1010 Room AcousticsRoom Acoustics室内声学室内声学

Room acoustics is concerned with the control of sou

nd within an enclosed space.

The general aim is to provide the best conditions for

the production and the reception 接受 接受 of desirable s

ounds.

Noise control was treated in chapter 9 but the exclus

ion of unwanted noise is an important element of roo

m acoustics

This chapter is concerned with

11.1 Acoustics Principles 11.1 Acoustics Principles 声学原理声学原理

11.2 Reflection 11.2 Reflection 声音的反射声音的反射

11.3 Absorption 11.3 Absorption 声音的吸收声音的吸收

11.4 Reverberation 11.4 Reverberation 混响声混响声

11.1 Acoustics Principle11.1 Acoustics Principle 声学原理声学原理

11.1.1 General requirements for good acoustics 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 timereverberation time suitable for the type of room

混响时间与房间类型匹配 Background noiseBackground noise and external noiseexternal noise reduced to acceptable l

evels 背景噪声和室外噪声降到规定值 Absence of echoesechoes 回声回声 and similar acoustic defectsacoustic defects 缺点 避免回声和类似的声学缺陷

11.1.2 the main purposes of auditorium?11.1.2 the main purposes of auditorium? auditoriumauditorium 听众席听众席 , , 观众席观众席

An auditorium is a room, usually large, designed to be occupied

by an audience.

the main purposes of auditorium can be divided into:

Speech 演讲演讲 Music 音乐音乐 Multi-purpose 多功能多功能detailed acoustic requirements vary with the purpose of the space,

Speech 演讲演讲

The requirement for a good speech is that the speech is intelligi

ble 可理解的可理解的 .

This quality will depend upon the power and the clarity of the so

und.

conference halls 会议厅会议厅 , law courts 法庭法庭 , theatres 剧院剧院 , and le

cture rooms 报告厅报告厅 .

Music 音乐音乐

These qualities are difficult to define but terms in common use includ

e

“fullness” of tone 声音的丰满度声音的丰满度 ,

“definition” of sounds 声音的清晰度声音的清晰度 ,

”blend ” of sounds 声音的混合声音的混合 and

“balance ” of sounds 声音的平衡声音的平衡 .

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

11.1.3 Sound paths in rooms 11.1.3 Sound paths in rooms 声音在室内的传播路径声音在室内的传播路径

reflection 反射反射 ,

absorption 吸收吸收 ,

transmission 透过透过 diffraction 绕射绕射 ,

Reflection and absorption play the largest roles in room acoustics

11.2 Reflection 11.2 Reflection 反射反射 Sound is reflected in the same way as light, provided provided

thatthat 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 usefulReflections 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 troublesoReflections at a distance from the source may be troubleso

me.me.

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

Concave surfaces 凹面 凹面 tend to focus sound Convex surfaces 凸面 凸面 tend to disperse sound

Figure 11.3 Reflection from room surfaces

The domed ceilings 穹顶穹顶 of

the Royal Albert Hall in Londonthe Royal Albert Hall in London 皇家爱尔伯特音乐厅皇家爱尔伯特音乐厅 ,

have often contributed to unsatisfactory acoustics and required re

medies.

Reflections at a distance from tReflections at a distance from the source may be troublesome.he 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

if a strong reflection is received later than 1/20th second after th

e reception of the direct sound.

There is a risk of a distinct echo

An echo is a delayed reflectionAn 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

Flutter echoes 多次回声多次回声 are rapid reflections which cause a

“buzzing” 嗡嗡嗡嗡 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 驻波驻波 or room resonances 共鸣共鸣 ,

which are detected as large variation in sound level at differ

ent positions.

Standing wave effectsStanding wave effects are most noticeable for low-frequen

cy sounds in smaller rooms and,

in general , parallel reflecting surfaces should be avoided

what Hall shapes would be better?

Rectangular 矩形矩形 Wind fan 宽的扇形宽的扇形 Horse shoe 马蹄形马蹄形 Raked seats 阶梯座位阶梯座位

11.2 Reflection 11.2 Reflection 反射反射11.3 Absorption 11.3 Absorption 吸声吸声

Different materials and constructions have different

absorption coefficients

the coefficient for any one material varies with the

frequency of the incident sound.

energy soundincident

energy sound absorbedt(a)coefficien absorption

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

t)coefficien AbsorptionArea( absorption Total

tcoefficienabsorptionArea

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 maxi

mum effect at different frequencies

(1) Porous absorbers for high frequencies

吸收高频的多孔吸声体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 fibreglass 玻璃纤维玻璃纤维 and mineral wool 矿棉矿棉 .

The cells should interconnect with one another

some foamed plastics 泡沫塑料泡沫塑料 is not the most effective form for so

und absorption.

The absorption of porous materials is most effective at frequencies

above 1kHz,

the low frequency absorption can be improved slightly by using incre

ased thickness of materials.

(2) Panel absorbers 平板吸声体

Panel or membrane 膜膜 absorbers resonant frequencyresonant frequency 共振频率共振频率

(md)

60 f

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 c

avity

(a/dV)55 f

Practical absorbers 实际的吸声体

Practical absorbers often absorb sounds by a

combination of several different methods

11.4 Reverberation 11.4 Reverberation 混响声混响声 An echo An echo standing waves 驻波驻波 or room resonances 共鸣共鸣 Reverberation is a continuation and enhancement of a sound c

aused by rapid multiple reflections between the surfaces of a ro

om.

混响声是房间表面间快速多次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反射引起的持续增强的声音

the same as an echo?

11.4.1 Reverberation time 11.4.1 Reverberation time 混响时间混响时间

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.

Areas of exposed surfaces 暴露表面的面积暴露表面的面积 Sound absorption at the surface 表面的吸声量表面的吸声量 Distance between the surfaces 表面间的距离表面间的距离 Frequency of the sound 声音的频率声音的频率

11.4.2 Ideal reverberation time 11.4.2 Ideal reverberation time 理想混响时间理想混响时间

Different activities require different reverberation time

Speech : 0.5 to 1 second Music: 1 to 2 seconds

Short reverberation timesShort reverberation times 短的混响时间短的混响时间 are necessary for clarity of speech, otherwise the continuing pr

esence of reverberant sound will mask the next syllable 音节音节 an

d cause the speech to be blurred 模糊模糊 .

Longer reverberation timesLonger reverberation times 长的混响时间长的混响时间 are considered to enhance the quality of music , otherwise soun

d “dry ” or “dead” if the reverberations time is too short.

Larger rooms are judged to require longer reverberation times

Optimum reverberation timesOptimum reverberation times 最佳混响时间最佳混响时间 Stephens and Bate formula

r= 4 for speech, 5 for orchestras 管弦乐队 , 6 for choirs 合唱团 Ideal reverberation timesIdeal reverberation times

can be presented in sets of graphs, such as those sh

own in figure 11.10

107.0(V)118.0rt 3

11.4.3 Reverberation time formulas

)a1(Slog-

16V.0t

e

Sabine’s formula 赛宾混响公式

Eyting’s formula 依林混响公式

A

16V.0t

A= total absorption of room surfaces (m2 sabins) 室内界面总吸声量室内界面总吸声量

=∑ （ surface area X absorption coefficient) 各表面面积各表面面积 XX 吸收系数吸收系数

S= total area of surfaces m2 房间的总表面面积房间的总表面面积

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

A

16V.0t

)a1(Slog-

16V.0t

e

11.4.4 Calculation of reverberation time

reverberation times are calculated by finding the total abs

orption units in a room and

then using a formula such as Sabine’s formula.

Do not directly add or subtract reverberation times witDo not directly add or subtract reverberation times wit

h one another.h one another.

Use sabine’s formula to convert reverberation times to abs

orption units, make adjustments by addition or subtraction

of absorption units, then convert back to reverberation time.

A

16V.0t )a1(Slog-

16V.0t

e

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 A

16V.0t

2A

500016.0 1.0

1A

500016.06.1

500sabinsA1

800sabinsA2

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

300m2 0.10

Calculate the reverberation time ( for a frequency of 500Hz) of this hall when it is occupied by 100 people.

Area Abs coeff

Abs.units

m2 sabins

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

300m2 0.10 30

occupants

100people

0.46

each46

Total 99sabins

42.299

150016

A

16V.0t

Worked example 11.3The reverberation time required for the hall in worked exa

mple 10.2 is 0.8s. Calculate the area of acoustic tiling吸声瓦吸声瓦 needed, on the walls to achieve this reverberation time( absorption coefficient of tiles = 0.4 at 500Hz)

Surface Area Abs coeff

Abs.units

m2 sabins

Tiles 吸声瓦吸声瓦 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

300m2 0.10 30

occupants

100people

0.46

each46

Total 99+0.38S99+0.38Ssabins

0.38S99

15000.16 0.8

A

16V.0t

2529mS So areas of tilesSo areas of tiles

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