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didge 26th July 2010 07:58 PM

Advice needed - using sub as microphone
The question I have is quite unusual and I really hope to connect with
someone who can advise me about the project I am attempting.

First of all, I am a didgeridoo maker and player. If you haven't already heard of it, a didgeridoo is a wind instrument coming from the Aboriginal people of Northern Australia. It is a hollowed log, basically.

What I want to do is use a subwoofer in reverse as a microphone in order to add bass to a 45hz didgeridoo that would be played in front of the subwoofer. I've already done this with a sub mounted on the back of a sympathetically-tuned kick drum. It works great for recording
(unbelievably nice bass, actually) but the feedback loop is severely reinforced by the drumhead and live performances are not really feasible.

What I would like to do as an alternative to the kick drum, is to create the added bass by taking advantage of Helmholtz resonance. My idea is
to mount the subwoofer on the top of a djembe drum (the goblet shaped drum from Africa). The bass tone on a djembe is created by Helmholtz
resonance because of the port on the bottom of the goblet, like a subwoofer enclosure. For this project, the djembe would have no drumhead, just the subwoofer and it would have the right volume and port size to create
Helmholtz bass at 45hz (how to calculate?).

When the 45hz didgeridoo is played against the subwoofer, the vibrations of the woofer should go through the port and oscillate from the
Helmholtz effect.

My main question is whether the Helmholtz oscillations would bounce back up through the drum to the speaker membrane and cause it to vibrate in such a way as to add the right kind of bass going to the input. Normally, if you
wanted to capture the bass tone of a djembe, you would place a regular microphone under the bottom of the drum, so I'm not sure if the sub
mounted on the top is going to receive a boost from the Helmholtz oscillations. And, if it does receive a boost, would it be out of phase or have
some other effect that would make the project unworkable?

More generally, I welcome any suggestions on alternative ways to do this.
I would appreciate any advice. I'm new to this forum and not sure if I've posted to the right sub-forum (no pun intended) and so I will be posting the same question to a couple other sub-forums that seem related.

If anyone is interested, I would love to send an mp3 of a recording I made with the 45hz didgeridoo played against the kick-drum-sympathetic-resonator-sub-mic!


Ferrit37 26th July 2010 10:00 PM

something like this?

Yamaha Gives Low End a "Subkick"

didge 27th July 2010 02:54 AM

Thanks for pointing that out. It is similar to the one I made, but the differences have inspired some brainstorming.

First, I wonder how such a small drum could really be a good way to go. When I made my sub mic, my approach was to get the volume of the drum and head tension to match the 45hz didgeridoo frequency. What I ended up using was a 22" kick drum, with about 4 or 5 inches trimmed off the end to bring the volume to the right amount (using the feedback loop as a tuner).

Second, when I made the sub-mic, I had read somewhere that the maximum amount of bass is focused toward the back of the drum. And so I mounted the subwoofer at the back. The drum has only a front head and is open (with a 15" subwoofer at the back). I wonder why the Yamaha Subkick has a subwoofer inside the drum, right behind the drumhead. The advertisement says the subwoofer is shock-mounted inside the drum shell and that the subwoofer is something like 6.5" and the drumhead is 10".

So this leads me to wonder: Is air able to pass around the subwoofer between the subwoofer and drum shell (10" - 6.5" = 3.5")? Or does the shock mounting completely block passage so that with subwoofer and shock mounting combined, there is a complete "wall" inside the drum splitting it into two separate chambers? I'm now thinking of ways to shock mount the 15" sub inside the 22" drum if I decide to modify my sub-mic set up. Maybe a used tire, assuming that I would want to create that "wall."

All that aside, though, I think I may have found the answer to the feedback problem: using mesh drum heads. The advertisement says: "The Mesh heads, although sonically invisible to those frequencies, add a small amount of sustain to the sound." After reading this, I quickly looked up mesh drum heads and found a great DIY article on how to make mesh drum heads using window screens.

Still though, I'd like to learn about the Helmholtz resonance and whether the djembe idea would work.

Ang 27th July 2010 02:57 AM

I appreciate the urge to reduce feedback, but why does this necessitate different solutions than amplifying output from any other acoustic (and bass heavy) instrument?

Ferrit37 27th July 2010 03:43 AM

Hey didge,
I think the yamaha thing looks like it does because yamaha sell drums :)

here's a link to a diy version
HOME - How to Build a Kick-Drum Mic from a speaker

didge 27th July 2010 03:52 AM

standard solutions
Not sure I understand the question. Until Ferrit pointed out the Yamaha Subkick, I was not aware of any sub-mics designed to be used in live situations. Since the drumhead was causing feedback, I was looking for another way to add bass to the 45hz didgeridoo. On it's own, the didge doesn't have much bass at all--but plenty enough to trigger the very sensitive sub-mic. The point of the sub-mic was not just to amplify, but to add extra bass utilizing sympathetic resonance, giving the didge a sound it does not have on it's own. The total instrument is the combination of didge and sub-mic. The effect is great, but impractical live. So, I figure an alternative way to add extra bass is to utilize helmholtz resonance which would not require a drumhead and hopefully eliminate the feedback problem (but maybe not).

"why does this necessitate different solutions than amplifying output from any other acoustic (and bass heavy) instrument?"

What are the standard solutions to which you are referring? I'm sure I could dress up the sound using electronics. I've been learning sound editing on Ableton live and getting to know compression, equalizer, etc. But the gist of it to me is the elegance of getting that kind of bass without needing to rely so heavily on the electronics (I don't know if that's what you are referring to or not). Plus it turns heads when you're playing a ten foot didge against a kick drum. "What the hell is he doing?"

didge 27th July 2010 04:35 AM

That's a good point. When I was checking out the Subkick, I wondered how much theory Russ Miller applied and how many different placements he experimented with before arriving at his setup.

I looked at that DIY article. Judging by the various setups that work well enough, it seems like the extra bass is mainly a function of the large circumference of the subwoofer itself. I wonder if there would be any benefit of having the subwoofer at the beginning of a pipe that runs about 24.6 feet long--the wavelength corresponding to 45hz. A friend suggested this, but I don't understand enough about sine waves and such to know why it would or wouldn't help the cause.

cbdb 27th July 2010 05:20 AM

I dont get it. There are lots of mics that are flat to 20 hz ( and you can even use the mics proximity effect to get more bass ). Your "speaker mic" is a regular mic with all the top end EQed out of it.

Ferrit37 27th July 2010 06:21 AM

Hey Guys,
A mic can't add frequencies that aren't there, having some kind of resonance system seems a bit hit-n-miss.
a better idea would be to use one of the "missing fundamental" devices Waves MaxxBCL or MaxxBass 107 for instance or sub harmonic synthesiser like DBX120A

davygrvy 27th July 2010 07:35 AM

I'm with cbdb. A hyper-cardioid close up to the end of the didge ought to add plenty of bottom from proximity effect. Also try an AKG D112 or EV RE20

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