Don’t keep it bottled up

by Stephanie Chasteen on June 26, 2007

If you fill a glass bottle partway with water, and hit it with a spoon, you’ll hear a pitch. If you dump out some of the water, and hit it again, you’ll get a higher pitch. Less water, higher pitch. That’s because the frequency of sound is related to how quickly the sound wave can make a “round trip” through the thing it’s traveling through. If there’s less water, it takes less time for it to make the round trip, and the pitch goes up. (Look up “resonance” if you want to know more.) You can actually make a whole xylophone this way if you tune the bottles just right — go to Phil Tulga’s website for instructions!

But now, instead of hitting the bottles, blow across the top to get a tone (like you used to do to annoy your parents when you had an empty coke bottle). The bottle with less water will now have a lower pitch. That’s because now the sound is traveling through the air in the bottle. The more air, the more time it takes the sound wave to do the round trip, so the lower the pitch.

This type of sound we get from blowing across a bottle is called Helmholtz resonance. Back in the 1800’s Herman von Helmholtz made sets of these “Helmholtz resonators ” (shown here in the picture). Helmholtz Resonators at St. Mary’s College for Women These were the first frequency analyzers! Each bottle will only resonate (make a sound) when the right frequency sound is played into it. So if an orchestra played a chord, Helmholtz could run around and put his ear to his resonators, and determine which notes were present in that sound.

So with the same set of bottles, with decreasing amounts of water, you can play two different sets of notes — one by blowing, which decreases in pitch, and one by hitting the bottles, which increases in pitch!

Addendum: Sarah Tulga, who collaborated with her dad on publishing the very helpfulwebsite with the tunings of the water bottle xylephone, now has her own website on the science of sound.  Check it out at sarahtulga.com.

{ 1 comment }

Elm February 10, 2009 at 11:18 am

Thanks. I was wondering why I got a higher pitch with less water, but it looks like I had not a Helmholtz resonator after all. I’ll try blowing now. Cheers.

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