Loudness and amplitude

In previous topics you learnt that sound is made when objects vibrate.

The loudness of a sound is a way to describe how quiet or loud a sound appears. Loudness depends on two variables:

• how much the object is vibrating – the greater the distance of each vibration, the louder the sound
• how far away the vibrating object is – the further away the quieter the sound that we hear

The greater the distance of each vibration in the object, the greater the distance that particles in air will be pushed and pulled.

Sounds get quieter when the distance from the vibrating object increases because of energy dissipation. You learnt about energy dissipation in Stage 7.

You learnt in Stage 7 that when a sound wave travels forward in air the particles move backwards and forwards, as shown in the diagram.

The movement of particles in air is difficult to draw in a sound wave, but is easier to draw in a graph. The graph shows the distance that the particles move forward and backward with time. The shape of this graph is sometimes called a waveform.

The maximum distance that particles move, either forward or backward in a sound wave, is called the amplitude.

You can measure the amplitude in two ways:

• The amplitude is the distance from zero to the top, or peak, of the graph.
• The amplitude is the distance from zero to the bottom, or trough, of the graph.

Both these distances are equal in a graph of the same sound wave.

The peak is the highest point of the graph and the trough is the lowest point.

As the distance or range of the vibration increases, the loudness increases. You can also say that as the amplitude of a sound wave increases, the loudness of the sound increases.

A piece of equipment called an oscilloscope displays waveforms of sound waves. An oscilloscope uses a microphone to detect the sound and then produces a waveform of the sound on a screen.

In this photo, the microphone is attached to the oscilloscope by the black wire.

The next diagram shows how a quieter sound and a louder sound compare when the waveforms are seen on an oscilloscope.
The louder sound has a larger amplitude than the quieter sound.

Pitch and frequency

When you vibrate a ruler while holding the ruler at the edge of a desk, the vibrations make sound.

When the vibrating ruler is moved so the length of the vibrating part is shorter, then the pitch of the sound changes. The pitch of a sound is how high or low the sound appears on a musical scale.

The reason that the pitch of the sound from the vibrating ruler changes is because the speed of the vibrations change.

The faster the vibrations, the higher the pitch of the sound.

The speed of vibrations is measured by the number of complete vibrations per second. In Stage 8 in the topic on speed, you learnt that per means ‘in each’. The number of vibrations in each second is called frequency. As the frequency of a sound wave increases, the pitch of the sound also increases.

Frequency is measured in a unit called hertz or Hz.

A frequency of 500 Hz means that 500 complete vibrations happen every second.

Look back at the graph that shows amplitude. The horizontal axis of this graph is time, so a wave with a higher frequency will have more waves in the same time.

As with amplitude, we can use an oscilloscope to compare the frequencies of sound waves of different pitches.

The next diagrams show how a higher pitch sound and a lower pitch sound compare when the waveforms are seen on an oscilloscope.

You will also notice from the waves of higher and lower pitch, that their amplitudes are the same. That means the loudness of both these sounds are the same.

Usually, vibrating objects that are shorter make higher pitch sounds than those that are longer.

This picture shows a musical instrument called a gayageum. The strings are plucked to make them vibrate. The pegs, called bridges, can be moved to change the length of each string that vibrates. The shorter the string, the higher the pitch.