Helmholtz Sound Absorption Demo

Helmholtz resonators can be used in acoustics to selectively absorb sound at a specific frequency, but DIY efforts often fail to achieve the intended results.

These failures could be due to several causes, such as tuning for the wrong frequency or building a device that resonates at the right frequency but which does not absorb effectively. This is unfortunate, because there is a lot of interest in using Helmholtz resonance to address low-frequency room modes (annoying resonances at a specific frequency caused by the room dimensions).

To better understand how and when absorption can occur, I did a simple experiment with a 500ml soda bottle (a well-known example of a Helmholtz resonator). The bottle resonates at around 200Hz if you blow on it, and the tests involved adding progressively more damping (in the form of cotton balls) to the bottle while flooding the room with a 200Hz test tone.

I used a measurement microphone to assess the impact of the "resonator" on the adjacent sound field, and the results were pretty clear. Without any absorbent damping, the "naked" resonator tends to amplify the local sound rather than absorbing.

After adding five cotton balls to the empty bottle, the local sound level actually increased a bit. This might have something to do with impedance matching, or it could just reflect slight differences in the position of the bottle mouth relative to the mic (I held the bottle by hand).

However, as more cotton balls were added there was a clear trend in the behaviour away from amplification and into absorption. The peak absorption occurred with 35 cotton balls added to the bottle, after which the absorption started to decrease with more balls. There is a small shift in the resonant frequency as more damping is added, but this does not significantly impact the result.

There are many web-based tools that can calculate the geometric parameters needed for a Helmholtz resonator to target a specific frequency, but to be useful for noise control or as a bass trap the device has to absorb. This demo shows that tuning the absorption is as important as tuning the frequency, and may explain why DIY efforts sometimes fail.

This is probably one of the most boring YouTube videos in history, unless you are somebody who has tried to used an online calculator to build a Helmholtz resonator and wondered why it didn't do anything.