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I was at a jazz concert tonight and the kick drum was facing the audience. It had a circular "window" on the lower right (from the audience's point of view), with a microphone in front of it. From where I was sitting, I couldn't tell if there was an actual hole there or some clear material. Or maybe even it just looked clear.

My guess is that it was a hole, and somehow that makes it sound better for the microphone. But then what is the rest of the membrane on that side for? Just looks? I always assumed the membrane on the opposite side from the head served some sonic purpose.

It was a Yamaha, I think, if that makes a difference.

Elements in Space
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Mark Foskey
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The question is more what the purpose of the second membrane with the hole in it is. It turns the drum into a Helmholtz resonator and increases the emitted sound energy. Sound waves travel by alternating in energy expressed in pressure differentials and energy expressed in the momentum of moving air. A drum membrane offers a lot more resistance to movement than air does which allows somewhat efficient transfer of energy from the beater to the membrane. The membrane then emits some amount of that energy through vibration and transfer to the drum body. When the drum is open, there is comparatively little pressure change contributing to a low-frequency sound wave, when the drum is closed, the pressure change is large but not allowed to escape.

Giving the drum a port hole both allows for significant pressure buildup and a resulting large escape velocity through the port hole, making for a quite more defined thump and quite faster decay of the drum membrane vibration. For a sound composition that is geared towards feeling the bass kick more than hearing it and delegating its contribution mainly to the low frequency spectrum, that is helpful.

Of course, ;ocations near the port hole also offer a high signal to noise ratio for highly resilient microphones, with a large focus on the low frequency thump and comparatively modest pickup of the subsequent membrane vibration.

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    I'd never heard of a ported kick drum being described as a helmholtz resonator, which intrigued me. It seems that I can't calculate the theoretical resonant frequency or bandwidth for this resonator because of how shallow the port is. Do you have any empirical data for this? – Edward Feb 18 '23 at 17:21
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You can have a bass drum with a complete rear head, with a hole in the rear head or with no rear head at all. In the latter two cases, there's access to insert a cushion or blanket. They all sound different, from the orchestral bass drum 'boom' to the drier 'thud' favoured by rock players.

Here's a pretty comprehensive run-down of the options.

https://drumheadauthority.com/articles/bass-drum-hole/

A hole lets you get a mic inside the shell. Here's more than you wanted to know about micing bass drums :-) It seems that the idea often isn't to mic the hole as a sound port, but use it as access to get a mic closer to the beater point.

Laurence
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  • “It seems that the idea often isn't to treat the hole as a sound port, but as access to get a mic closer to the beater point.” - only people who don’t understand kick drums think it’s mainly about the mic. It is not mainly about the mic. – Todd Wilcox Feb 18 '23 at 13:31
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    It's about both. If it was all about the 'live' sound of the kick drum we'd use a stereo pair a few feet back. But we prefer to close-mic, sometimes focusing on the strike point, sometimes on the rear head. But not particularly, it seems on the puff of air coming out of the actual hole. – Laurence Feb 18 '23 at 13:54
  • I think I misunderstood one of your sentences. I thought you were saying the main reason to cut a hole is for miking, but that’s not your point, is it? – Todd Wilcox Feb 18 '23 at 14:00
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A drum, when hit on one side, produces a pressure wave within. When there's a smallish hole on the other side, the air will be concentrated out of that hole. So that's a good place to point a mic, but were the whole resonator head missing, there would not be that concentration of air pressure, so the sound would be weakened, at least as far as a mic in the hole is concerned.

Tim
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