Sound Notes

Seite 1

Describing sound waves

Sound waves are formed by the vibration of particles, moving back and forth, and transferring energy to the particles next to them.

The particles themselves do not move along with the wave, but vibrate (forward and back) about their original position.

Sound waves are longitudinal waves. This means the movement of particles (forward and back) is in the same direction that the wave of sound energy is transferred.

They are made up of compressions where there is a high density of particles, and rarefactions where there is a low density.

Sound travelling through different mediums

Sound travels at different speeds through different materials. In general, the denser the material, the faster the sound wave travels.

Sound therefore travels faster through solids than it does through liquids and air.

A vacuum is a volume of space with no particles.

Sound waves cannot travel through a vacuum as there are no particles to vibrate and pass on the sound energy.

Features of a sound wave

The loudness of a sound refers to the amplitude of a wave.

This means how far from the rest position the vibrating particles travel.

A loud sound has a large amplitude and so the particles in the wave move a large distance from their original position.

The pitch of a sound refers to the frequency of a wave.

Frequency means how many vibrations / oscillations completed each second.

The higher the pitch of a sound, the higher the frequency. The particles in the wave move forwards and back more times each second.

This is shown on an oscilloscope by having more complete waves on the screen.

All instruments make something vibrate, for example the strings on a guitar and the reed in a clarinet vibrate.

The longer or thicker the vibrating object, the lower the pitch.

Structure and function of the ear

The ear is split into three sections, inner, middle and outer ear.

The outer ear contains the ear flap (pinna) and ear canal which channel sound waves to the ear drum.

The middle ear contains three small bones which transfer and amplify the vibrations of the ear drum to the inner ear.

The inner ear contains the fluid filled cochlea which changes sound wave vibrations to electrical signals. These are then taken to the brain, via the auditory nerve, which interprets the electrical signals as sounds.

The decibel scale, dB

The ‘loudness‘ of a sound is measured on the decibel scale.

A normal conversation has a loudness of about 50 dB, heavy traffic 90 dB and a plane taking off 140 dB.

The scale is not linear and an increase in 10 dB, for example from 70 to 80 dB, actually means the noise is x10 louder.

Permanent damage to hearing occurs in the inner ear due to a long term exposure to high sound levels.

Examples include working in noisy environments or regularly listening to loud music on headphones.

Louder sounds cause the eardrum to vibrate with a bigger amplitude.

Permanent damage occurs when exposure to extremely high sound levels, around 160 dB and above, causes vibrations with a big enough amplitude to ruptured ear drum.

Echo location and ultrasound

Echoes are formed when sound particles reflect from objects such as walls.

Animals such as bats use echo location to ‘see’ by emitting sound waves, detecting the echoes sensing how far away objects are.

Ultrasound waves are sound waves above the frequency of human hearing

Typical human hearing has an upper limit of 20,000 Hz.

Ultrasound waves can be reflected off the boundaries between different tissues as they pass through a person. The reflections can be detected and an image of internal organs such as a heart or a foetus can be formed.

Ultrasound vibrations can also be used to clean delicate objects like jewellery and your teeth without the risk of causing chemical damage.