Seeing and hearing spectra

What the Activity is for

Building sounds from a range of frequencies.

This activity allows you to introduce the idea that most sounds are made up of many frequencies, and this provides us with a very powerful way of describing and identifying sounds.

What to Prepare

• a prepared selection of sounds, either recorded with the means to replay them, or a selection of devices to make the sounds
• a microphone, linked to a computer, running sound analysis software

What Happens During this Activity

Few sounds consist of one pure note. In fact what we hear as a pure note is a very special kind of vibration – it varies sinusoidally (following a smooth sine curve) with time.

First invite pupils to listen to the sounds, then ask them to see, on the screen, what frequencies they are made of.

You might like to draw the parallel with seeing a range of colours and seeing how these are made up from the different spectral colours.

As the different sounds come up, note the different soundprints that could be used to identify the sounds.

In looking at a selection you'll want pupils to notice:

• The balance between low and high frequencies and how this relates to the pitch they hear.
• That you can hear two notes at once, making sense of many different frequencies arriving all at the same time.
• Changes in loudness and how these are recognised.

Such soundprints are the basis of voice recognition techniques.