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How do we perceive sound?

We hear sounds very nonlinearly.

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These files are recordings of the sound of hairdryers:

Initially, there are 10 hairdryers all running at once. When 5 of them are switched off, the sound energy produced is halved (5÷10), but we barely hear a reduction in noise. Listen to 10 hairdryers followed by 5. (you will need RealPlayer to listen to this sound file)
When 9 of the 10 hairdryers are switched off, we do perceive a difference. Now there is only 10% of the original sound power. Listen to 10 hairdryers followed by 1. (you will need RealPlayer to listen to this sound file)

Because our ears respond so nonlinearly to sound, sound power is measured on a logarithmic scale - the decibel. A reduction to one-tenth of the sound energy is a reduction of 10 decibels (or 10 dB).

The aim of the Silent Aircraft Initiative is to reduce aircraft noise by about 25 dB relative to current aircraft. This means reducing the sound energy to three thousands of its original level. Listen to the sound of a current aircraft, followed by a reduction of that sound by 25 dB. (you will need RealPlayer to listen to this sound file)

Last updated:06/11/06