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Author Topic: Sound created by wind instruments  (Read 2857 times)

Offline Exodus

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Sound created by wind instruments
« on: 18/04/2006 19:01:56 »
Hello all, long time no see!

I have a question;

Why does the length of the 'tube' of wind instruments effect the deepness of the sound that they make AND more importantly, why does opening holes along the tube, i'e the keys, change the note that the instrument produces?

thanks
Rich


 

Offline daveshorts

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Re: Sound created by wind instruments
« Reply #1 on: 18/04/2006 21:36:42 »
The wind instrument is basically a tube with a hole at one end - strangely if you send a sound wave down an open tube it will reflect off the open end - this is a bit like the way that if you hold a piece of rope hanging down and send a wave down it, the wave will reflect off the end, even though the end isn't attached to anything.

Right so the thing you make the noise with - reed, whistle - is making loads of different frequencies these all go down the tube and bounce off the end. Some frequencies coming back up the tube will cancel out the stuff going down the tube - these are inhibited. Other frequencies coming back up will reinforce the ones going down, these will get stronger. The longer the tube the longer it takes for the sound to get to the end and back so the lower the fundamental note.

Another way of thinking about it is imagine the air as water sloshing up and down a bath - the longer the bath the more time it will take for the water to slosh up and down hence the lower the note.

You will find that the other notes that work are 2x, 3x, 4x... the fundamental frequency. Basically if one wave will fit in a space without cancelling out, 2,3 etc waves will also fit in and not cancel out, these other frequencies are called the harmonics or overtones of the note and give it it's shape.

The simpleist thing that opening a hole can do is essentially make the tube shorter, so making the note higher. I think as with most things everything is slightly more complex than this, but this is a start.
 

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Re: Sound created by wind instruments
« Reply #1 on: 18/04/2006 21:36:42 »

 

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