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Consider a plastic soft drinks bottle. Shake it up so that it fizzes and pressurizes the environment inside the bottle. Tap the side of the bottle, and it'll make a low bing sound.

Now open the bottle and release the pressure, so the inside and outside equalize. Tap the side again - now it makes a hollow thunk.

Why does the bottle resonate like this when it's pressurized, and doesn't when it isn't?

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When it's pressurized, the bottle wall is under tension which stiffens its responses to being vibrated and produces a higher frequency resonance. In addition, when under pressure there are few bubbles contained within the bulk of the fluid to absorb vibrations and damp them out. So, when you release the pressure, the bottle wall relaxes and shifts it resonance to a lower frequency, and all the bubbles swimming around in the fluid damp the vibrations. The result is a low, dull thump when you strike the bottle.

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