Class 8 Science


Sound is a kind of wave which gives us the sense of hearing.

Can you imagine a life without sound? Sound helps us in gathering vital information about our surroundings. Moreover, we also let others know about our presence by producing sound.

Sound is produced by a vibrating body. A vibrating object produces sound. When a bell is hit by a gong, the bell starts to vibrate and produces sound.

Vibration: The to and fro or back and forth motion in an object is called vibration or oscillation. Traditional musical instruments produce sound because of vibration in some of their parts.

Sound Produced by Humans

Sound is produced by voice box or larynx, in human beings. The voice box or larynx is situated in the upper part of wind pipe. There are two vocal cords stretched across the larynx in way that there is a small gap between them. When air is forced through the gap, vocal cords begin to vibrate and sound is produced. Muscles which are attached to the vocal cord enable us to make the vocal cords tight or loose as per need. Sound quality varies according to tension or slack in the vocal cords.

structure of larynx

The vocal cords in men are longer (about 20 mm), but they are shorter in women and children. Due to this, voices of men, women and children are different from each other.

Sound Needs a Medium for Propagation

Sound needs a medium through which it can travel. Sound cannot travel through vacuum. Sound can travel through solid, liquid. Propagation of sound happens in all directions in a medium.

Human Ear

Human ear, which gives us the sense of hearing is a complex structure. It can be divided into three main parts, viz. external ear, middle ear and internal ear.

Structure of Human Ear

External Ear

The external ear or pinna appears like a funnel. Its function is to catch sound waves and to direct them towards middle ear.

Middle Ear

The middle ear is composed of a stretched membrane and three small bones. The stretched membrane is called the ear drum and small bones are called bony ossicles. These are named sequentially from outside to inside as; malleus, incus, and stapes (hammer, anvil, and stirrup). When sound wave comes to the middle ear, it sets vibrations in the ear drum. After that, sound waves are transferred from ear drum transfers these vibrations to the three bones.

Internal Ear

The internal ear is composed of cochlea and semicircular canals. Cochlea appears like a snail from outside. Vibrations from middle ear reach the cochlea. Signals from cochlea reach the brain and we hear a sound. Semicircular canals have no role in sense of hearing, rather they maintain the balance of the body.

Characters of Vibrations

characters of sound wave

Vibration is also called oscillatory motion or oscillation. It has certain unique characteristics which are as follows:


Number of oscillations in unit time is called frequency of oscillation. Frequency is expressed as Hertz (Hz). When an object is vibrating 1 time in a second, its frequency is 1 Hz.


Maximum displacement of a wave on either side from mean position is called amplitude. Thus, amplitude shows how far the vibrating object moves from the mean position.

Loudness of Sound

Loudness of sound depends on amplitude of vibration. Loudness of sound is directly proportional to square of amplitude of vibration. Louder sound has higher amplitude, while quieter sound has lower amplitude. Loudness is expressed in terms of decibel. Loudness of some common sounds is in the following table.

Normal breathing10 dB
Soft whisper (at 5 m)30 dB
Normal conversation60 dB
Busy traffic70 dB
Average factory80 dB

The noise becomes physically painful above 80 dB.

Pitch of Sound

Pitch of sound depends on frequency of vibration. A high pitched sound has high frequency, while a low pitched sound has low frequency. Children and women generally produce sound with high pitch.

Audible and Inaudible Sounds

Human beings can hear sounds between frequencies 20 Hz and 20,000 Hz. This range of frequencies is called the hearing range for humans. The sound with frequency below 20 Hz is called infrasound. On the other hand, the sound with frequency above 20,000 Hz is called ultrasound.

Noise and Music: A sound which is pleasant to ears is called music. But any unpleasant sound is called noise.

Noise Pollution

Presence of excess noise in environment is called noise pollution. Automobiles, factories, loud music, construction works, firecrackers, stone quarry, etc. are sources of noise pollution.

Effects of Noise Pollution

Continuous exposure to noise pollution can result in lack of sleep (insomnia), hypertension (high blood pressure), anxiety and many other disorders. Noise pollution can also lead to partial loss of hearing; in some cases.

Measures to Limit Noise Pollution:

Hearing Impairment

Loss of hearing is called hearing impairment. It can be total or partial, but total hearing impairment is rare. Total hearing impairment is usually congenital, i.e. by birth. A person with hearing impairment can learn sign language to communicate with others. Hearing aids can be used by people who are suffering from partial hearing impairment.