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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.

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room acoustics

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
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
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

11 1 2 the main purposes of auditorium auditorium
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
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
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
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
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,
slide10
Reflection and absorption

play the largest roles in room acoustics

11 2 reflection
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

slide12
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

slide13

Figure 11.3 Reflection from room surfaces

  • Concave surfaces 凹面 tend to focus sound
  • Convex surfaces 凸面 tend to disperse sound
slide14

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.

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

Reflections at a distance from the source may be troublesome.远离声源的反射可能是不利的
slide16
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
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 11 3 absorption
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
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
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
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
(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
(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
(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
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
  • 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
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
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
11 4 2 ideal reverberation time
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
slide31
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

11 4 3 reverberation time formulas
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

slide33
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
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
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
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
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
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,
slide42
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
Today’s key sentences
  • within an enclosed space封闭空间内的
  • play the largest roles in ( )

room acoustics

exercise in class
Exercise in class

1 Porous absorbers for ( )

A high frequency

B lower frequency

C specific lower frequency

D specific high frequency

slide45
(2) Panel absorbers for ( )

A high frequency

B lower frequency

C specific lower frequency

D specific high frequency

slide46
(3) Cavity absorbers for ( )

A high frequency

B lower frequency

C specific lower frequency

D specific high frequency