Sound

Checkpoint Physics

Sound

You have probably performed some experiments on sound without knowing it.

At some time most people have made a ruler vibrate by holding one end over the edge of a desk and twanging it.

The end of the ruler moves up and down rapidly and a low whirring sound is heard which becomes higher as you pull in the ruler from the edge of the desk.

From vibration to sound wave

Any object can make a sound wave when it vibrates. In practical work on sound you might use an elastic band, a guitar string or a tuning fork because they all vibrate easily.

A vibration is a movement about a fixed point. This may be described as a to-and-fro movement or a backwards and forwards movement.

From vibration to sound wave

Sound waves can travel in a gas, a liquid or a solid because they all contain particles.

When an object vibrates it makes the particles next to it in the gas, liquid or solid vibrate too.

Sound waves cannot pass through a vacuum because it does not contain any particles.

Describing the wave

The amplitude is the height of the crest or the depth of the trough.

The wavelength is the distance from a point on one wave to the exact same point on the next wave.

Detecting sound waves

The ear is the organ of the body for detecting sound waves.

It is divided into three parts – the outer ear, the middle ear and the inner ear.

The outer ear

When sound waves reach the outer ear some pass directly down the middle of the tube called the auditory canal. Some waves which strike the outer part of the ear are reflected into the auditory canal.

At the end of the auditory canal is a thin membrane which stretches across it. This is called the ear drum.

When sound waves reach the ear drum they push and pull on it and make it vibrate.

The middle ear

In the cavity of the middle ear there are three bones. They are called the hammer, anvil and stirrup, after their shapes. The ear bone forms a system of levers. When the eardrum vibrates its movements are amplified by the lever system.

The middle ear also has a tube, the Eustachian tube, which connects to the throat. This balances air pressure when we swallow allowing the ear drum to vibrate as freely as possible.

The inner ear

The inner ear is filled with fluid. The vibrations of the stirrup set up waves in the fluid. There is a membrane with delicate fibres in the cochlea. Each fibre only vibrates in response to a sound wave with a particular pitch. When a fibre vibrates it stimulates a nerve ending and a nerve impulse or message is sent to the brain where we become aware of the sound.

The cathode ray oscilloscope

The cathode ray oscilloscope or CRO can be used to investigate sound waves. A CRO conatins a cathode ray tube. At one end of the tube is a screen. The cathode rays make a spot on the screen which can be made to move from left ro right. When a micorphone is attached to the CRO, sound waves can be displayed on the screen.

The loudness of sounds

The loudness of a sound is related to the movement of the vibrating object. If an object only moves a short distance to and fro from its rest position, it will produce a sound wave with only a small amplitude and the sound that is heard will be a quiet one.

If an object moves a large distance to and fro from its rest position, it will produce sound waves with a large amplitude and the sound that is heard will be a loud one.

Sound Loudness / decibels

The sound hurts 140

A jet aircraft taking off 130

A road drill 110

A jet plane overhead 100

A noisy factory floor 90

A vacuum cleaner 80

A busy street 70

A busy department store 60

Normal speech 55

Voices in a town at night 40

A whisper 20

Rustling leaves 10

Limit of normal hearing 0

Loudness and energy

Sound energy passes through the air as the particles move to and fro. When a wave with a small amplitude is generated, a small amount of energy passes through the air.

When a wave with large amplitude is generated a large amount of energy passes.

The energy of a sound wave is converted into other forms such as movement energy in the eard drum and ear bones.

Loudness and deafness

The vibrating air particles of a very loud sound can produce such a strong pushing and pulling force on the ear drum that a hole is torn in it. The ear drum is said to be perforated. It no longer vibrates efficiently and the person loses his or her hearing. The ear drum can heal and normal hearing can be restored.

If a person is exposed to a very loud sound or a particular note for a long period of time they will no longer be able to hear it. This is due to permanent damge to a nerve ending in the cochlea.

Loudness and deafness

A common form of partial deafness, which is not related to loudness of sound, is the developement of ear wax in the outer ear. This stops sound waves reaching the ear drum.

Some people have growths of tissue in their middle ears which stop the ear bones moving freely. They amy be prescribed a hearing aid. This contains a microphone and amplifier and compensates for some of the loss of amplification that was provided by the ear bones.

The pitch of a sound

The pitch of the sound an object makes depends on the number of sound waves it produces in a second as it vibrates.

This number of waves per second is called the frequency. The frequency of a sound is measured in hertz (Hz).

The higher the frequency of the wave, the higher the pitch of the sound.

Experiments on sound

In the past many scientists have performed experiments to find the speed of sound.

Isaac Newton (1642-1727) investigated the speed of sound by measuring the time between a sound being made and its echo from a distant wall being heard.

Other scientists measured the time taken between seeing a distant cannon fire and hearing its sound.

Experiments on sound

The speed of sound in water was found using the apparatus shown in the picture. The experiment was performed at night. When the lever was pulled down both the arm carrying the bell hammer and the device carrying the match moved.