What is going on?
This is a simple experiment on vibrations! To keep it simple, the shorter the instrument the higher the sound. Why? As you shorten a sound wave you are effectively increasing the number of times a wave vibrates per second. Inside your straw you had a wave called a standing wave which is similar to the animation below:
Animation Courtesy of Dr Dan Russell, Kettering University
Notice that the particles in the animation move up and down, but they don't travel with the wave itself?
The animation above represents a transverse wave where the material moves up and down only.
Standing waves such as this can be found in the straw tube as you blow the triangular reed that you cut with the scissors. In this wave some of the air was moving rapidly up and down and some locations were not moving at all.
- The locations that were not moving are known as nodes.
- The locations that move the most are known as antinodes.
As you shorten the straw you shorten the wavelength of the wave (i.e. you shorten the frequency of the wave) and so you hear a higher pitch. If you lengthen the straw by joining two straws together, you lengthen the wavelength which lowers the frequency and thereby lowers the pitch.
Having trouble getting the sound?
Unfortunately this is a bit of a practice thing!
Put the points of the straws just behind your lips and gently pull forward whilst you blow. You’ll eventually find the ‘sweet spot’ where you'll get the straw to vibrate. Usually most people blow too hard on their first go.
The thickness of the straw does have an impact – buy the cheaper, thinner straws wherever possible. You can also buy the larger bendy straws and create a slide trombone by joining the two straw types together - kids love it!
How to teach science, the way primary kids love
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