9 Principles of sound9 Principles of sound声学原理声学原理

The abilities of speaking and hearing are very important The abilities of speaking and hearing are very important

factors in our personal lives and in our environment.factors in our personal lives and in our environment.

Areas such as music, sound recording and reproduction, Areas such as music, sound recording and reproduction,

noise controlnoise control is also important. is also important.

The The main sources of environmental noisemain sources of environmental noise issues are issues are

transportation noise,transportation noise,

industrial noise, industrial noise,

construction noise and construction noise and

noise from leisure and entertainment.noise from leisure and entertainment.

波长波长

Common topics of concern areCommon topics of concern are

• 11 ）） the exclusion of external noise the exclusion of external noise 隔断外部噪声隔断外部噪声• 2) the reduction of sound passing between roo2) the reduction of sound passing between roo

ms and ms and 减少房间之间的声音传递减少房间之间的声音传递• 3) the quality of sound inside rooms3) the quality of sound inside rooms 室内音质室内音质

Before these topics are studied , this chapter outlines Before these topics are studied , this chapter outlines

the basic principles of sound and its measurement.the basic principles of sound and its measurement.

These basic principles are indicated below:These basic principles are indicated below:

9.1 Nature of sound 9.1 Nature of sound 声音的特性声音的特性

9.2 Sound levels 9.2 Sound levels 声级声级

9.3 9.3 AttenuationAttenuation of sound of sound 声音的衰减声音的衰减

9.4 Nature of hearing 9.4 Nature of hearing 听觉特征听觉特征

9.1 Nature of sound9.1 Nature of sound

what is sound ?what is sound ? Sound is a sensation produced in the ear and brain by variation

s in the pressure of the air.

Sound is a Mechanical Wave. These pressure variations tran

sfer energy from a source of vibration .

What can be Origin of sound What can be Origin of sound 声源声源 ?? Moving objects: loudspeakers 扬声器 , guitar strings 吉他弦 , v

ibrating walls, and human vocal chords 人声 . Moving air: horns 号角 , organ pipes 管风琴 , mechanical fans

风机 and jet engines 喷气发动机 . Vocal 声音的 , 有声的 , 歌唱的 Chord 弦 , 和音 , 情绪

The sound waves travel in a longitudinal 纵向的 type

How do the sound waves travel?How do the sound waves travel?

Its corresponding Transverse Waves 横波

左右分工小能人 ( 横波与纵波 )

Water Waves Water waves are an example of waves that involve a combination of both longitudinal and transverse                                                         

The sound waves can travel through solids, liquids and gases

Sound is a mechanical wave and cannot travel through a vacuum.

Light is an electromagnetic wave and can travel through the

vacuum of outer space

A vacuum?

It is convenient to represent the wave as follows

plot （绘图） the vibrations ag

ainst time.

For a pure sound of one frequ

ency ， the plot takes the for

m of a sine wave 正弦波 .

can be specified in terms of :

Wavelength

Frequency

Velocityfv

How is the Velocity of sound?How is the Velocity of sound?

The velocity of sound is affected by the properties of the materi

al through which it is traveling.

The velocity in air increases as the temperature or humidity incr

eases.

An indication of the velocity of sound in different materials is giv

en in table 9.1 P181

5th row pine 松树松树 , , 树木树木 Last row granite 花岗岩花岗岩

How is the Frequency of sound?How is the Frequency of sound?

If an object that produces sound waves vibrate 100 times a sec

ond, then the frequency of that sound wave will be ??

The human ear hears this as sound of a certain pitch 音调音调 .

Pitch Pitch is the frequency of a sound as perceived by human hear.

Low-pitched notes are caused

by low-frequency sound wave

high-pitched notes are caused

by high-frequency waves.

Note 笔记笔记 , , 短信短信 , , 音符音符

The frequency range to which the human ear responds i

s approximately 20 to 20, 000Hz20 to 20, 000Hz

The range is too wide, what should we do?

To divided it into several parts

How to divide?

one method isone method is

An octave bandAn octave band 倍频程 倍频程 Octave bands commonly used in frequency analysis have the

following centre frequencies. 31.5 63 125 250 500 1000 2000 4000 8000Hz31.5 63 125 250 500 1000 2000 4000 8000Hz

Octave Octave 八个一组的物品八个一组的物品 , , 八度音阶八度音阶

中心频率Hz

63 125 250 500 1000 2000 4000 8000

频率范围Hz

45~

90

90~

180

180~

355

355~

710

710~

1400

1400~

2800

2800~5600

5600~11200

How to describe the Quality of sound How to describe the Quality of sound 音质音质

sound with only one frequency. pure tone 纯音纯音 sound with more than one frequency.

440Hz = fundamental 基准音 or 1st harmonic 谐音 880Hz = 1st overtone 泛音 or 2nd harmonic

1320Hz = 2nd overtone or 3rd harmonic etc.

OvertonesOvertones 泛音泛音 and harmonicsharmonics 谐音谐音 are frequencies equal t

o whole-number multiples of the fundamental frequency

音调的高低取决于基频，而音色取决于谐频分量的构成

Basic frequency Basic frequency 谐频谐频 harmonicsharmonics

880, 1320, 1760, 2200, 880, 1320, 1760, 2200,

2640, 3080, 3520……2640, 3080, 3520……

difference between pure tone, music and noise

When you sing in a bathroom, what would you find?When you sing in a bathroom, what would you find?

Resonance Resonance 声音的共鸣声音的共鸣Every object has a natural frequency

depends on factors such

as the shape, density and

stiffness of the subject.

Resonance occurs when the natural frequency of an obj

ect coincides with the frequency of any vibrations applied

to the object.

The result of resonance is extra large vibrations at this fr

equency.

9.1 Nature of sound 9.1 Nature of sound 9.2 how to measure sound? 9.2 how to measure sound?

Sound power 声功率 Unit : watt

声源单位时间内向外辐射的声能Sound intensity 声强 Unit: w/m2

is the sound power distributed over unit area.

单位面积的声功率 - 单位时间单位面积所通过的声能Sound pressure 声压 Unit: Pa

is the average variation in atmospheric pressure caused by the s

ound

声音所引起的空气压强的平均变化量 Intensity –pressure relationship v

pI

2

I = 100 w/m2

p =200 Pa

Thresholds of pain 痛阈 is the str

ongest sound that the human ear c

an tolerate.

If you want to detect a sound at 1000Hz, it’s s

ound intensity must be greater than

Io = 1X10-12 W/m2

Po = 20X10-6 Pa

Thresholds of hearing 听阈 is the weakest s

ound the average human ear can detect.

What is Decibels ?What is Decibels ? 分贝分贝 Decibels is a logarithmic 对数的对数的 ratio of two quantities.

2

1

210

1

210 )

p

p(10log)

I

I(10logN

Sound intensity level 声强级

Sound pressure level 声压级

)I

I(10logSIL0

10

)p

p(20logSPL

010

v

pI

2

Now, here is a question.

A clock with a alarm of 60dB

When there are two of such clocks,

Ring at the same time,

Do you think what the dB would be ?

L = 3 dBL = 3 dB

Worked example 9.3Worked example 9.3

Calculate the change in sound level when the intensity of a

sound is doubled.

Let I=initial intensity , so 2I= final intensity

Let L1=initial sound level and L2=final sound level

310log2I

I

I

2I10log

I

I10log)

I

2I(10logLL

00

001012

Therefore: double the energy gives a 3 dB increase in sound level

L = 3 dBL = 3 dB

double the energy gives a 3 dB increase in sound level

decibels can be directly added?

Intensities can be added

decibels cannot be directly added?

21 III

Sound pressure can be directly added?

22

21PP P

3dB increase in sound level is caused by doubling the sound energy

10 dB increase in sound level seems approximately twice as loud.

L = 3 dBL = 3 dB

Worked example 8.4Worked example 8.4 Calculate the total sound level caused by the combination of

sound levels of 95dB and 90dB. Threshold of hearing intensity

=1X1-12W/m2

Let I1=intensity of 95dB

I2=intensity of 90dB and

I3=intensity of the combined sounds

Using )

I

I(10logSIL0

10

)I

I(10log950

110 )

I

I(10log900

210

213 III

)I

I(10logSIL0

310

Please check the following website

www.sengpielaudio.com/calculator

The addition of decibel values is made easier with

the aid of a scale such as that shown in figure 9.7.

3dB increase in sound level is caused by doubling the sound energy

10 dB increase in sound level seems approximately twice as loud.

L = 3 dBL = 3 dB

9.3 can sound be attenuated 9.3 can sound be attenuated 衰减衰减as spread out from a source? as spread out from a source?

their amplitude 振幅振幅 decreases and the sound level drops.

does the total energy of the sound wave decrease a lot?

Except for some absorption by the air, the total energy of the

wave nearly remains constant

but the area of the wave front constantly increases.

The energy therefore spreads over larger areas and the densi

ty of this energy or the sound pressure measured at any poi

nt must decrease.

9.3.1 How does a Point source attenuate?9.3.1 How does a Point source attenuate?

Inverse Square Law Inverse Square Law 平方反比定律平方反比定律 The sound intensity from a point source of sound decreases ii

n inverse proportion ton inverse proportion to the square of the distance from the

source.

21

22

2

1

d

d

I

I

Worked example 9.5Worked example 9.5

A microphone measures sound at a position in a free field 5m from a point source. Calculate the change in SPL if the microphone is moved to a position 10m from the source.

Let L1=SPL at distance d1=5m

L2=SPL at distance d2=10m Using

The SPL decreases by 6 dB each time the distance is The SPL decreases by 6 dB each time the distance is double from a point source of sound in free space.double from a point source of sound in free space.

65

1010log

d

d10lo)

I

I(10logLL

2

2

21

22

2

121

A roadway can be a line source of air and noise pollution and need not be a straight line.

9.3.2 how does a Line source 9.3.2 how does a Line source attenuate?attenuate?

The attenuation of sound from a line source in a free field can be

shown by general relationship

Sound intensity from a line source decreases in simple inverse

proportion to the distance from the source.

1

2

2

1

d

d

I

I

In free space

Figure 9.9 Attenuation of sound from a line source

McMullan

9.4 Nature of hearing 9.4 Nature of hearing 听觉特性听觉特性

9.4.1 do you know your ear?9.4.1 do you know your ear?

can be divided into three main parts. Outer ear 外耳外耳 Middle ear 中耳中耳 Inner ear 内耳内耳

9.4.2 longer exposure to noise, you may h9.4.2 longer exposure to noise, you may have deafnessave deafness 耳聋耳聋

(1) Middle ear deafness 中耳耳聋(2) Nerve deafness 神经性耳聋 Temporary threshold shift (TTS) Temporary threshold shift (TTS) 暂时性失聪暂时性失聪 is a temporary loss of hearing which recovers in 1-2 da

ys after the exposure to noise.

Permanent threshold shift (PTS)Permanent threshold shift (PTS) 永久性失聪永久性失聪 is a pe

rmanent loss of hearing caused by longer exposure to

noise.

9.4.4 how to judge the Loudness of a soun9.4.4 how to judge the Loudness of a soundd ？ 响度？ 响度

a 50Hz tone at 70dB

a 50Hz tone at 85dB

a 50Hz tone at 78dB

a 1000Hz tone at 60dB

a 200Hz tone at 60dB

As the intensity of a sound increases it is heard to be “louder”.

Which one is heard to be louder?

It can be seen that the ear is most sensitive in the frequency

range between 2~5kHz , and least sensitive at low frequencies or

at extremely high frequencies.

The unit of loudnessThe unit of loudness

a 50Hz tone at 78dB

a 1000Hz tone at 60dB

a 200Hz tone at 60dB

Their loudness are 60 phone

Another unit:Another unit:

sone is a renumbering 重编号重编号 of the phone scale so that sone

values are directly proportional to the magnitude of the loudne

ss.

One sone is equivalent to 40 phons.

2 sones is twice as loud as 1 sone.

Sone is doubled each time the loudness level is increased by

10 phons

A sound with 60 phone is twice as loud as 30 phone?

Exercises in class

1 The sound waves can travel through （ ）solids, liquids and gases

A solids

B liquids

C gases

D a vacuum

2 The frequency range to which the human ear

responds is approximately ( )

A 20 to 2,000Hz

B 20 to 20, 000Hz

C 200 to 2,000Hz

D 200 to 20,000Hz

3 double the energy gives a （ ） increase in sound level

A 3 dB

B 10 phones

C 6 dB

D 10 dB

4 （ ） increase in sound level seems approximately twice as loud.

A 3 dB

B 10 phones

C 6 dB

D 10 dB

5 The SPL decreases by （ ） each time the distance is double from a point source of sound in free space.

A 3 dB

B 10 phones

C 6 dB

D 10 dB

6 Sone is doubled each time the loudness level is increased by ( )

A 3 dB

B 10 phones

C 6 dB

D 10 dB