The Sound Stretcher - Sourcing Stereo Sound

18 April 2013

Ingredients

An exhibit being built for the
Cambridge Science Centre plays with your perception of where sound is coming from

Sound stretcher
The device being used

It consists of a pair of ear defenders with stretchable vacuum cleaner tubes connected to each earpiece. You get someone to talk into both of the other ends of the tubes and then stretch one or the other.

Result

You should find that if you stretch the right hand tube it sounds like the noise is coming from the left and vice versa.

To illustrate the effect, we've recorded this audio clip.  It works best when you're wearing headphones.  Here's the sound stretcher in action:

Explanation

This works because it confuses one of the major ways you work out where sounds are coming from. If a sound is coming from the left it will reach your left ear before the right, and vice versa. If it is coming from straight ahead it will reach both ears at the same time.

Sound coming from left If the sound is coming from the left it will reach your left ear first
Sound from right If it is coming from the right it will reach the right ear first
Sound from the front If it is coming from straight ahead or behind it will reach both at the same time.

This means that if you wear the ear defenders and both tubes are the same length it sounds like the sounds are coming from in front of you, but if you stretch the right hand tube the sound takes longer to get to your right ear, so it sounds like it is coming from the left.

Hearing through the tubes If both tubes are the same length the sound reaches both ears at the same time.
Listening through stretched tubes if you stretch the right hand tube it takes longer to get to your right ear, so it sounds like it is coming from the left.

This system will only tell you which side of your head sounds are coming from, not whether it is in front or behind, which is why it is sometimes very hard to work out is coming from in front of you or behind you.

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