Detecting sound

Ears

We can detect sound using our ears. An ear has an eardrum inside, connected to three small bones. The vibrations in the air make the eardrum vibrate, and these vibrations are passed through the three small bones (called ossicles) to a spiral structure called the cochlea. Signals are passed from the cochlea to the brain through the auditory nerve, and our brain interprets these signals as sound.

The ear, featuring the pinna, ear canal, eardrum, three small bones (hammer, anvil and stirrup), cochlea and auditory nerve.A diagram to show the inner structure of the ear

Microphones

Mobile phones and telephones contain microphones. These devices contain a diaphragm, which does a similar job to an ear drum. The vibrations in air make the diaphragm vibrate, and these vibrations are changed to electrical impulses. In the lab, the electrical impulses can be sent to an oscilloscope, which represents them as a graph on a screen.

Oscilloscope traces

The graphs shown by an oscilloscope are called oscilloscope traces. The diagrams show some typical oscilloscope traces for sound:

A snapshot from oscilloscope traces of three sounds - 1 is a shallow wave with 2 peaks and 1 trough, 2 is a normal wave with 2 peaks and 1 trough, 3 is a deep wave with 3 peaks and troughs.Oscilloscope traces for sound waves

For a wave:

  • the amplitude is the maximum height of the wave from its resting position – the greater the amplitude, the louder the sound
  • the wavelength is the distance between the crests (tops) of two waves next to each other (or any other two identical point on waves next to each other)
  • the frequency is the number of waves per second – the higher the frequency, the closer together the waves are and the higher the pitch

Diagrams 1 and 2 show two sounds with the same wavelength and frequency, so they will have the same pitch. The sound in diagram 2 has a greater amplitude than the one in diagram 1, so it will be louder.

Diagrams 2 and 3 show two sounds with a different wavelength and frequency. The sound in diagram 3 has a higher frequency than the one in diagram 2, so its pitch will be higher.