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.

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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.

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).

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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.

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

Uses of ultrasound waves

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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 $$\underline{340\,m/s}$$​.