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

Sound / Light
Production / Mechanical
vibrations / Electric and magnetic vibrations
Type of
waves / Longitudinal / Transverse
Travelling
medium / Solids, liquids and gases / Transparent materials and a vacuum
Travelling
speed / 330 m s-1 (at 0 oC air) / 3 x 108m s-1 in air

(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.

(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.

(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’.

Noise Level in dB / Sources of noise
Very loud / 120 / Near roar of aeroplane
110 / Violent hammering of steel plate
100 / Disco
Loud / 90 / Loud motor horn
80 / Loud radio music
70 / Busy office
Moderate / 60 / Urban district
50 / Normal conservation
40 / Inside a running car
Quiet / 30 / Suburban street
20 / Quiet room
10 / Faintest audible 'sound
Silent / 0 / 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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Sound Waves