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# Sound

## What is sound?

Sound is produced when an object vibrates in a material like water or air.

The material in which sound travels is the medium. A medium can be any gas, liquid or solid.

Sound travels because of the vibration of particles in the medium. Sound cannot travel through a vacuum as there are no particles to vibrate and carry the sound along.

When the string of a violin is played it vibrates and produces a sound. The vibrations travel through the air to the audiences' ears where they are detected.

The vibration of a violin string produces a sound.

## Sound waves as waves of high and low pressure

Sound waves are longitudinal waves. The particles inside the medium vibrate parallel to the direction the wave travels.

When the blue stick in the image vibrates to the left:

• Particles on the left hand side are pushed away and a compression is formed. Compressions have slightly higher pressure than their surroundings.
• Particles on the right hand side rush to fill the gap left behind by the stick and a rarefaction is formed. Rarefactions have slightly lower pressure than their surroundings

When the blue stick vibrates to the right, the opposite happens.

The particles in the medium push each other and the compression and the rarefaction move away from the source.

The vibrating stick produces sound waves that travel both left and right. $C$ stands for compression and $R$ stands for rarefaction.

## Speed of sound

Sound waves are carried along by collisions between particles in the medium. The speed of the wave depends on the interactions between these particles.

Sound travels fastest in solids, slower in liquids and slowest in gases.

 Medium Speed of sound (m/s) Air Water Iron Diamond 343 1500 5130 12000

The speed of sound in air can be calculated by taking two measurements:

• The distance between the source of the sound and the receiver
• The time taken for the sound wave to travel from the source to the receiver

The speed of sound can be calculated using the formula $$\text{speed of sound} = \frac{\Tred{\text{distance}}}{\Tblue{\text{time taken}}}$$

A man uses a stopwatch to time how long it takes between seeing the sound being made and hearing the noise.

## Sound waves on a graph

Sound waves are often displayed as sine waves on a graph to show the amplitude and wavelength more clearly.

Compressions correspond to crests (maximum points on the graph). Rarefactions correspond to troughs (minimum points on the graph).

The wavelength is the distance between two crests (or two troughs).

The amplitude is the height of the wave. It is measured from the centre of the wave to a crest. It is not measured from a trough to a crest.

A sound wave can be shown on a graph as a transverse wave. The upper lines represent particles. C stands for compression. R stands for rarefaction.

## Loudness and pitch

The human ear can detect the loudness and the pitch of a sound.

The loudness of a sound depends on the amplitude of the sound wave. The larger the amplitude, the greater the pressure of one compression of a sound wave and the louder the sound.

The standard unit of sound level is the decibel.

The pitch of a sound describes whether it is high (like a flute) or low (like a bass drum). The pitch depends on the frequency of the sound wave. The higher the frequency the higher the pitch.

Humans can hear sounds with frequencies between 20 and 20,000 Hz. Some species of bats can hear sounds with frequencies up to 200,000 Hz.

The sound carried by the blue wave is louder and lower-pitched. The sound carried by the red wave is quieter and higher-pitched.

## Echo

An echo is a reflection of sound. A sound hits an object and is reflected back to its source.

If you stand inside a large empty room with smooth walls and begin talking you can produce an echo with your voice.

Echoes are used for measuring distances.

Ships use echoes to measure the depth of the ocean. The ship sends a sound wave down towards the sea bed. It is reflected back towards the ship and detected.

The time difference between sending the sound and receiving the echo is recorded.

The speed of sound in water is already known so the depth of the ocean is calculated using $$\text{depth of ocean} = \frac{1}{2} \times \text{time difference} \times \text{speed of sound}$$

A ship uses echo to calculate the depth of the ocean.

## Ultrasound

Ultrasound is a type of sound with a frequency higher than 20,000 Hz. Ultrasound cannot be heard by humans.

Ultrasound is used in pre-natal scanning (making images of babies inside the womb).

A beam of ultrasound is partially reflected by different materials in the baby's body. These reflections are used to form an image of the baby.

Ultrasound is used instead of X-rays as ultrasound is safer and does not damage body tissue.

Ships use ultrasound to locate submarines underwater. This is called sonar.

The sound wave sent out by the ship is reflected back to the ship by the submarine. This reflection or echo is then detected by the ship.

This image of a baby inside the womb was created using ultrasound.

## Comparison of sound and light

Sound and light share many properties.

• They are both kinds of waves.
• They can both be reflected and refracted.
• They both obey the formula $\text{speed} =\text{wavelength} \times \text{frequency}$.

However, sound and light also have some key differences.

Sound Light
Type of wave Mechanical, longitudinal Electromagnetic, transverse
Speed in different materials Faster in solids than in gases Slower in solids than in gases
Speed in air $343 \umps$ $3 \times 10 ^{8} \umps$
Travels through vacuum? No Yes
Effect of changing frequency Sound changes pitch Light changes colour

In a thunderstorm you see the lightning before you hear the thunder. This is because light travels faster than sound.