Room Acoustics室内声学 Room acoustics is concerned with the control of sound within an enclosed space. The general aim is to provide the best conditions for the production and the reception接受 of desirable sounds. Noise control was treated in chapter 9 but the exclusion of unwanted noise is an important element of room acoustics
This chapter is concerned with 11.1 Acoustics Principles 声学原理 11.2 Reflection 声音的反射 11.3 Absorption 声音的吸收 11.4 Reverberation 混响声
11.1 Acoustics Principle声学原理 11.1.1 General requirements for good acoustics • Adequate levels of sound 足够的声级 • Even distribution to all listeners in the room 使每位听众都能听到 • reverberation time suitable for the type of room 混响时间与房间类型匹配 • Background noise and external noise reduced to acceptable levels 背景噪声和室外噪声降到规定值 • Absence of echoes回声and similar acoustic defects缺点 避免回声和类似的声学缺陷
An auditorium is a room, usually large, designed to be occupied by an audience. the main purposes of auditorium can be divided into: Speech 演讲 Music音乐 Multi-purpose 多功能 detailed acoustic requirements vary with the purpose of the space, 11.1.2 the main purposes of auditorium?auditorium听众席, 观众席
Speech演讲 • The requirement for a good speech is that the speech is intelligible可理解的. • This quality will depend upon the power and the clarity of the sound. • conference halls会议厅, law courts法庭, theatres剧院, and lecture rooms报告厅.
Music音乐 Music Hall Vienna • There are more acoustic requirements for music than for speech. These qualities are difficult to define but terms in common use include “fullness” of tone声音的丰满度, “definition” of sounds声音的清晰度, ”blend” of sounds声音的混合and “balance ” of sounds声音的平衡.
Multi-purpose 多功能 Compromise of speech and music • Churches, town halls, conference centres, school halls, and some theatres are examples of multi-purpose auditoria.
11.1.3 Sound paths in rooms 声音在室内的传播路径 • reflection反射, • absorption吸收, • transmission透过 • diffraction绕射,
Reflection and absorption play the largest roles in room acoustics
11.2 Reflection 反射 • Sound is reflected in the same way as light, provided that the reflecting object is larger than the wavelength of the sound concerned. • reflection is useful to obtain good room acoustics? Reflecting surfaces in a room are used to help the even distribution of sound
The following general rules apply Reflections near the source of sound can be useful 靠近声源的反射有用 Reflections at a distance from the source may be troublesome. 远离声源的反射可能是不利的 Plane reflector 平面反射板 Curved reflector 曲面反射板
Figure 11.3 Reflection from room surfaces • Concave surfaces 凹面 tend to focus sound • Convex surfaces 凸面 tend to disperse sound
The domed ceilings 穹顶of the Royal Albert Hall in London皇家爱尔伯特音乐厅, have often contributed to unsatisfactory acoustics and required remedies.
if a strong reflection is received later than 1/20th second after the reception of the direct sound. There is a risk of a distinct echo An echo is a delayed reflection 回声是延迟的反射声 Reflections at a distance from the source may be troublesome.远离声源的反射可能是不利的
in smaller rooms + smooth parallel surfaces Flutter echoes多次回声are rapid reflections which cause a “buzzing” 嗡嗡 Each frequency of a sound has a wavelength. If the distance between parallel surfaces equal the length of half a wave, or a multiple of a half wavelength. standing waves 驻波or room resonances共鸣, which are detected as large variation in sound level at different positions. Standing wave effects are most noticeable for low-frequency sounds in smaller rooms and, in general , parallel reflecting surfaces should be avoided
what Hall shapes would be better? • Rectangular 矩形 • Wind fan 宽的扇形 • Horse shoe 马蹄形 • Raked seats 阶梯座位
11.2 Reflection 反射11.3 Absorption 吸声 Different materials and constructions have different absorption coefficients the coefficient for any one material varies with the frequency of the incident sound. 11.3.1 Absorption coefficient 吸声系数
Table 11.1 • lists the average absorption coefficients of some common materials at the standard frequencies used in acoustic studies. • Clinker 煤渣;炉渣;煤渣块 • Clinker blocks 煤渣砌块
11.3.2 Total absorption 总的吸声 The total Absorption of a surface The total absorption of a room Is the sum of the products of all areas and their respective absorption coefficients Unit：m2 sabins or “absorption units”
11.3.3 Types of absorber 吸声体的类型 • can be classified into three main types which have maximum effect at different frequencies (1) Porous absorbers for high frequencies 吸收高频的多孔吸声体 (2) Panel absorbers for lower frequencies 吸收低频的平板吸声体 (3) Cavity absorbers for specific lower frequencies 吸收超低频的空心吸声体
(1) Porous absorbers 多孔吸声体 • fibreglass 玻璃纤维and mineral wool矿棉. • The cells should interconnect with one another • some foamed plastics 泡沫塑料is not the most effective form for sound absorption. • The absorption of porous materials is most effective at frequencies above 1kHz, • the low frequency absorption can be improved slightly by using increased thickness of materials.
(2) Panel absorbers平板吸声体 Panel or membrane膜absorbers resonant frequency共振频率 m _ the mass of the panel ( kg/m2) d_ the depth of the airspace (m) A panel absorber is most effective for low frequencies in the range 40 to 400Hz.
(3) Cavity absorbers 空腔吸声体 Helmholtz resonators亥姆霍兹共振器 are enclosures of air with one narrow opening. The maximum absorption occurs at the resonant frequency of the cavity
Practical absorbers 实际的吸声体 • Practical absorbers often absorb sounds by a combination of several different methods
11.4 Reverberation 混响声 • An echo standing waves 驻波or room resonances共鸣 • Reverberation is a continuation and enhancement of a sound caused by rapid multiple reflections between the surfaces of a room. 混响声是房间表面间快速多次 反射引起的持续增强的声音 the same as an echo?
11.4.1 Reverberation time 混响时间 • Reverberation time Is the time taken for a sound to decay by 60 dB from its original level.
The time taken for this decay in a room depends upon the following factors. • Areas of exposed surfaces 暴露表面的面积 • Sound absorption at the surface表面的吸声量 • Distance between the surfaces 表面间的距离 • Frequency of the sound 声音的频率
Different activities require different reverberation time Speech : 0.5 to 1 second Music: 1 to 2 seconds Short reverberation times短的混响时间 are necessary for clarity of speech, otherwise the continuing presence of reverberant sound will mask the next syllable音节and cause the speech to be blurred模糊. Longer reverberation times长的混响时间 are considered to enhance the quality of music , otherwise sound “dry ” or “dead” if the reverberations time is too short. Larger rooms are judged to require longer reverberation times 11.4.2 Ideal reverberation time 理想混响时间
Optimum reverberation times最佳混响时间 Stephens and Bate formula r= 4 for speech, 5 for orchestras管弦乐队, 6 for choirs合唱团 Ideal reverberation times can be presented in sets of graphs, such as those shown in figure 11.10
Sabine’s formula赛宾混响公式 Eyting’s formula 依林混响公式 11.4.3 Reverberation time formulas A= total absorption of room surfaces (m2 sabins)室内界面总吸声量 =∑（surface area X absorption coefficient)各表面面积X吸收系数 S= total area of surfaces m2房间的总表面面积
The Sabine formula is suitable for rooms without excessive absorption. If the average absorption in a room is high, such as in a broadcasting studio, Eyting’s formula
11.4.4 Calculation of reverberation time reverberation times are calculated by finding the total absorption units in a room and then using a formula such as Sabine’s formula. Do not directly add or subtract reverberation times with one another. Use sabine’s formula to convert reverberation times to absorption units, make adjustments by addition or subtraction of absorption units, then convert back to reverberation time.
Worked example 11.1 A hall has a volume of 5000 m3 and a reverberation time of 1.6s. Calculate the amount of extra absorption required to obtain a reverberation time of 1s . Know t1=1.6s A1=? t2= 1.0s A2=? V=5000 m3 Using
Worked example 11.2 A lecture hall with a volume of 1500m3 has the following surface finishes areas and absorption coefficients at 500Hz Calculate the reverberation time ( for a frequency of 500Hz) of this hall when it is occupied by 100 people.
Worked example 11.3 The reverberation time required for the hall in worked example 10.2 is 0.8s. Calculate the area of acoustic tiling吸声瓦needed, on the walls to achieve this reverberation time( absorption coefficient of tiles = 0.4 at 500Hz)
Today’s key words • Room acoustics 室内声学 • reverberation time 混响时间 • Background noise 背景噪声 • External noise 室外噪声 • Echoes 回声 • Plane reflector 平面反射板 • Curved reflector 曲面反射板 • standing waves 驻波or room resonances共鸣,
Porous absorber 多孔吸声体 • Panel absorber 平板吸声体 • Cavity absorber 空腔吸声体 • Absorption coefficient 吸声系数
Today’s key sentences • within an enclosed space封闭空间内的 • play the largest roles in ( ) room acoustics
Exercise in class 1 Porous absorbers for ( ) A high frequency B lower frequency C specific lower frequency D specific high frequency
(2) Panel absorbers for ( ) A high frequency B lower frequency C specific lower frequency D specific high frequency
(3) Cavity absorbers for ( ) A high frequency B lower frequency C specific lower frequency D specific high frequency