# Sound levels and damage to the ears

It is possible that listening to very loud music over long periods can damage the ears. Playing in a rock group may cause problems of this type.
The level of sound intensity is defined by the decibel. The ratio of the intensities (I/Io) of two sounds is given in decibels (dB) by the formula:

Decibel level = 10 log (I/Io)

The human ear has a threshold of hearing which at 1000 Hz is taken as 10-12 W m-2. The table below gives the decibel values and power densities for a number of different sounds.

 Sound intensity (dB) Power density (Wm-2) Sound intensity (dB) Power density (Wm-2) Threshold of pain 120 1 Quiet car 50 10-7 Riveter 95 3.2x10-3 Quiet radio in a house 40 10-8 Elevated tram 90 10-3 Whisper 20 10-10 Busy street traffic 70 10-5 Rustle of leaves 10 10-11 Ordinary conversation 65 3.2x10-6 Threshold of hearing 0 10-12

Although the noise level produced by the high-power amplifiers of a rock group may be 90 dB or more, a personal stereo may give levels at your ears that can rise to as much as 100 dB, and are therefore even more dangerous.

### Physics and music

Physics has had many impacts on music over the years. Just consider a few interesting effects.

The increase in volume and brilliance of the sound of a violin over the centuries has been due partly to the skill of the violin maker and partly to the much greater understanding of the Physics of the instrument. Those of you who play a brass instrument will know that a much clearer sound can be produced on an instrument without valves. Holes in the tube tend to reduce the clarity of the sound produced.

When you next see a clarinet being played, think about where the sound is coming from. Most comes from the side holes with only a few high frequencies coming from the bell bottom. In fact the instrument acts as a simple diffraction grating with a set of sources. The sound that you hear, if any, depends on where you sit!

The use of instruments such as the sampling piano means that many new and interesting sounds may be created. For example, you may blow across a bottle top or twang a ruler and the sampling piano will record the sound. The 'shape' or quality of this sound is stored digitally and it may then be replayed at the same frequency, or the frequency may be changed, by pressing the keys of the piano, to produce a tune. The development of computers has led to the production of computerised sound, some computers have even been made to sing. One of my favourite pieces of computer music is a recording of the Queen of the Night aria from Mozart's opera The Magic Flute which is sung superbly by a computer in Paris.

Sound and music, of whatever kind and however produced, depend critically on the physics of their sources. Even a full- sized symphony orchestra could be regarded as an experiment in practical physics with some superb results.