DRUMMER CAFE

Very, very, very cool.

So here's the scoop. A transducer is something that takes acoustic energy and converts it to electrical energy ... and vice versa. A microphone takes the acoustic and converts it to electrical; a speaker takes the electrical and converts it to acoustic. Both microphones and speakers do this by way of a  diaphragm ... much like our ear drum.

I've seen mics and speakers work in a reverse role before ... but not really to the benefit as Keith has shared.

As far as switching the polarity ... this is just to prevent phase cancellation. If you don't do this, the sound picked up from the mic and the sound picked up by the speaker would cancel each other out. Since you are basically micking the same sound source (ie. kick drum), you've got to elements tranducing the sound ... mic and speaker. By putting one of the elements out of phase with the other ... you prevent phase cancellation.

Phase cancellation is really easy to hear, and I highly recommend that if given the chance, you should listen to it.  As an experiment, you could mic your kick drum (or anything else you want) with two mics ... much like what the engineer did with Keiths kick drum. Monitor the two individual mics on their own tracks, but run them MONO or centered up on the board. You will notice that the sound almost all midrange frequencies; virtually no highs or lows. This is what phase cancellation sounds like ... as if you were in a tunnel or narrow tube.

Typically you can prevent this kind of cancellation by using the 3:1 ratio rule. Whatever the distance the closest mic is from the sound source ... the second mic should be three times that distance away. Obviously you can't really do that in this situation because you are wanting the speaker to pick-up the air movement. When micking different instruments, like one mic per tom tom, the closer the mics are to each other, the closer you will have to place them to their own sound source.

I my opinion, it's always good to check and see if you have phase cancellation when placing two mics on the same sound source ... like top/bottom snare, top/bottom djembe ... or in the case of Keith's cool story!

Thanks for sharing that bro!  

Sorry if I was unclear. The amp was NOT making any sound. Instead, the speaker acted as a diaphragm (sp?). Air moved by my bass drum would move the speaker. The speaker, acting as a tranducer, converted that physical motion into electric "information."  

There was a cable running out of the amplifier (I assume there's some kind of output jack - help me out, those who know more about amps). This cable was plugged into the recording console, where the electric information was converted back to sound - extremely low-end sound.

Bart - can you help me clarify this?

a great and easy example to understand the principle of a speaker acting as a mic is headphones.  plug a pair of headphones (not earbuds) into a mic input on a mixer.  one side will act as a microphone.  its essentially a stripped down, transducer, cable, jack, example of the guitar amp as a mic trick.   ive built kick drum mics out of 2" diameter tv speakers wired directly to an xlr jack.  the main issue with using an amp is whether youre going through the amp portion, or bypassing it.

side note:  only one will work because the headphone cable ends in a tip-ring-sleeve plug.  with one side wired to the tip, the other to the ring, and the ground to the sleeve.  a mic input is just tip-sleeve.  so the ring portion of the headphone plug (either right or left) is not being sent through.

When you get some mp3's, we'd love to hear what it sounds like!

Alright, I apologize upfront about my denseness  , but I still don't understand how things are hooked up here.

Is there a mic on the amp?  If so, is it coming as an input from the mixing board or as an input from the amp itself?

Second, you mentioned that there was a cable running as an output from the amp to the mixing board.  So, in all, you have two cables associated with the amp alone, right (one with the mic and the other running from amp to mixing board)?  If that's the case, how is this managed by the engineer?  How many channels on the mixing board are associated with the kick drum (3?)?

Aaargh......!!!  Sorry guys......  

Okay ...

Forget about the amp being an amp/speaker; think of it being a HUGE microphone. You hook it up to the board just like you would a microphone ... more than likely with a 1/4" cable or XLR ... whatever the amp supports.

A microphone does not have a power source on it's own unless it's a condenser, in which it would receive Phantom power, a small battery inside the mic itself, or a power supply that would come with the mic (ie. tube mic) ... but this is for condenser mics that work VERY differently than dynamic mics, which have a diaphragm. So ... with that out of the way, forget everything I just said.  

You hook the HUGE mic (aka guitar amp) up to the board. You do not turn the amp on. You do nothing but connect it to the board. From the board you do any adjustments ... just like you would on a normal dynamic mic.

Dynamic mics have a real diaphragm, just like a really small speaker cone. This diaphragm moves when acoustic energy (sound) hits it. Sound waves cause the pressure to increase/decrease at the mic diaphragm in the audible range (usually 20-20,000 Hz).  These increases/decreases in the background pressure push/pull on the diaphragm causing it to move.  The diaphragm is connected to a coil of wire, which moves in an external magnetic field supplied by a permanent magnet within the microphone ... as well as in the speaker.

There is a law of physics called Faraday's Law that says that if the magnetic field passing through a loop of wire changes, it induces a voltage drop around that loop.  In the case of the dynamic microphone (and speaker), as the coil moves, causing the magnetic field passing through the coil to change.  This, in turn, induces a voltage.  The microphone converts a pressure fluctuation into a time varying voltage. If a guitar amplifier is turned on, it's doing the opposite; converting varying voltage into pressure fluctuation, making the speaker move ... thus creating acoustic energy or sound.

By leaving the amp/speaker OFF you are turning it into a huge mic because it is not receiving varying voltage information as it normally would ... but is now open to be influenced by surrounding sound waves ... thus the kick drum.

Sorry for the long detailed explanation. I used to teach Music Technology courses at the college level, and got an Associates degree in Electronics before I got into music heavily. I'm glad I did, but it makes me a little ... hmmm ... anal? Anyway, I figure if we are going to explain something, let's do it right!

So ... if you are going to try this, don't turn the amp/speaker on. Just plug it in to the mic-pre or mixing board ... like you would a mic. There's no need to apply Phantom power as this is dynamic mic technology ... not condenser mic technology, which operates like a capacitor. You should not have to rewire anything regarding the amp/speaker. The only thing you may have to do is flip the phase on either the amp/speaker or the mic in the kick drum ... so they don't cancel each other out in the mix.

Incidentally, I keep saying amp/speaker because the music population at large refers to the guitar cabinet as just an amp ... which is really just a small part of the whole picture. The amp (amplifier) powers the speaker, which is made up of the cone (diaphragm), coiled wire, and magnet.

Wow Bart.  For a guy with long hair, you're purdy smart!  lol

Hey, the power is in the hair man!

One thing I didn't mention ... you'll need to use the speaker jack on the cabinet when trying to do all this. You're not going to get good results if you run it through all the circuitry. Most cabinets have a 1/4 inch jack where you physically plug the speaker into the amp. What you'll want to do is bypass the amp and just use the speaker. Perhaps that's what Felix means by "hot wiring".  If you don't have a way to plug JUST THE SPEAKER into the mixing board ... they you'll have to create your own connections.

Just take some alligator clips, wire them up to your 1/4" wire/plug, clip them to the contacts on the speaker, then plug the other end into the snake, board, mic-pre, or whatever you're using to get the sound to tape.

Remember the speaker is just acting like a HUGE mic, that's all. You don't want all the electronics from the amp in series ... so we are bypassing. But like I said, and I'm not a guitar player, but many rigs allow you to unplug the speaker ... so you could power from another amp if you wanted.

Also, I don't claim that this will work or sound good; I've never done it (on purpose that is). I'm only explaining physics behind the how and why it would work.  

First of all, thanks to Bart for helping clarify/explain this concept.

As far as what you listen for, you turn up that channel and see what you've got, just like you would with any mic. In my case, we got TONS of low end. Not as much click or punch as a conventional kick drum, but way more low-end - like that subwoofer bass drum sound that's popular in hiphop. I'm guessing that the size of the speaker influences the sound, and the fact that the speaker wasn't built for its properties as a microphone - it's more like a happy accident that it emphasizes the low end so nicely. Again, just guessing.

For the final mix, we'll blend the more conventional sound of the regular mic in the kick and the sound of this speaker/mic - should be pretty manly. (Ironically, these were fairly light pop tunes, so I doubt the final mixes will be TOO heavy. But they should have a nice thump to them.)

If you're working with an inexperienced or close-minded engineer, don't even suggest this. Lots of engineers I've worked with only believe in ONE way to do anything. This guy was pretty hip, and used lots of room mics, didn't ask me to muffle my kick drum, etc.  Although I was not dazzled with what he accomplished with my snare. We worked on it for a while, using half a dozen drums, and he never really rocked my world, although the drums sounded great in that room.

We recorded on 2" tape, and then transferred it to ProTools. So I'll have more chances to work on the snare sound later on. But we've got a good sonic foundation - the kick and toms KILLED.

hmmm ... (gears turning) ... I might actually build a kick "subwoofer" mic cabinet combining that idea with another ... bear with me here.

build an 18x18 square cabinet about 16" deep with a closed back.  have a 12" (maybe 15?) speaker mounted in the center wired to an xlr jack in back.  on the top and two sides of the cabinet have "flaps" that can fold back and out of the way or forward to essentially extend the speaker cabinet out toward the front of the kick while keeping the speaker far enough away.  this would help isolate the kick sound and focus it in on the speaker thats going to be picking up the sound.   kinda like having a chair in front of the kick and laying a blanket over the chair and drum.  

combine this woofer mic with a batter side mic and youd have all the punch, attack, and tone that you hear when youre playing with the added FAT from the woofer mic.  without phase issues.

(drools)