How do sound waves get louder when they are bouncing around a resonating chamber, like in a guitar?

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How do sound waves get louder when they are bouncing around a resonating chamber, like in a guitar?

In: Physics

The sounds keep bouncing around in the chamber until they find their way out (the hole in the guitar). Since there is only one way out of the chamber, the soundwaves get aimed in one direction; a person standing in that direction will hear the sound more loudly because the sound is less spread out and more hits them.

It’s like a water hose with an adjustable nozzle, the more narrow you make the stream, the more intense (or loud in the case of sound) the stream will be.

A guitar works very similar to a drum, which you will notice when you slap a guitar – it sounds like a drum. A banjo even has a skin like a drum, and no hole.

The difference is that rather than slapping the drum with sticks, the vibrations from the strings are transferred to the thin wood of the body through a part called the “bridge”, that is the piece of wood at the very bottom of the strings. The resonances inside the body help with absorbing the vibrations from the wood and turning them into sound energy, which is why the shape and material of the guitar is so important for its sound. It does not amplify the sound energy from the string, which don’t produce much sound at all.

The hole serves two purposes: It lets out the sound waves and directs them towards the front. But it also works the same way as a bass-reflex hole in a subwoofer: Air rushing out of the hole creates an underpressure inside, which then sucks air in, creating an overpressure, pushing air out… This effect is known as [Helmholtz resonance](https://en.wikipedia.org/wiki/Helmholtz_resonance), the effect that also makes instruments such as flutes work. In a guitar, they serve the same purpose as in a subwoofer: They allow the guitar to produce very deep sounds with a fairly small body.