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Helmholtz Resonators


Where does all that bass come from?

Jon Gilchrist (c) 2003

Taken from a posting on the Yahoo Goblet Drumming group and therefore refers to the darbouka and tonbak drums but can also be applied to Djembes and any other goblet-shaped drums

In laymans terms, there are two things going on with goblet drums. First, there is a membrane that vibrates (the head) and causes sounds. In doumbek-speak, this would be your "tek" and "ka" sounds. The bulk of the drum does not come into play in determining how these sound, and that's why they still sound ok when you have the drum setting on the floor with the bell (the opening at the end of the drum) covered up. This sound is determined by a number of things, such as the head material, tightness, size and some others that escape my memory at the moment.

The second thing that goblet drums do is act as a big air spring to cause an air column to vibrate. The bowl of the drum is a big cavity for air, and the neck/body of the drum forms an air column. When the head vibrates, it causes the air in the bowl to expand and contract,
which causes the air in the body to vibrate and cause sound. The relationships are that the bigger the bowl, the more air you have to expand and contract. The end result should be the bigger the bowl the more resonant the drum or the longer your "doum" sound will last. The
body comes into play in that the *smaller* the body diameter, the *lower* the sound will be. This is somewhat opposite to what you would normally think, but you can easily verify it. Play a doum, then stick your arm in the body up against one side. As you play with your
arm in the drum, the doum sound gets lower.

In relation to a Tonbak, they usually have very large bowl volumes (compared to an Alexandria style aluminum doumbek) and very small diameter necks or bodies. I would expect them to be very low and very resonant.

The doum sound should be relatively independant of the head material, though if the head is too tight it can cause a doum sound to be very short because the air doesn't compress much. If your drum sounds "dead", try loosening the head a bit.

I'm working on a spreadsheet that will allow you to enter the dimensions of your drum and it will predict the pitch of the doum sound. So far it is at best an approximation, as it isn't very
accurate for any of the drums I've tried it on. But it is consistant in that it is always off by about the same percentage and in the same direction. If I ever get it to work, I'll post it.

Along these lines, I have a couple projects I've wanted to work on. One is a drum with a body that can be adjusted for length and/or diameter (something like a trombone), and one would be a drum with a body that can have a variable volume. The first one could be done
with PVC pipe. I'm not sure about the second type. You don't need to know any of this stuff to be a decent drummer. But if you understand *how* your drum works, you can sometimes use that knowledge to make your drum do interesting new things.

Ok, enough geek stuff. -Jon

Here's a link to a web site that explains some of how a goblet drum works.