Sound waves

In a nutshell

Sound waves are longitudinal waves. Sound can reflect off of surfaces which results in echoes. Frequency is the amount of wavelengths passing through a point per second.

Properties of sound waves

The two types of waves are transverse and longitudinal. Sound waves vibrate (or undulate) in the same direction as they travel. This is different to water waves, or transverse waves. Therefore sound waves are classed as longitudinal waves.

Longitudinal waves

A longitudinal wave is a wave which vibrates in the same direction as it travels. They are made up of compressions and rarefactions.

1.	Wavelength
2.	Compression
3.	Rarefaction
4.	Direction of travel
5.	Vibrations

Medium	A material that waves travel through.
Wavelength	The distance from one rarefaction to the next, or from one compression to the next - they will give the same measurement.
Compression	The point on a wavelength where the particles are closest together.
Rarefaction	The point on a wavelength where the particles are furthest apart.

Note: For transverse waves, a wavelength is measured from one crest to the next crest, or one trough to the next trough. In fact, a wavelength can be measured between any two consecutive identical points of a wave, although it is easier just to stick to crest-to-crest or trough-to-trough!

Frequency

Definition

The number of wavelengths passing through a point per second. Frequency is measured in units of hertz ( $Hz$ ).

Frequency is a useful measurement which can be used to calculate the speed of a wave. For sound waves, a higher frequency means a higher pitch.

Sound waves in different mediums

Sound waves travel through different mediums by vibrating the particles inside a medium. Sound needs a medium to travel through.

Example

In air: sound can travel through air by vibrating the particles in it

In a vacuum: a vacuum is a region of space with no particles in it. Sound cannot travel through space as there are no particles to vibrate through.

Sound waves travel at different speeds in different mediums.

Medium	Speed of sound
Air	$343 \,m/s$
Water	$1480\,m/s$
Solid	$6000\, m/s$

Note: $m/s$ refers to metres per second.

Longitudinal waves reflect just like transverse waves. Repeated reflections of sound waves create echoes. Sound waves can also be absorbed by materials. Smooth, hard surfaces reflect sound best, whilst soft, rough surfaces absorb sound.

Example

In a cave: you can hear echoes as the walls are smooth and hard.

In a bedroom: you cannot hear echoes as the carpet and curtains are soft and rough.

Producing and detecting sound waves

Loudspeakers

Loudspeakers are able to produce sound waves by converting electrical energy into kinetic energy by vibrating the cone. The vibrating cone then vibrates the air particles nearby, creating sound waves. The electrical signal controls how fast the air particles vibrate, which controls the sounds produced.

1.	Permanent Magnet
2.	Coils of Wire
3.	Electrical Signals
4.	Cone
5.	Vibrations

Microphone

A microphone has a diaphragm (thin sheet of material) which vibrates when sound waves in the air hit it. These vibrations are turned into electrical signals, which can be connected to an oscilloscope to measure the waves.

Tip: A microphone is just the opposite of a loudspeaker! It turns a sound wave into electrical signals.