Two lips make sound
Paul plays a straw oboe.
By cutting two lips into the flattened end of a soda straw and blowing with just the right pressure you can make sounds resonate in the straw.
Flatten one end of the soda straw by sticking the end in your mouth, biting down with your teeth and pulling it out. Do this several times to make a nice flexible flat ended straw.
Cut a piece of straw from each side of the flat region making the straw have two lips at the end.
Cut two edges off the flattened end of a straw.
To Do and Notice
Put the straw in your mouth, bite down on it gently with your front teeth just beyond the cut end.
Experiment with blowing hard and soft while biting down hard and soft until you make the straw sing.
What's Going On?
When you blow, a pulse of compressed air flows
down the straw.
The pulse travels down the straw at mach 1, the speed of sound, and
bounces off the distant open end.
When the sound bounces off the open end the
compressed air changes into a low pressure expansion.
When the expanded air reaches the lips they are forced closed then bounce open to admit more air.
Thus the sound bounces back and forth inside the straw and the lips of the straw open and close creating a sound.
If you shorten the straw, the sound takes less time to travel down the straw and back and the frequency of the sound increases. You can show this by cutting off the end of the straw with scissors. As you snip the end off the frequency increases.
You can also find one straw that fits into another to make a longer straw with a lower frequency.
If you make holes in the straw by melting them open with a soldering iron you can make a straw oboe which you can play by covering the holes with your fingers. An uncovered hole acts as an end of the straw.
You can find two straws that fit well together one inside the other and make a multiple-straw oboe which will play really low pitch notes.
If you can find straws that fit inside each other and yet still slide back and forth you can make make a straw oboe trombone.
You can roll up a piece of paper into a cone and tape the cone onto the end of the straw to make a straw oboe with a bell. The bell makes less sound bounce off the end of the straw so that more sound comes out into the air. The straw oboe thus becomes much louder.
A paper cone added to the end of the straw oboe makes it much louder.
Use poser paper to make a larger bell and the straw oboe will become very loud.
A huge bell makes it louder still.
The straw oboe with a bell acts like an old fashioned ear trumpet in reverse. Instead of collecting sound it broadcasts sound.
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Scientific Explorations with Paul Doherty
1 June 2000