谱。文中并将半消声室内的测试结果与混响室内的测试结果相对比,探讨测试环境对测试结果的影响。民族乐器发声强度及其频谱特性的获得是开展民族音乐厅堂音质研究的基础。     

混响室内测定声功率的问题

用简正振动方式理论分析了混响室内声功率测定问题,求得了严格公式和统计公式,解决了测试中多年未有定论的问题。     

线阵声强缩放方法在水下声源辐射功率估算中的应用

为了解决水下声源辐射声功率难以计算的问题,利用线阵声强缩放方法在波束形成声源识别的基础上,根据波束输出结果与声源辐射声功率之间的换算关系来获得相应的声功率。为了提高线阵声强缩放方法的水下声功率估算精度,给出了一定动态范围限制的主瓣区域积分方法,并通过仿真验证了该方法的有效性。在消声水池中开展了水下声功率估算的实验研究。在不同的测试距离下,对双声源条件下的单频以及宽带声源在阵列侧的辐射声功率进行了估算,以混响法的测量结果为参考值,研究了估算误差随声源频率、测试距离等影响因素的变化规律。实验结果表明,无论是单频还是宽带声源,声功率的最大估算误差不超过2.6 dB,在高频时不超过1.6 dB。验证了线阵声强缩放方法应用于水下声源辐射声功率估算的正确性与可行性。      

混响室中单极子的声功率发射

在本文中推出了混响室内一个简单声源的功率发射的准确公式,并用简单分析方法求得声功率发射的重要特性。其一就是声源在混响室内的平均功率并不等于其在自由声场中的功率,而是大于后者,这是前人所未料及的。还进一步推出一平滑的统计公式,后者一直可用于波长接近于室内平均自由程的低频率。由这公式可看到室中有很多功率发射峰值的点,在室内形成规则的立体网格,接近周围墙壁则为高发射区。统计公式在预测声功率发射沿与混响室某长、宽、高方向的相对变化时,基本是准确的,只有平滑时引起的误差,用于室内声场的一般估计时可能有较多误差。为说明问题,文中举了几个数字实例以比较简正波理论和统计理论的结果。混响室内声功率发射的变化影响声源的使用和测量,必须予以注意。     

水下翼型结构流噪声实验研究

为测量流激水下翼型结构的流噪声,提出了一种混响箱测量方法。在重力式水洞中搭建了一套实验测量系统,利用混响箱法测量了水下翼型结构模型的辐射声功率。在此基础上研究了流速及结构参数(厚度、肋、声学覆盖层)对其辐射声功率的影响。结果表明:当流速小于5 m/s时,辐射声功率随流速的6次方增长,符合偶极子的辐射规律;当流速大于5 m/s时,辐射声功率随流速的10土1次方规律增长,不再按偶极子的规律辐射;若对水下翼型结构模型加厚、加环肋及外部敷设黏弹性材料,均可在一定程度上抑制流噪声。此研究方法可对水下复杂结构的辐射声功率测量及结构优化设计提供一定的参考。      

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。     

混响室条件下临界辐射干扰场强计算模型

为解决均匀场与混响室内辐射敏感度测试结果相关性较差的问题,利用统计学理论对混响室内场强直角分量及天线接收功率的统计特性进行分析,理论推导出基于受试设备干扰概率的混响室条件下临界辐射干扰场强计算模型。为验证该模型的正确性,以ETS 3142E型天线为受试设备分别在混响室及均匀场中进行临界辐射干扰场强测试,实验结果表明,利用该计算模型得出的临界辐射干扰场强值与均匀场测试结果吻合良好,平均相对误差可控制在2 dB以内,可将该模型应用于实际辐射敏感度测试。     

瓦状阻尼橡胶块对高铁车轮减振降噪的影响分析

依据声学测试标准,为了评价某型高铁车轮在安装不同形式橡胶块装置后的减振降噪效果,在半消声室内基于B&K振动噪声测试分析系统,对裸轮和橡胶块车轮开展振动声辐射室内测试实验,并基于有限元方法对车轮模态进行了仿真分析。测试结果可知:相比裸轮,WA、WB车轮模态阻尼比显著增加,车轮的减振效果明显,其中WA车轮的减振效果略优于WB车轮。径向激励下,WA车轮声功率级降低了8 dB(A),WB车轮声功率级降低了5.5 dB(A);轴向激励下,WA车轮声功率级降低了8.2 dB(A),WB车轮声功率级降低了6.2 dB(A)。分析可知橡胶块装置能有效抑制车轮的滚动噪声和曲线啸叫,对车轮的减振降噪有积极作用。     

混响室条件下环境场强测量位置选取准则

针对在混响室条件下进行辐射敏感度测试时电磁环境场强难以度量的问题,分析了混响室内电场强度的统计特性,理论推导了有损混响室内场强空间相关系数表达式,并通过仿真分析验证了表达式的正确性。根据表达式得出有损混响室内相对距离大于半波长的两点间场强相互独立的结论,并由此推断出,混响室内环境场强测试位置与受试设备(EUT)距离应大于0.5倍波长,且应以此作为测量位置选取准则。采用仿真计算与实验验证相结合的方法分析了相对误差随测试点与EUT间相对距离的变化规律,对该准则进行了验证,结果表明,当测试点与EUT相对距离大于0.5时,相对误差能够保证在1.5 dB以内,可应用于实际混响室环境场强测试。     

声学国际标准的新信息

IISO（国际标准化组织）和thC（国际电子学委员会）最近批准公布的新标准和原标准的修订版。1．11Stl：（1）ISO4869－2：1994，护耳器——第fu部分：戴着护耳器时，有效A计权声压级的估计．（2）ISOlo844：1994，声学——对道路车辆辐射噪声的测量路径的规定。（3）ISO3743－1：1994，声学——噪声源的声功率级的测量——在混响场中对较小的、可移动的声源的工程测量法——第一部分：硬墙测试房间的比较法．（4）IS03744：1994，声学——用声压级测量噪声源的声功率组在反射面上的理想自由声场中测量的工程法．（5）IOS389．3：19…     

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 characterization of on-stage performers in performing spaces

The influence of on-stage performers on the acoustic characteristics of performing spaces is significant because the musicians are absorptive and close to the sound source. However, the acoustic information of the musicians and their effect on absorption, scattering, and diffusion remain unclear. The acoustic characteristics of musicians were measured in a reverberation chamber and a semi-anechoic chamber while varying the type of clothes, the instruments, and seating density. The clothing worn showed a larger impact on the absorption per person, whereas the addition of cellos resulted in low-frequency absorption per person. The addition of cellos also increased the scattering and diffusion characteristics. Finally, the total absorption by the musicians under various conditions was analyzed in a concert hall using the simple Sabine equation where the influence of the musician’s instruments resulted in reverberation time of more than 0.1 s decrease with about 90 musicians on stage. However, the influences of additional musical instruments resulted in no significant difference in the reverberation time compared to the people alone. In addition, scattering due to the various conditions of orchestra such as the seating density and the addition of musical instrument was not a significant predictor of reverberation time.     

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.     

Evaluations of output from room acoustic computer modeling and auralization due to different sound source directionalities

Evaluations of the predictions and auralizations from the room acoustic modeling program, ODEON, have been run using three directional source types with the same sound power: (a) an omni-directional source; (b) three sources with realistically-directional characteristics based on measurements from real instruments (grand piano, violin, and singing voice); and (c) an artificial, extremely directional beaming source. Objective analyses have been run for nine source/receiver combinations in a simple hall on three acoustic parameters: relative sound pressure level (SPL), reverberation time (T30), and clarity index (C80). Auralizations were subsequently created for two source/receiver combinations and used in subjective testing with 28 subjects. Results show that, objectively, differences in SPL were negligible for the majority of cases. Some differences in T30 and C80 were found between the omni-directional and realistically-directional sources; however, subjects did not perceive any corresponding differences when comparing the auralizations, possibly due to the limited directional octave band data available. Subjects did significantly differentiate between auralizations from the omni-directional source and the extreme beaming source. Subjective results from comparing these two sources in terms of reverberation, clarity and realism were generally consistent with objective data, although source/receiver combination and musical track had some influence on the outcomes.     

Reduction of acoustic radiation by perforated board and honeycomb layer systems

A perforated system, proposed previously for reducing the radiated sound from a plate at arbitrary frequencies, is applied to three-dimensional problem. Plates are assumed to be supported in a duct of a finite cross-section and excited by a harmonic point force. The sound radiation is investigated from the viewpoint of acoustic power and it is discussed whether the attenuation effect shown previously in the one-dimensional system can be obtained with the three-dimensional system. The effect of support conditions on attenuation characteristics is discussed by using clamped and simply supported circular models. Allowing for the effect, a simply supported rectangular model is studied in detail and its problems are revealed. In order to overcome the problems, a new system including subdivided air cavities in the form of a honeycomb layer instead of a undivided backing cavity is proposed. Each of the honeycomb cells can create local one-dimensional sound fields. Calculated theoretical results are compared to data obtained in a 1/5th scale reverberation chamber. The results for the reduction effect, which are in good agreement, show that the honeycomb layer system can achieve the same reduction of the radiated sound power at arbitrary frequencies as the one-dimensional perforated system.     

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.     

Effects of edge screens on the absorption of blocks of theatre chairs

This paper reports new measurement results investigating the use of screens around samples of theatre chairs to minimize edge effects when measuring theatre chair absorption in reverberation chambers. The absorption measurements included both full scale and scale model measurements in reverberation chambers and a model recital hall. The use of screens has been proposed to better approximate the sound absorption of the larger blocks of chairs in auditoria. The method of measuring the absorption of blocks of chairs with screens around their edges and located in the corner of a reverberation chamber did not give results indicative of the values obtained for larger areas in auditoria. The addition of screens around samples of chairs did not eliminate the variation of absorption coefficients with perimeter/area ratio. The results of extrapolations from measurements of blocks of screened chairs to infinite samples gave lower absorption coefficients than found for blocks of unscreened chairs. The absorption of chairs in large performance halls can best be predicted using the P/A method to extrapolate from reverberation chamber measurements of smaller samples of unscreened chairs, with a range of P/A values, to the larger samples and lower P/A ratios of blocks of chairs typical of performance spaces.