Sound – visualising sound waves

Sound is a form of energy that is caused by the vibration of matter. Sound is transmitted through waves, which travel through solids, liquids and gases. We are most used to the sound travelling through air, but sound is able to travel faster and further in solids and liquids.

‘Seeing’ sound

If we could see the molecules that make up the air around us, we would see sound as a series of more and less dense areas of air that are moving away from the source of the sound at about 340 metres per second. We say sound is a wave because the air molecules move back and forth while the sound travels along. The air behaves much like a longitudinal or compression wave on a spring.

It is difficult to draw compression waves, so waves are generally represented as transverse waves for simplicity. The dense areas of the compression wave are the peaks of the transverse wave and the sparse areas are the troughs.

Microphone – transforming sound energy into electrical energy

In order to visualise a sound wave, we can use a microphone to transform sound energy into electrical energy. A simple microphone is made up of a very thin membrane with a coil of very fine wire attached. A magnet is positioned so that it is just inside the coil of wire but not touching it. When a sound wave strikes the membrane, it jiggles (vibrates) back and forth because of the high and low pressure areas of the wave. This causes the coil to jiggle, and when a coil moves in a magnetic field, an electrical current is produced. If we look at the electrical current using an oscilloscope, we can see the sound as a series of peaks and troughs.

The sound from single pitch or note will make a simple sine wave on the screen. The wave will change as you change the volume or pitch of the note.