Presentation on theme: "10Room Acoustics 室内声学 Room acoustics is concerned with the control of sound within an enclosed space. 接受 The general aim is to provide the best conditions."— Presentation transcript:
10Room Acoustics 室内声学 Room acoustics is concerned with the control of sound within an enclosed space. 接受 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 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 reverberation time suitable for the type of room 混响时间与房间类型匹配 Background noiseexternal noise Background noise and external noise reduced to acceptable levels 背景噪声和室外噪声降到规定值 echoes 回声 acoustic defects Absence of echoes 回声 and similar acoustic defects 缺点 避免回声和类似的声学缺陷
the main purposes of auditorium? auditorium 听众席, 观众席 An auditorium is a room, usually large, designed to be occupied by an audience. the main purposes of auditorium can be divided into: 演讲 Speech 演讲 音乐 Music 音乐 多功能 Multi-purpose 多功能 detailed acoustic requirements vary with the purpose of the space,
演讲 Speech 演讲 可理解的 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. 会议厅法庭剧院 报告厅 conference halls 会议厅, law courts 法庭, theatres 剧院, and lecture rooms 报告厅.
音乐 Music 音乐 These qualities are difficult to define but terms in common use include 声音的丰满度 “fullness” of tone 声音的丰满度, 声音的清晰度 “definition” of sounds 声音的清晰度, 声音的混合 ”blend ” of sounds 声音的混合 and 声音的平衡 “balance ” of sounds 声音的平衡. There are more acoustic requirements for music than for speech. Music Hall Vienna
Multi-purpose 多功能 Churches, town halls, conference centres, school halls, and some theatres are examples of multi- purpose auditoria. Compromise of speech and music
Reflection and absorption play the largest roles in room acoustics
11.2 Reflection 反射 provided that 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 Reflections near the source of sound can be useful 靠近声源的反射有用 靠近声源的反射有用 Reflections at a distance from the source may be troublesome. Reflections at a distance from the source may be troublesome. 远离声源的反射可能是不利的 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
凹面 Concave surfaces 凹面 tend to focus sound 凸面 Convex surfaces 凸面 tend to disperse sound Figure 11.3 Reflection from room surfaces
穹顶 The domed ceilings 穹顶 of the Royal Albert Hall in London 皇家爱尔伯特音乐厅 the Royal Albert Hall in London 皇家爱尔伯特音乐厅, have often contributed to unsatisfactory acoustics and required remedies.
Reflections at a distance from the source may be troublesome. 远离声源的反射可能是不利的 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 An echo is a delayed reflection 回声是延迟的反射声 回声是延迟的反射声
in smaller rooms + smooth parallel surfaces 多次回声 嗡嗡 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. 驻波共鸣 standing waves 驻波 or room resonances 共鸣, which are detected as large variation in sound level at different positions. Standing wave effects 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? 矩形 Rectangular 矩形 宽的扇形 Wind fan 宽的扇形 马蹄形 Horse shoe 马蹄形 阶梯座位 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 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
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 ： m 2 sabins or “absorption units”
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
Figure 11.5 Response of different absorbers
(1) Porous absorbers 多孔吸声体 玻璃纤维矿棉 fibreglass 玻璃纤维 and mineral wool 矿棉. The cells should interconnect with one another 泡沫塑料 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 膜 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 亥姆霍兹共振器 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 驻波共鸣 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?
Reverberation time 混响时间 Reverberation time 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. 暴露表面的面积 Areas of exposed surfaces 暴露表面的面积 表面的吸声量 Sound absorption at the surface 表面的吸声量 表面间的距离 Distance between the surfaces 表面间的距离 声音的频率 Frequency of the sound 声音的频率
Ideal reverberation time 理想混响时间 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 Short reverberation times 短的混响时间 音节 模糊 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 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
Optimum reverberation times 最佳混响时间 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 Ideal reverberation times can be presented in sets of graphs, such as those shown in figure 11.10
Reverberation time formulas Sabine’s formula 赛宾混响公式 Eyting’s formula 依林混响公式 室内界面总吸声量 A= total absorption of room surfaces (m 2 sabins) 室内界面总吸声量 各表面面积 X 吸收系数 =∑ （ surface area X absorption coefficient) 各表面面积 X 吸收系数 房间的总表面面积 S= total area of surfaces m 2 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
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. 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 m 3 and a reverberation time of 1.6s. Calculate the amount of extra absorption required to obtain a reverberation time of 1s. Know t 1 =1.6s A 1 =? t 2 = 1.0s A 2 =? V=5000 m 3 Using
Worked example 11.2 A lecture hall with a volume of 1500m3 has the following surface finishes areas and absorption coefficients at 500Hz AreaAbs coeff Walls, plaster on brick 墙体，砖外铺石膏板 400m Floors, plastics tiles 地板，塑料瓷砖 300m Ceiling, plasterboard on battens 天花板，木龙骨上铺 石膏板 300m Calculate the reverberation time ( for a frequency of 500Hz) of this hall when it is occupied by 100 people.
AreaAbs coeff Abs.units m2 sabins Walls, plaster on brick 墙体，砖外铺石膏板 400m Floors, plastics tiles 地板，塑料瓷砖 300m Ceiling, plasterboard on battens 天花板，木龙骨 上铺石膏板 300m occupants 100people 0.46 each 46 Total 99sabins
Worked example 11.3 吸声瓦 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)
Surface AreaAbs coeff Abs.units m2 sabins 吸声瓦 Tiles 吸声瓦 S S Walls, plaster on brick 墙体，砖外铺石膏板 400-Sm S Floors, plastics tiles 地板，塑料瓷砖 300m Ceiling, plasterboard on battens 天花板，木龙骨 上铺石膏板 300m occupants 100people 0.46 each 46 Total S Ssabins