Introduction

In old westerns, cowboys often put their ear to a railway track to hear if a train is coming. This is because the speed of sound waves depends upon the material through which they are travelling. Sound travels 15 times faster through the metal railway tracks than through air, giving the cowboy an early warning of the train's arrival.

Have you ever been swimming in the sea when a boat goes past a long way off? You can hear the motor sooner and much more clearly underwater than above the water. Look at the table in Fig.1 to work out why.





Sound travels fastest in solids. This is because the molecules in solids are close together, which makes it easier for the vibrations to pass from one

molecule

A molecule is a group of atoms bonded together.molecule to the next.

The relationship between the

speed

Speed is a measure of how fast something is moving. It is calculated by dividing the distance travelled by the time taken.speed,

frequency

In general, the frequency of an event describes how often it occurs. When talking about waves, the frequency is a measure of how many waves go past a fixed point in a given time.frequency, and

wavelength

The wavelength of a wave is the distance from one peak to the next, or from one trough to the next.wavelength of sound is the same as for other waves:


We can use this equation to work out the wavelength of a particular sound wave, or a certain note.

The frequency of the musical note middle C is 261 Hz.


The wavelength is found by dividing the speed by the frequency:



The frequency of the musical note A is about 440 Hz. It has a higher pitch and a higher frequency than middle C.

A common way to measure the speed of sound is to set up two microphones that are both attached to an electronic timer. A loud noise is made near the first microphone and the timer starts. The sound wave then travels towards the second microphone. When the wave reaches the second microphone, the timer stops. If you know the distance between the two microphones you can work out the speed of sound.

The distance between the two microphones is 20 m. Sound travels at 330 m/s in air. Click on the t in the equation above to rearrange it. Then use the equation to work out how long the sound takes to travel between the two microphones.

Nothing can travel faster than light. However, some aeroplanes can fly faster than sound. We say that they can pass through the sound barrier. When planes fly this fast, they create a sonic boom. This is a very loud booming sound wave that can be heard for miles. It is very similar to the wake made behind a fast-moving boat.



The animation in Fig.2 shows a plane flying slower than the speed of sound. You can see that all the circles (wavefronts) remain inside each other. The plane in Fig.3 is moving faster than the speed of sound. The wavefronts generated by this plane overlap and form a cone in which the waves pile on top of each other (interfere constructively). This leads to the high intensity sound of a

sonic boom

A sonic boom is the loud booming noise created when a moving object travels faster than the speed of sound. It is caused by constructive interference of overlapping sound waves.sonic boom.

Summary


Sound waves travel fastest in solids and slowest in gases.

The speed of sound can be measured in a laboratory using microphones and an electronic timer.

A sonic boom is created when a moving object travels faster than the speed of sound.

A method of measuring the speed of sound in air involves using two microphones both connected to an electronic timer. The timer measures how long the sound takes to travel between the two microphones. The experiment is repeated 10 times and the results are shown in the table below.