DRUMMER CAFE

there is little difference between a mic and a speaker..  there's a fixed magnet, a coil which moves within the fixed magnet's field.. and a cone (very simplified)  when an change in voltage is applied to a coil...  it changes the magnetic field of the coil, therefore producing motion...  which moves the speaker.

conversely....  when a cone moves a coil through a magnetic field... it produces a change in voltage....

not only should the amplifier be off.... but the speaker should not be connected to the internal amplifier.  Instead...  wire it to a jack (1/4" probably) straight to an input on the board...  the large speaker will resonate at low frequencies... and produce low frequencies...  the only problems I can see...  are....

an impedance mismatch...  8 ohms is not typically what a board wants to be plugged into...  perhaps  you'd be better with a custom made matching transformer...

and second...  a lack of shielding..  causing buzzes and pops...  one would think  that a sound man or a crossover...  would be able to squelch all of that

Trouble is, there are many different ways to mic a kit properly.
Also, you see a lot of engineers mic a kit in a usual fashion, using the 'usual' mics, but the drums still sound bad. So, mic choice and positioning are very important, but there are other factors at work I think.

Yamaha makes one of these...I think they call it a sub-kick. It looks like a snare drum mounted vertically in front of the front head although I gather that inside there is a huge speaker (15"?) that can then be plugged into an amp or PA or recording console.

Judging by the discussion, anyone could make this out of an old speaker lying around. I think I might try it when we next record.

Seems to me that if you played a big bass drum (24" or better) you could get some serious low end from a big "mic"

Yamaha makes a manufactured mic ( I believe they call it a sub enhancer or something) which is basically just a 6.5 inch speaker that mounts facing your reso head.  You are supposed to use it in addition to another mike.  Of course, much more expensive than just buying a cheap speaker and rigging it yourself.

Here's a link to Bart's review of it.
http://community.drummercafe.com/index.php?action=loadpage&subdir=reviews&content=yamaha_skrm100.htm

Edit:  Yeah, I missed page 2 before I replied as well, sorry for the old news.

Ya know, Yamaha MAKES a transducer mic specifically for this purpose.  Looks like a little snare drum, set up right in front of the kick.  It's in all the drum magazines now.

We do, as it was mentioned a couple times already in this thread.   I highly recommend reading an entire thread before replying so you can get a feel for the discussion and whats been said.  



You might also note the "quote" and "modify" buttons on the top of posts (others and yours respectively).  "quote" will allow you to quote a previous post so replies have some context, and "modify" will allow you to edit a post by making necessary changes or adding on forgotten thoughts.

Yes, we know. I posted this thread nearly 2 years ago - probably a year before Yamaha introduced this.

I just saw this thread for the first time.  

Lots of great info, and a very interesting discussion.  there are a few things, however, that I'd like to clarify.  It took me quite a while to write this, even so, it may ramble a bit.  Hopefully it will all make sense.
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First of all:  As several people mentioned,  a speaker is exactly the same as a dynamic mic.  However, this would be a dynamic OMNI mic, not a cardioid, and it would have an unbalanced output.  Both of these things need to be addressed:

REGARDING THE PICK-UP PATTERN:

I haven't seen the new Yamaha "sub" mic up close, but my guess is that is is optimally mounted, with side porting, to have a cardioid pattern.  That's probably a better choice when mic'ing a full kit, due to leakage considerations.

Oddly, from pictures it looks like the Yamaha is "simply" a round, sealed speaker cabinet, which should give it an omni pickup pattern. Thus, it would also pickup output from the bass amp, lower toms, etc.  That's odd. I'll have to take a closer look one of these days.

One advantage of an omni design would be that LF pickup remains constant no matter how far you get from the source. With cardioids, you must be careful not to get too far away, as cardioids will start dumping LF after a few feet. A very interesting trade-off in this case, and something to consider if you are "rolling your own."

another oddity:  With no side-ports, there will be a lot of proximity effect. If you get really clsoe to the kick, this proximity effect could actually give too MUCH bottom, and that bottom becomes very muddy.  The guitar-speaker trick works because that speaker is in "open air" and thus has no proximity effect. Thus, the large cone gives lots of LF, without proximity muddiness.
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REGARDING THE UNBALANCED OUTPUT

You should probably run the output of that speaker through a direct-box, not just straight into the board.  This is because the guitar-amp's speaker is unbalanced.  It might work OK, but over long runs you could pick up a ton of noise.  

Also, it may not be the right impedance to drive long lines without LF loss.  
-I don't actually know.

BG wrote, ".... 8 ohms is not typically what a board wants to be plugged into...  perhaps  you'd be better with a custom made matching transformer..."    

A typical speaker has an internal impedance of 8 ohms, true, but that doesn't mean that it would send an 8-ohm output down the line.  If it did, that would be great! (too low is much better than too high)     My guess is that the actual output is quite high, as with an unbalanced mic, and thus you definitely need to worry about LF losses.  Use a REALLY good direct box or transformer, as small trannies also dump LF.  

Bart:  "Balanced 1/4 inch mic inputs are not as common, obviously, but they are out there. Typically if you want a balanced mic input, you have to go with XLR inputs. "

-Again Bart raises an important issue that need further clarification:  The connector often does indicate whether you have a balanced source or not, but not always, and it is certainly not the determining factor.  ( I have seen plenty of XLR jacks hooked-up to unbalanced circuits, so don't ever just assume!  A trs 1/4" jack can be either mono-balanced or dual-unbalanced. )  A balanced circuit has to send the signal down two paths, one being electronically (180 degrees) out of phase with the other.  This can be done electronically, or with a transformer.

Regardless of all that, your speaker output will DEFINITELY be unbalanced. therefore, as mentioned above, use a direct-box or large transformer for best results.
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ON HOW IT WORKS:

MR A wrote:  " Air moved by my bass drum would move the speaker."
And RLRR wrote:  "Actually, I think air movement *is* sound."

First,  Sound is the sympathetic vibration of billions of air molecules.  Air movement is the movement of those molecules from one place to another.  These are definitely not the same.

However, What is interesting is that in this case, as Mr A brought up,  BOTH sound and air movement are probably contributing to the overall output of the speaker into the console.   You could move the speaker off to the side, so no air actually hit it, and it would still output lots of low frequencies, but it's quite possible that in this case the actual air pressure also contributes. Surely it must have SOME effect, and perhaps that's the secret of this whole trick.   (or maybe not)

When an engineer puts a U47 outside a kick, it is always with LOTS of popper-stoppers or a thick foam cover, but that is primarily to protect the diaphram from damage.  You're not going to damage a 12" speaker.   I would like to mount a few panels of nylon mesh in front of the speaker, to see how much affect the actual air movement has vs the sound waves.  One could then add / remove panels of mesh to get the most optimum balance.   Heavy.

RLRR:  "The reason why it picked up the low frequencies and didn't pick up any highs probably has to do with the natural frequency of the speaker. Larger diameters have larger natural frequencies."

With this I agree, sort of.  The heavy 12" speaker would simply resist mid and HF energy due to it's mass, thus sounding "bass heavy."  Realize, though, that even a 1/2" diaphram can easily transmit 20 hertz, so it's not a resonant-fequency issue.  (well, maybe just a little bit.)
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HOW TO MIX IT IN:

Bart wrote:  "switching the polarity ... 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"

This deserves some clarification:  

Bart is of course absolutely correct about the problem, and if you go back and read his early posts he explains it in some detail.  However, simply switching the electrical polarity of the speaker will not likely give optimum results in this particular case.  Switching polarity is the equivalent of a 180 degree mechanical shift, at all frequencies. This is possibly better than nothing if you are recording analog.  -I say "possibly" because a 22" kick drum's fundamental frequency is around 24 feet long.  Your speaker-mic would have to be 12' away for the electrical flip to perfectly compensate.

If you are recording digitally, you will likely do better to mechanically shift the second track "into alignment" afterwards.  Even this doesn't work for all of the upper harmonics, it only aligns  the fundamental frequency. but that's a seriously-heavy discussion best left for another time.  You could certainly try both methods and see which works best in your particualr situation.

There is also a small analog box made by some company (IBP, I think?) that claims to be able to align the phase of two mics in real time, and for all frequencies.

I readi the manufacturer's explanation once, and thought that it was a load of cow dung, but you may want to investigate further as I am not 100% sure.    To the best of my knowledge no such device exists as a plugin, and probably because it can't actually ever be done.    Again, IMO the entire concept is manure, but who knows......

So I run the cable from the amp's input into the mixing board's input or do I need to run it from a line out or headphone jack to the board?

Mr. A, I don't know if this has already been posted, but do you have any sound clips of that kick drum when it is miked up like that?

I would love to hear it!