Sound waves

In a nutshell

Sound waves are longitudinal waves. They can be reflected or refracted. Certain frequencies of sound waves cannot be heard by the human ear. Ultrasound waves are high frequency sound waves that can be used for cleaning, imaging and deep water scans.

Longitudinal waves

Sound waves are longitudinal waves. This means they oscillate in the same direction as they travel. Sound waves are created by vibrating objects and are made up of compressions and rarefactions.

Physics; Waves and the electromagnetic spectrum; KS4 Year 10; Sound waves - Higher

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

Longitudinal waves need a medium to travel through. This is because they travel by transferring vibrations between particles and need them to propagate (move).

Properties of sound waves

Sound waves can be reflected by smooth, hard surfaces. Reflections of sound waves results in echoes.

Sound waves are also refracted at the boundary between two mediums. They speed up when entering denser materials. The frequency of a wave does not change during refraction, but the wavelength and wave speed do.

Note: Light waves behave oppositely to sound waves during refraction. They slow down when entering denser materials. Don't get the two mixed up!

The frequency of a sound wave is related to its pitch. The amplitude of a sound wave is related to the volume of the sound.

Hearing sound

Humans hear sound using the ear system. Sound waves enter the ear and travel through the ear canal until they reach the eardrum. The waves are absorbed by the eardrum which causes it to vibrate. These vibrations are passed along the ossicles (three small bones) to the cochlea. The cochlea converts the vibrations to electrical signals and sends them to the brain using the auditory nerve. Sound is then heard in the brain.

Physics; Waves and the electromagnetic spectrum; KS4 Year 10; Sound waves - Higher

	Part	Description
1.	Ear canal	A tube which carries the sound waves from the outer ear to the inner ear. It contains ear wax which helps keep out infections and traps dirt.
2.	Eardrum	A thin piece of skin which vibrates when sound waves absorbed by it.
3.	Ossicles	Made up of the anvil, the hammer and the stirrup. These are small, fragile bones that carry vibrations.
4.	Cochlea	Converts vibrations from the ossicles to an electrical signal that the brain can process.
5.	Auditory nerve	Sends the electrical signal to the brain for processing.

Note: Humans can hear frequencies between $20\,Hz$  and $20\,kHz$ . This is limited by the shape of the eardrum and the structure of the ear system.

Ultrasound waves

Sound waves above the frequencies humans can hear (above $20\,kHz$ ) are known as ultrasound waves. These are a high frequency sound wave which can only be heard by certain animals, including dogs and dolphins (and the evulpo fox!)

Uses of ultrasound waves

Use	What it does	How it works
Cleaning jewellery	Dirt is shaken off materials.	Vibrations caused by ultrasound are fast and high-energy.
Treating kidney stones	Breaks down kidney stones.	The fast, high-energy vibrations break the kidney stones up.
Ultrasound imaging	Produces image of areas that can not be seen with the human eye.	Ultrasound is partly reflected at the boundary between materials. The areas which reflect can be detected and an image can be produced.
Echo sounding	Detects objects in deep water.	The time taken for the wave to be transmitted, reflected and detected can be used to work out the depth of objects. This is done using the speed-distance-time equation.

Sound wave calculations

The speed of a sound wave in a certain medium can be calculated using the wave speed equation.

$wave \space speed = frequency \times wavelength\\ \ \\ v = f\times\lambda$

The speed of a wave will change in different mediums.

Tip: Remember the frequency of a wave never changes between mediums!

Example:

A sound wave travels through water with a speed of $1500\,m/s$ . It has a wavelength of $5\,m$ .

a) What is the sound wave's frequency?

First, write out the quantities given and check they are in the correct form:

$v = 1500 \,m/s \\ \ \\ \lambda = 5\,m$ 

Then, write down the equation you need to use:

$v = f \times \lambda \\ \ \\ f = \frac{v}{\lambda}$ 

Next, substitute the values into the equation:

$f = \frac{1500}{5} = 300$ 

Make sure you include units:

$frequency, f = 300\,Hz$ 

The frequency of the sound wave is $\underline{300\,Hz}$ .

b) The sound wave now travels through air. It's wavelength changes to $114\,cm$ . What is the speed of the sound wave through air?

First, write out the quantities given and check they are in the correct form:

$f = 300\,Hz \\ \ \\ \lambda = 114\,cm = \frac{114}{100} = 1.14\, m$ 

Then, write down the equation you need to use:

$v = f\times\lambda$ 

Next, substitute the values into the equation:

$v = 300 \times 1.14 = 342$ 

Make sure you include units and round to two significant figures:

$wave\ speed, v = 340 \,m/s$ 

The speed of the sound wave through air is $\underline{340\,m/s}$ .