1. 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

2. 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

3. 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 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

4. 11.1.2 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,

5. 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.

7. 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.

8. 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.

9. 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,

10. Reflection and absorption
play the largest roles in room acoustics

11. 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

12. 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

13. Figure 11.3 Reflection from room surfaces
Concave surfaces 凹面 tend to focus sound
Convex surfaces 凸面 tend to disperse sound

14. 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.

15. Reflections 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 echo
An echo is a delayed reflection
回声是延迟的反射声

16. 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

17. 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

18. 11.2 Reflection 反射 11.3 Absorption 吸声
Absorption coefficient 吸声系数
Different materials and constructions have different absorption coefficients
the coefficient for any one material varies with the frequency of the incident sound.

19. Clinker 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

20. Unit：m2 sabins or “absorption units”
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”

21. 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

22. Figure 11.5 Response of different absorbers

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 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.

24. (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.

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

26. Practical absorbers 实际的吸声体
Practical absorbers often absorb sounds by a combination of several different methods

27. 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?

28. 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.

29. 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

30. 11.4.2 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
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

31. Ideal reverberation times
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

32. 11.4.3 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

33. 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

34. 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.

35. 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

36. Worked example 11.2
A lecture hall with a volume of 1500m3 has the following surface finishes areas and absorption coefficients at 500Hz
Area
Abs coeff
Walls, plaster on brick
墙体，砖外铺石膏板
400m2
0.02
Floors, plastics tiles
地板，塑料瓷砖
300m2
0.05
Ceiling, plasterboard on battens 天花板，木龙骨上铺石膏板
0.10
Calculate the reverberation time ( for a frequency of 500Hz) of this hall when it is occupied by 100 people.

37. 100people Abs.units m2 sabins 8 15 30 46 99sabins Area Abs coeff
Walls, plaster on brick
墙体，砖外铺石膏板
400m2
0.02 8
Floors, plastics tiles
地板，塑料瓷砖
300m2
0.05 15
Ceiling, plasterboard on battens 天花板，木龙骨上铺石膏板
0.10 30
occupants
100people
0.46
each 46
Total
99sabins

38. 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)

39. S 100people Surface Abs.units m2 sabins 0.4S 8-0.02S 15 30 46
Area
Abs coeff
Abs.units
m2 sabins
Tiles吸声瓦 S
0.40
0.4S
Walls, plaster on brick
墙体，砖外铺石膏板
400-Sm2
0.02
8-0.02S
Floors, plastics tiles
地板，塑料瓷砖
300m2
0.05 15
Ceiling, plasterboard on battens 天花板，木龙骨上铺石膏板
0.10 30
occupants
100people
0.46
each 46
Total
Ssabins

40. So areas of tiles

41. 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,

42. 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

43. Today’s key sentences within an enclosed space封闭空间内的
play the largest roles in ( )
room acoustics

44. Exercise in class 1 Porous absorbers for ( ) A high frequency
B lower frequency
C specific lower frequency
D specific high frequency

45. (2) Panel absorbers for ( )
A high frequency
B lower frequency
C specific lower frequency
D specific high frequency

46. (3) Cavity absorbers for ( )
A high frequency
B lower frequency
C specific lower frequency
D specific high frequency