# From MAKE 30: How Does the “Sound Sucker” Work?

In the current issue of MAKE, Vol 30, we have an easy project by William Gurstelle for building a “sound sucker” using a mug, Jello, and hollow stirring straws. The instructions for making it are shown here (click thumbnail image below).

In the article William says:

Using this “sound sucker” device allows you to experience a curious sensation: it’s as if sound is not only being blocked, but actually sucked away from your ear.

The sound sucker works on a narrow range of frequencies. My testing showed it most effective a few cycles to either side of 660Hz (depending on the amount of gelatin), and the effect is most noticeable in a room with a wide spectrum of ambient noise frequencies.

We don’t know how the sound sucker works. Can you explain this acoustic phenomenon? If you can, post your explanation here.

### 3 Responses to From MAKE 30: How Does the “Sound Sucker” Work?

1. I believe it’s a demonstration of Helmholtz resonance plus baffling/dampening

“A tuned cavity which is reflective can enhance the sound at the resonant frequency, as in a bass-reflex speaker, but if the cavity is covered with absorbing material, it will selectively absorb sound energy at that frequency.”

2. Greg has it right: This is a tuned acoustic absorber, AKA a “Bass Trap.”

http://en.wikipedia.org/wiki/Bass_trap#General_description.E2.80.94types

This tuned absorber is tuned to a very high pitch (660 Hz) because it is not shaped like a Helmholtz resonator. Helmholtz resonators, which are shaped like coke bottles, have a small diameter neck and a large diameter volume on the bottom. These straws are all neck, because they are blocked at the bottom by the Jello(!) That means they are acting like tiny organ pipes. The resonance frequency of pipes that are closed on one end (plugged by Jello) and open on the other is given by the equation: frequency = (speed of sound) / (4*Length of straws), which is ~670 Hz, based on a standard 5” long coffee straw. This is nominally identical to the 660 Hz value listed in the article. The absorption of sound is due to (both thermal relaxation and) friction from the air moving along the huge amount of surface area of many, many straws. This friction removes energy from the sound wave (thereby making it quieter at 670 Hz) and converts the energy to heat.