Presentation on theme: "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."— Presentation transcript:
1Room 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
2This 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
311.1 Acoustics Principle声学原理 General requirements for good acousticsAdequate 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
411.1.2 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:Speech 演讲Music音乐Multi-purpose 多功能detailed acoustic requirements vary with the purpose of the space,
5Speech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.
7Music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.
8Multi-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.
911.1.3 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,
10Reflection and absorption play the largest roles in room acoustics
1111.2 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
12The 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
13Figure 11.3 Reflection from room surfaces Concave surfaces 凹面 tend to focus soundConvex surfaces 凸面 tend to disperse sound
14The 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.
15Reflections at a distance from the source may be troublesome 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 echoAn echo is a delayed reflection回声是延迟的反射声
16in 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
17what 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
1811.2 Reflection 反射 11.3 Absorption 吸声 Absorption coefficient 吸声系数Different materials and constructions have different absorption coefficientsthe coefficient for any one material varies with the frequency of the incident sound.
19Clinker 煤渣;炉渣;煤渣块 Clinker blocks 煤渣砌块 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
20Unit：m2 sabins or “absorption units” Total absorption 总的吸声The total Absorption of a surfaceThe total absorption of a roomIs the sum of the products of all areas and their respective absorption coefficientsUnit：m2 sabins or “absorption units”
2111.3.3 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
23(1) Porous absorbers 多孔吸声体 fibreglass 玻璃纤维and mineral wool矿棉.The cells should interconnect with one anothersome 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.
24(2) Panel absorbers平板吸声体 Panel or membrane膜absorbersresonant 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 frequenciesin the range 40 to 400Hz.
25(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
26Practical absorbers 实际的吸声体 Practical absorbers often absorb sounds by a combination of several different methods
2711.4 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?
2811.4.1 Reverberation time 混响时间 Reverberation time Is the time taken for a sound to decay by 60 dB from its original level.
29The 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
3011.4.2 Ideal reverberation time 理想混响时间 Different activities require different reverberation timeSpeech : 0.5 to 1 second Music: 1 to 2 secondsShort 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
31Ideal reverberation times Optimum reverberation times最佳混响时间Stephens and Bate formular= 4 for speech, 5 for orchestras管弦乐队, 6 for choirs合唱团Ideal reverberation timescan be presented in sets of graphs, such as those shown in figure 11.10
3211.4.3 Reverberation time formulas Sabine’s formula赛宾混响公式Eyting’s formula 依林混响公式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
33The 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
3411.4.4 Calculation of reverberation time reverberation times are calculated by finding the total absorption units in a room andthen 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.
35Worked example 11.1A 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 m3Using
36Worked example 11.2A lecture hall with a volume of 1500m3 has the following surface finishes areas and absorption coefficients at 500HzAreaAbs coeffWalls, plaster on brick墙体，砖外铺石膏板400m20.02Floors, plastics tiles地板，塑料瓷砖300m20.05Ceiling, plasterboard on battens 天花板，木龙骨上铺石膏板0.10Calculate the reverberation time ( for a frequency of 500Hz) of this hall when it is occupied by 100 people.
37100people Abs.units m2 sabins 8 15 30 46 99sabins Area Abs coeff Walls, plaster on brick墙体，砖外铺石膏板400m20.028Floors, plastics tiles地板，塑料瓷砖300m20.0515Ceiling, plasterboard on battens 天花板，木龙骨上铺石膏板0.1030occupants100people0.46each46Total99sabins
38Worked example 11.3The 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)