Effects of periodic type diffusers on classroom acoustics

The present study investigates the application of periodic type diffusers for improving the acoustics in classrooms and proposes an optimum combination of absorptive and diffusing treatments for achieving preferred acoustics in classrooms. The measurements were carried out in a 1/10 scale model classroom, adding different amounts of absorptive and diffusing materials to one or more of five surfaces of the room. Adding absorptive materials on the entire ceiling area and diffusers on each wall increased early-to-late arriving sound ratio (C₅₀) values corresponding to higher clarity. Adding diffusers on 25% of the ceiling area with the other 75% absorptive was more beneficial for achieving higher G₅₀ and G values than adding absorptive materials on the entire ceiling. Diffusers were beneficial for enhancing the early arriving reflection energy (G₅₀) at the most distant seats and they resulted in achieving more uniform acoustical conditions across the room. The treatment of the front wall with diffusers was found to lead to better acoustical conditions than treating the rear and side walls with diffusers. Adding diffusers on the ceiling and lower front wall or side wall and adding diffusers on the ceiling and absorptive materials on the lower front wall were selected for consideration as the more successful treatments for classroom acoustic design.     

薄膜超构表面声扩散体扩散性能研究*

针对传统的声扩散体结构复杂、加工不便的缺点,该文提出了一种薄膜型声学超材料扩散体,通过有限元软件建立了扩散体的有限元模型。分析了薄膜张力、质量块大小及背腔深度对声波相位调控的影响,并与传统施罗德扩散体的扩散性能进行比较,发现调控后的薄膜超构表面声扩散体对声能的吸收降低,并使入射声能更均匀地向全方向扩散。在相同的尺寸情况下,与传统施罗德扩散体相比,薄膜超构表面声扩散体在所需频段内扩散性能更优,结构更优,体积更小,质量更轻,加工更方便,更适用于工程应用。     

Study on echo suppression effect and coloration due to periodic-type diffusers

An echo is one of the obstacles in halls and theatres. In order to suppress this undesirable echo, walls and ceilings have been often given absorptive surfaces which absorb sound energy or corrugated surfaces which provide wave diffusion. While it is not difficult to predict the degree of echo suppression effects in the case of absorptive surfaces, it is generally difficult to do that in the case of corrugated surfaces. In addition, if the surfaces have periodicity, unintended effects called coloration can distort tonal characteristics of the sound field. Thus, many acoustic designers would hesitate to employ this kind of periodic-type diffusers. However, there are some cases where it is necessary to suppress echoes without any energy loss. Periodic-type diffusers have the advantages of easy estimation of diffusion properties, simple method of design, and ready availability. In this study, a subjective experiment with simulated stimuli was carried out to clarify the echo suppression effect of the periodic-type diffusers from the viewpoint of reflected energy level. Although coloration interfered subjective judgments in some cases of the experiment, it is shown that the periodic-type diffusers have echo suppression effects if the coloration does not occur. This study is also devoted to discussion about causes and features of the coloration.     

阶梯状声扩散体形体优化方法

应用时域有限差分法计算阶梯状声扩散体扩散系数,结合免疫遗传算法分别对周期排布的六阶和十二阶阶梯状声扩散体的形体进行优化。首先,验证了时域有限差分法计算声场的有效性,之后采用两种方式进行优化设计:第一种是直接对多周期重复排布的扩散体形体进行优化;第二种首先对单周期扩散体形体进行优化,再将该扩散体重复周期排布。结果显示:第一种优化方式能获得扩散性能更优的周期排布的扩散体。单周期扩散性能最优的扩散体重复一定周期后,其性能不会保持最优,相反,扩散性能并非最优的单周期扩散体重复一定周期后的扩散性能却优于前者;扩散体总长度一定条件下,增加扩散体的阶次、降低重复周期数,可有效提升扩散体的扩散性能。因此,直接对多周期重复排布的高阶次扩散体形体进行优化,能够获得扩散性能最优的周期重复排布的扩散体。      

Development of scattering surfaces for concert halls

The degree of diffusion, characterized by the “scattering coefficient” of surface materials, has been known to be one of the most important factors in determining the acoustical qualities of concert halls. Based on the suggested ISO method, which measures the random-incidence scattering coefficient of surfaces in a diffuse field, the scattering coefficients of different sizes and densities of wooden hemispheres and cubes were measured in model-scale reverberation rooms. The results show that hemispheres with a height of more than 15 cm have the highest average (500 Hz to 4 kHz) scattering coefficient. It was also found that the scattering coefficient becomes higher when the diffuser density reaches about 50% for hemispheres and 30% for cubes.     

音乐厅音质设计进展述评

本文对近年来音乐厅音质设计中若干问题的进展作一述评。（1）在已建立的众多独立音质参量基础上，如何作出大厅综合评价，就需要考虑到它们的互动性和贡献权，方法学将成为关键。（2）空间感已确认由声源视在展宽度ASW和听者环绕感LEV组成，对于影响两者的诸多复杂因素有了较深入研究，这声场中的细节又将如何与大厅设计相联系亦备受关注。（3）听众及座椅吸声这个老问题有了新的研究成果，对于空、满场的声学关系已积累了更多资料，使大厅音质的估算更为精确可靠。（4）常说扩散对大厅音质很重要，但其主观效应仍然处于玄虚状态，一些新的主观试听实验结果还不足以说明问题。     

Characterizing diffusive surfaces using scattering and diffusion coefficients

This paper presents the effects of height and coverage density of diffusive surfaces through the measurements of scattering and diffusion coefficients using scale models. Measurements were conducted using a 1:10 scale model. Scattering coefficients of 1D, 2D, and 3D diffuser profiles as well as diffusion coefficients of the 3D diffuser profile (hemisphere) were measured. It was found that the structural height of the diffuser is closely related to the frequency characteristics of the scattering coefficients, while the coverage density of the diffuser is closely related to the values of the scattering coefficients. Diffusion coefficients were less sensitive to structural height and coverage density than were the scattering coefficients. Using the measurement results, scattering and diffusion coefficients were estimated. In addition, the relationship between scattering and absorption coefficients was discussed.     

Ternary and quadriphase sequence diffusers

A room acoustic diffuser breaks up reflected wavefronts, and this can be achieved by presenting a spatially varying surface impedance. In hybrid surfaces, varying impedance is achieved by patches of absorption and reflection, giving reflection coefficients nominally of 0 and 1. These surfaces are hybrids, absorbing some of the incident sound while diffusing any reflected energy. A problem with planar hybrid surfaces is that specular energy is only removed by absorption. By exploiting interference, by reflecting waves out-of-phase with the specular energy, it is possible to diminish the specular energy further. This can be achieved by using a diffuser based on a ternary sequence that nominally has reflection coefficients of 0, -1, and +1. Ternary sequences are therefore a way of forming hybrid absorber-diffusers that achieve better scattering performance without additional absorption. This paper discusses methods for making ternary sequence diffusers, including giving sequence generation methods. It presents prediction results based on Fourier and boundary element method models to examine the performance. While ternary diffusers have better performance than unipolar binary diffusers at most frequencies, there are frequencies at which the performances are the same. This can be overcome by forming diffusers from four-level, quadriphase sequences.     

音乐厅音质设计

本文简要介绍音乐厅建筑实践的发展史和四个最近设计成功的大厅。简要叙述现在于大厅内能测量且与主观响应有关的客观参数，即混响时间、初始时间延迟间隙、响度、扩散、侧向反射和早期对后期能量比。最后，简短地报道近来声场模拟方法的发展，用这些方法可以预测某些声学特性。     

An investigation of the sound field above the audience in large lecture halls with a scale model

Measurements of steady-state sound pressure levels above the audience in large lecture halls show that the classical equation for predicting the sound pressure level is not accurate. The direct field above the seats was measured on a 1:10 scale model and was found to be dependent on the incidence angle and direction of sound propagation across the audience. The reverberant field above the seats in the model was calculated by subtracting the direct field from the measured total field and was found to be dependent on the magnitude and particularly on the placement of absorption. The decrease of sound pressure level versus distance in the total field depends on the angle (controlled by absorption placement) at which the strong reflections are incident upon the audience area. Sound pressure level decreases at a fairly constant rate with distance from the sound source in both the direct and reverberant field, and the decrease rate depends strongly on the absorption placement. The lowest rate of decay occurs when the side walls are absorptive, and both the ceiling and rear wall are reflective. These consequences are discussed with respect to prediction of speech intelligibility.     

Optimizating the seats surrounding the platform in a recital hall when considering source directivity

The influence of source directivity can be significant for solo and chamber music performances, particularly for arena type halls that have improved visual intimacy for the audience. Computer simulation was performed to analyze the possibility of optimizing the acoustical qualities of a 4000-m³ recital hall with a significant portion of seats surrounding the platform when considering source directivity. The effects of surface treatments, length-to-width proportion, seating arrangement and platform location were evaluated. Most acoustical parameters derived from a baritone singer source were categorized as one factor that can be best represented by 2-kHz band early sound strength G_E. Regardless of room proportion, front-back 2-kHz band energy difference derived from optimized models can be controlled within 4 dB, a value much smaller than the difference in direct component.     

Absorbing surfaces in ray-tracing programs for coupled spaces

Nowadays architects commonly use the ‘coupled space concept’; examples are mezzanines, half-open office spaces and exhibition rooms. There is a need to predict acoustical quantities for this category of spaces, since half-open spaces may be a cause of noise annoyance. The transmission of sound between coupled spaces depends on design decisions like position, shape and dimensions of the surfaces and on the reflection characteristics. This paper deals with some problems related to the application of absorbing surfaces in coupled rooms, especially when they are modelled in a ray-tracing program. Absorption coefficients from meausurements in reverberation chambers may exceed 1.0 and they do not bear any information about angle dependent behavior, so an extra conversion must be made into input values for the ray-tracing model. Therefore ray-tracing calculations have been performed in a computer model of a reverberation chamber. From a comparison study between measurements and calculations in three coupled rooms it is found that the accuracy is good, provided that the sound reflections on the walls are introduced as angle dependent. Care should be taken in choosing a diffusion factor of flat surfaces.     

室内声学中散射研究进展

近20年,界面散射研究在声场预测、定量测量和产品研发等方面取得了重要进展,加深了我 们对散射及其作用的认识。人们可以定量地评估、设计和优化散射产品,用更加丰富的手法进行室内 音质设计。     

Relation of acoustical parameters with and without audiences in concert halls and a simple method for simulating the occupied state

Five acoustical parameters-reverberation time RT, early decay time EDT, clarity C80, strength G, and interaural cross-correlation coefficient IACC-were measured using identical procedures with and without audiences in six concert and opera halls. Reverberation times without audiences were measured in 15 additional halls using the same measuring techniques as for the six halls above, but for full occupancy the data were taken from musical stop chords at symphonic concerts. This paper shows that in all halls (1) the occupied RT can be predicted from the unoccupied RT using a linear regression equation, y = a - b exp(x), within acceptable limits, at low- and mid-frequencies. It is also shown for the six halls that (2) occupied C80's are predicted accurately from unoccupied values by the newly proposed equation; (3) G's with and without audiences are highly correlated by a first degree linear regression equation; and (4) IACCs have nearly the same value in both occupied and unoccupied halls. As a separate subject, the successful use of a cloth covering for seats in a concert or opera hall to simulate the occupied condition has been developed.     

Some preliminary acoustical considerations in the design for the proposed Wellington (New Zealand) town hall

The preliminary design for Wellington Town Hall, New Zealand, is an evolution from the successful design for Christchurch Town Hall. It includes, however, the results of a decade of experimental work on the effects of early lateral reflections on listener preference for orchestral music. The basic form of the auditorium remains an arena, roughly elliptical in plan, and containing within its boundaries large suspended surfaces to ensure appropriate reflection sequences. The acoustical design objectives include the combination of a long reverberation time, “envelopment” due to the lateral reflections and clarity as encourtered in Christchurch Town Hall. Advances in the provision of diffusion and understanding of the acoustical requirements for ensemble are included in the proposals. The experimental work and other advances in knowledge relating to concert hall design are reviewed, and their application in the design of Wellington Town Hall is discussed.     

扩压器入口面积比对自由旋涡气动窗口性能的影响

 针对采用主动式扩压器的自由旋涡气动窗口,实验考察了扩压器入口面积比对扩压器压力恢复性能及自由旋涡气动窗口密封性能的影响。结果表明：入口面积比是影响扩压器压力恢复性能的重要因素;通过优化扩压器入口面积比,可以提高气动窗口密封压比和稳定性。主动式扩压器比传统扩压器具有更小的最佳入口面积比,使气动窗口射流更接近于理想自由旋涡射流;主动式扩压器最佳入口面积比约为2.03。在最佳入口面积比情况下,扩压器内端壁吹气临界驻室压力最低,外端壁吹气压力对密封压力基本没有影响。     

室内粗糙表面声散射体掠入射角度散射估计

针对经典边界元方法对掠入射角度下的散射估计精度较低的问题,发展了一种边界无网格模型.此模型将散射体视为具有无限延伸的形式,避免了经典边界元法中薄板形式所导致的声压差问题,而且更加符合实际房间中散射体的存在形式;模型利用无网格算法实现数值仿真,可对任意表面形状特别是曲面散射体具有更高的仿真精度。利用边界无网格模型计算了不同形状散射体的散射系数及散射声场,并将结果与解析方法、测量实验进行了对比.对比结果表明,边界无网格模型可以准确预测散射体的散射性质,特别是对掠入射角度的估计要优于经典边界元法.研究结论可应用于室内声学散射体特征预测及优化设计,对提高声场扩散及室内音质水平具有重要意义.      

有叶扩压器的流场分析

本文利用计算流体力学通用软件了进行离心压气机扩压器的初步设计工作。参照有关文献[1~8]中的设计公式,本文进行了以下几种扩压器的几何造型和数值计算: (1)双圆弧中心线机翼型叶片扩压器; (2)楔形扩压器;(3)用优化程序设计的扩压器,给出了各扩压器造型在给定边界条件下流场的马赫数、静压、总压的分布,计算出各扩压器造型的静压比和总压恢复系数。最后对设计的扩压器与叶轮一起连算,以取得更接近实际情况的计算结果。通过对计算结果进行的分析比较,得到了离心压气机扩压器设计中有价值的初步性结果,为进一步的设计提供了改进途径和相应的基础数据。     

Engaging concert hall acoustics is made up of temporal envelope preserving reflections

Strong, exciting, and engaging sound is perceived in the best concert halls. Here, it is shown that wideband early reflections that preserve the temporal envelope of sound contribute to the clear and open acoustics with strong bass. Such reflections are fused with the direct sound due to the precedence effect. In contrast, reflections that distort the temporal envelope render the sound weak and muddy because they partially break down the precedence. The presented findings are based on the earlier psychoacoustics research, and confirmed by a perceptual evaluation with six simulated concert halls that have same monaural room acoustical parameter values according to ISO3382-1.