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Sound Waves
(A) Wave nature of sound
Sound shows all properties of waves. It is a kind of waves.
(B) Properties of sound
Some properties of sound:

Sound is produced by vibrations.

Sound is a longitudinal wave, which consists of a series of compressions and
rarefactions.

Sound can travel through solids, liquids and gases, but not in a vacuum.
(Vsolid > Vliquid > Vgas)
(C) Comparison of sound and light
Production
Sound
Light
Mechanical
Electric and magnetic vibrations
vibrations
Type of
Longitudinal
Transverse
Solids, liquids and gases
Transparent
vacuum
330 m s-1 (at 0 oC air)
3 x 108m s-1 in air
waves
Travelling
medium
Travelling
materials
and
a
speed
(D) Ultrasonic waves
Audible frequency ranges between 20 Hz and 20 kHz. Ultrasound is sound of frequency
above 20 kHz.
Ultrasound have wide applications in industry and medicine, such as sonar, ultrasound
cleaner, ultrasound flaw detector and ultrasound cleaner.
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(E) Musical notes and noise
A note is a sound produced by regular vibrations of a source, while a noise is produced
by irregular vibrations.
(1) Musical Notes
Musical notes have the following characteristics.
(i) Loudness
Loudness increases with the amplitude of the sound.
(ii) Pitch
Pitch increases with the frequency of the sound.
(iii) Quality
Two different musical instruments sound very differently even though we are playing
notes of the same pitch and same loudness by them. They are said to have different
qualities and their waveforms are different.
Each sound from a musical instrument has a strong fundamental frequency, which
determines the pitch of the note. Weaker frequencies are also produced. They are
called overtones or harmonics, the frequencies of which are integral multiples of the
fundamental frequency. Notes of the same pitch and loudness from different musical
instruments have different qualities because they have different numbers and
amplitudes of overtones accompanying the fundamental note.
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(2) Noise
Loudness or sound intensity level is measured in decibels (dB) by a sound intensity level
meter. The lowest sound intensity level that can be heard is called the threshold of
hearing. It should be noted that the zero point on the decibel scale is not ‘no sound’.
Very loud
Loud
Moderate
Quiet
Silent
Noise Level in dB
120
110
100
90
80
70
60
50
40
30
20
10
0
Sources of noise
Near roar of aeroplane
Violent hammering of steel plate
Disco
Loud motor horn
Loud radio music
Busy office
Urban district
Normal conservation
Inside a running car
Suburban street
Quiet room
Faintest audible 'sound
Threshold of hearing
Noise can be irritating and unbearable. It affects our work and health. Prolonged
exposure to noise will damage hearing.
In Hong Kong, there are noise control measures enforced by law.
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