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The “Other” Absorber

March 19, 2013

As mentioned in Sound Absorbers’ Dirty Secret, there is another type of absorber, which is particularly well suited for low frequency applications. This is the Helmholtz resonator, named after Hermann von Helmholtz, the fine German fellow who invented the modern version of it. He also gave us lots of devices and discoveries in other areas as well. Quite a renaissance man!

The incarnation of his resonator that is most useful to sound folks is a simple rigid box with a hole in it (known as the “port”). If you built a ported speaker cabinet without cutting holes for the speakers, you would have a Helmholtz resonator.

The absorbers I spoke of previously are called resistive absorbers, where the kinetic energy of air particles in motion is changed into heat by the material resisting their movement. The Helmholtz resonator is what is called a volumetric absorber, where the internal volume interacts with the port to absorb sound around a certain frequency.

This frequency is determined by the relationship of the internal volume of the box to the size of the port.


Forget Shape, Now SIZE Matters
Now we turn the tables around. Since this is a volumetric absorber, what matters most is the internal volume, or size, of the resonator. The shape has virtually no influence on the absorption. Unlike my previous blog post on room shape, where we’re adjusting the dimensions to change the relationship between the room resonances, what we’re interested in here is just the cubic volume of the box.

So this gives us a particularly handy property that makes them easy to implement.

Since shape doesn’t matter, you can make them into any shape you want. That allows you to make them fit into spaces that might otherwise be wasted, like under a console meter bridge, under a desk, or even built into a speaker wall below the speakers. They still work just fine; let your imagination run wild.

Once you figure out the size you want, you adjust the port size to tune it to the desired frequency.


But Wait, There’s More!
The Helmholtz resonator is handy for another reason.

When you get into the low frequencies (below ~300 Hz.), you enter a region where the variations in level between resonant frequencies are usually their greatest. This is because the room resonances are at their fewest, resulting in both large gaps between resonances, as well as resonances piling up around one frequency (see the “Shape Matters” post). This gives you peaks and dips in the response that are responsible for significant coloration of the room’s sound.

So in this region it is actually counterproductive to absorb a broad range of frequencies like the resistive absorbers. A broadband absorber in this region will absorb at the frequency peaks like we want, but it also absorbs the dips we don’t want to make deeper.

This is like a bass tone control that turns EVERYTHING down. But what we really want to do is reduce the peaks while preserving everything else, so that we’re smoothing out the frequency response. This why I have a strong preference to use resonators rather than bass traps in critical listen spaces.

By smoothing out the response in this region, where a lot of a room’s coloration occurs, you’re making critical progress in having a “neutral sound” in your room.


The Light Goes On
So now I bet you’ve made the connection between resonators and the good sound they can provide. If you can identify the frequencies of the troublesome room resonances, you can build resonators tuned to those frequencies to smooth out the response.

And that makes your room sound better.

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