Turning a speaker into a microphone

[snaresmacker]

Hey all,

 

I'll like to use a 15" P.A. speaker I've got lying around as a low-frequency microphone (for bass drum) by reversing the polarity of the speaker. I've heard of this technique, and that it works quite well (a la the Yamaha Subkick), but I don't konw exactly what it means to reverse a speaker's polarity (how do I do it??)

 

Thanks!

[mamberg]

to reverse the polarity, you just hook the wires up to the opposite connections.

 

 

but to do this, you would be MUCH better off with a smaller speaker. The 15 is going to have to much mass to give you a good signal.

[snaresmacker]

Oh, ok sweet. How about a fender hotrod 1x12 amp?

[The_Idiom]

 

Originally posted by snaresmacker Oh, ok sweet. How about a fender hotrod 1x12 amp?

 

 

 

think smaller....

 

I beleive its usually a 6 inch speaker that would be used

 

Brandon

[blue2blue]

 

Originally posted by mamberg to reverse the polarity, you just hook the wires up to the opposite connections...

 

 

 

The reason you might want to reverse the polarity of the speaker-turned-mic is if you are also using a beater-side mic (for example, conventional mic on the drummer's side and speaker-mic on the audience side... )*

 

You would do so if you determined that the phase of the signal being picked up by the two mics was more out of phase with each other than in phase. (And, of course, out of phase material will sound gauzey, "phasey," washed out and/or have various kinds of tonal anomalies, ie, comb filter type effects.)

 

Same principle as two-miking a snare (top and bottom).

 

 

If the two mics are exactly (and nothing is) the same distance from a single drumhead but on opposite sides of it, their signals will be 180 degrees out of phase. Reversing the polarity of ONE of the mics will put them back in phase.

 

 

In the real world, no two mics will be exactly equidistant -- and even if they were their respective sound fields would likely be somewhat different, so you would not get a complete cancellation -- but you might well lose a whole lot of "oomph."

 

And, then, many snares and kicks in the studio these days do have both skins on them, so the two mics will be picking up substantially different signals, no matter what the polar phase of the mics is.

 

 

For that reason, when two-miking a drum, it's always best to experiment with different positions and distances, as well as flipping the polarity of one of the mics as you do so.

 

Let your ears be your guide.

 

And when you've got what you want, make a note of how you did it, where the mics were, polarity, etc, to give you a starting place for next time. That said, it's just a starting place. Always use your ears.

 

____________

 

* Some/many engineers will try reversing polarity on individual drum mics if they're getting phase cancellation issues when combining the sounds from all the mics. As a rule of thumb, keeping mics close to their target drums can help minimize phase issues.

 

Many engineers use the so-called 3-to-1 rule to minimize multi-mic phase issues. they try to keep the mic no farther from its target than 1/3 the distance to the next miked drum/sound source.

 

IOW, if you're miking two toms with two mics, and the left mic is, say, 6 " from the right tom -- you'll want it to be no more than 2" away from its own left tom.

 

But, again, that is only a rule of thumb. Every kit miking is different.

[where02190]

I've had very good success with 15" speakers as speaker mics.

 

You need to invert the polarity (connecting the + to XLR pin 3 and the - to XLR pin 2) as the positive polarity of the speaker is created when the speaker moves out. Putting it in front of a kick drum, it would first move in, or negative. Inverting the +/- corrects this issue.

 

If you have another kick drum around, remove one head, and, using small bungee cords, suspend the speaker (facing out) using them wrapped around the lugs. If not, go to your local Home Depot or Lowes, and get a 24" concrete construction tube. They are very inexpensive. Again, use small bungees, cut a couple small holes to hook them into, and suspend the speaker in it. A couple pillows will keep the tube in place.

 

If you have a sleeping bag or packing blanket, toss it over the kick and the speaker mic mount for isolation.

 

Low pass the speaker mic track at around 100hz or so. All you really get from it is low end, and much above 100hz I find yields some major phase anomolies with traditional mics incorporated.

[blue2blue]

Again, and with all due deference to my more experienced friend, it seems to me that the only reason the polarity of the mic/speaker is an issue is in its relation to other mics -- particularly a mic on the opposite side of the skin.

 

 

If there is only one mic on the kick, chances are, reversing polarity will have little, and not entirely predictable effect on that signal's relationship with the signal from other other mics.

 

 

Let's take a real world example with one mic (speaker, conventional, it doesn't matter) on the side away from the drummer.

 

For simplicity's sake, let's say it's a single head kick and there's only one other mic, on the snare, precisely 2 and a half feet from the kick drum center. Let's say the kick's fundamental tone is 110 Hz.

 

We might think, since the snare mic will be picking up the drummer side of the kick, that we'll get the best phase coherence if we reverse polarity of our mic on the opposite side of that skin.

 

Let's also stipulate that that kick mic is really close to the skin. (Okay, I'll admit it, I'm winging this as I go along.)

 

Now, while our mics are looking at opposite sides of the skin -- ONE of them is 2.5 feet away.

 

And THAT is about 1/4 of the wavelength of a 110 Hz wave. (If my math is right; and I had to reteach it to myself just now, so... )

 

So it's 90 degrees out of phase with what a mic would be getting if it was directly in front of the kick skin (drummer side) or 270 degrees out of phase with what the mic in our example above would be picking up.

 

If we reverse the polarity of either of the mics, we've accomplished... essentially nothing.

 

Now, granted, I've picked my example to make the math simple and clear.

 

In the real world, you're going to be worried about more than the 110 Hz fundamental, the distance won't be a precise quarter of a wavelenth of the fundamental, the sound will not radiate from a single point on the kick drum (though the impact sound will but not reverberation which is, after all, the drum effect).

 

And, of course, there will be a number of mics. And some of the mics, like the rack tom mic(s) will tend to 'see' signals from both sides of our kick.

 

 

But I don't think there's much merit to the theoretical notion that because you're miking from the "other" side of the drum head you should reverse polarity on that mic.

 

Like so much else, what happens will depend on complex circumstance.

 

Use your ears.

 

_____________

 

PS... in rereading where's post, I'm wondering if he's suggesting that speakers used as mics will produce an inverted signal when faced with the same wave as a mic.

 

That's beyond my specific knowledge -- but for the reasons cited above, I continue to suggest that reversing polarity will have unpredictable effects and will not necessarily deliver a more phase coherent mix of mic signals. Of course, that's for a single side kick miking.

 

Actually, I'm guessing that if a speaker DOES output inverted polarity (compared to a mic) that you would then NOT invert polarity if you were using a mic on one side of the kick head and a speaker-mic on the other.

 

Damn... it's a more interesting topic than I thought.

[where02190]

 

Again, and with all due deference to my more experienced friend, it seems to me that the only reason the polarity of the mic/speaker is an issue is in its relation to other mics -- particularly a mic on the opposite side of the skin.

 

 

A front mic and a non inverted speaker mic will be out of phase with eachother, since the speaker is facing the kick, and it's first movement will be in, not out, which will result in a negatively polarized waveform with respect to the mic.

 

I've tried it both ways, it definitely works better with the speaker wired out of phase.

[blue2blue]

Well, I for sure agree that phase matters when you're using two mics (of any kind) on either side of a drum skin.

 

Certainly, if you had two identical mics on either side of a drum skin, the most reasonable place to start would be inverting polarity on one of them.

 

 

My comments were actually more directed to a situation where there was only a mic-speaker on a kick and how the distance between that single mic-speaker and, in my example, a snare mic would be far enough that the phase relationship of the mic-speaker (in either polarity vis a vis the snare mic would probably be highly ambiguous.

 

You know... never mind.

 

[where02190]

Since a speaker mic has a very limited frequency response, it being the only mic would be very unlikely.

[snaresmacker]

Holy crap... thanks! Well I did try it, and it did work--quite well. but I haven't really tinkered yet. ...

[wooden]

 

Originally posted by where02190 I've had very good success with 15" speakers as speaker mics. You need to invert the polarity (connecting the + to XLR pin 3 and the - to XLR pin 2) as the positive polarity of the speaker is created when the speaker moves out. Putting it in front of a kick drum, it would first move in, or negative. Inverting the +/- corrects this issue. If you have another kick drum around, remove one head, and, using small bungee cords, suspend the speaker (facing out) using them wrapped around the lugs. If not, go to your local Home Depot or Lowes, and get a 24" concrete construction tube. They are very inexpensive. Again, use small bungees, cut a couple small holes to hook them into, and suspend the speaker in it. A couple pillows will keep the tube in place. If you have a sleeping bag or packing blanket, toss it over the kick and the speaker mic mount for isolation. Low pass the speaker mic track at around 100hz or so. All you really get from it is low end, and much above 100hz I find yields some major phase anomolies with traditional mics incorporated.

 

 

And what do you put in the third xlr pin?

[snaresmacker]

The third is just a ground-wire I believe.

 

WOODEN-- In my attempt to reverse the polarity, I just cut the two wires coming out of the speaker (red and black), stripped, and "twist tied" them back together backwards (red to black and black to red). It was the best I could come up with in my confusion of 2 quarter-inch ins and outs and RCA outs.

 

Yeeeeeah I don't know anything about electronics.

[alcohol]

Why not just switch the polarity on your mic preamp?

[snaresmacker]

I just have a tascam 4-track recorder with built in pre's.

[offramp]

I had a thread about this recently; you can see photos, waveforms, and hear examples of what happened in my case. I called it my "sub-par kick project".

[Vix Bass Paul]

This reminds me of an article about the Beatles in the studio.

They actually did this to record Paul Mc Cartney.

 

http://www.avrev.com/music/revs/beatles/

[where02190]

 

Originally posted by wooden And what do you put in the third xlr pin?

 

 

Pin1 is open, it's only a ground, which is not needed in this circuit.

Pin 2 connects to the speaker minus lead.

Pin 3 connects to the speaker plus lead.

 

 

Why not just switch the polarity on your mic preamp?

 

 

You can do that also, however if you wire the speaker out of phase, then the signal comes to the pre in phase. You can also invert the recorded track as well in your DAW with a plugin. Many ways to skin this cat. I prefer to correct at the source, since the reason it must be inverted is due to the position of the speaker in relation to the source.

[TheDarkBrown]

regarding the polarity thing.. i think i see where he's coming from by reversing it.

 

when you have a "thud" in a song, you want the speaker producing the sound to push or throw the soundwaves out to you.

 

if you keep the polarity the same on the speaker, the speaker will record the thud moving inward, thus reproducing a negative thud, which in turn will cause the speaker playing the recorded sound to move IN rather than OUT. reversing the polarity would fix this issue.

 

but, you dont necessarilly have to due this at the source, it can be done after it's recorded into the daw if thats what youre using.

 

hope i made sense.

 

btw i too have had success recording with a 15" speaker. in it's cabinet even, with a tweeter. a great big 15" pa speaker and cabinet that weighs more than 50lbs slapped right up against the kick with no resonant head.

[toddlans]

Anybody have any thoughts on what kinds of speakers to use? I know we established that a big is ok (15in, etc) and I think the yamaha subkick is only like 6", right? But, what about the material (kevlar, paper, etc.), ohms, and all that? How will different factors affect the tone. I'm going to look for something on ebay probably, just wondering what to look for.

 

Also, WHERE, I do have an extra kick drum around and I'm wondering exactly what you're talking about. So you have your actual kick drum you are using with a resonant head ON, right? And, then the other kick drum right up against it? Or how far away? And then you have the head off of the speaker kick drum on the side closer to drumset right? And then where does the speaker hang exactly, if its on the front lugs it seems like it would be right on front edge of the drum. I'm just a bit unsure of what you meant, but I'd like to know so I can try it when I finally pick up a speaker (I've been meaning to this since last fall, but I keep forgetting about it).

[tedcrilly]

 

Originally posted by TheDarkBrown regarding the polarity thing.. i think i see where he's coming from by reversing it. when you have a "thud" in a song, you want the speaker producing the sound to push or throw the soundwaves out to you. if you keep the polarity the same on the speaker, the speaker will record the thud moving inward, thus reproducing a negative thud, which in turn will cause the speaker playing the recorded sound to move IN rather than OUT. reversing the polarity would fix this issue. but, you dont necessarilly have to due this at the source, it can be done after it's recorded into the daw if thats what youre using. hope i made sense. btw i too have had success recording with a 15" speaker. in it's cabinet even, with a tweeter. a great big 15" pa speaker and cabinet that weighs more than 50lbs slapped right up against the kick with no resonant head.

 

 

Sorry mr. school of earth wind and fire you are completely wrong.

[where02190]

No tedcrilly, the poster is completely right. Speaker polarity is positive when the speaker moves out, negative when it moves in. If the speaker is facing the kick drum forward, then the first motion of the speaker will be in, or negative. Inverting the speaker polarity, or inverting the recorded signal, will correct the polarity to keep in in phase with any conventional mics also facing the same direction.

 

toddlians, I use an old 24" kick drum, with one head on. the other side, I have only the rim, no head (cut off leaving only the rim to allow for mounting the rim properly.

 

I use small bungee cords wrapped around the lugs on this side with no head to hold the speaker in place.

 

The played kick drum has a front head, depending on the style it may or may not have a modest mic hole in the lower left (viewed from the front). I place the speaker mic kick as close as possible, typcially about 4-5" from the played kick, just leaving enough room to sneak a front mic in.

 

I tune the back head on the speaker mic for some resonance usually, unless a very short attack kick is desired, then I use a folded piece of flannel sheet to dampen this head.

[TheDarkBrown]

 

Originally posted by toddlans Anybody have any thoughts on what kinds of speakers to use? I know we established that a big is ok (15in, etc) and I think the yamaha subkick is only like 6", right? But, what about the material (kevlar, paper, etc.), ohms, and all that? How will different factors affect the tone. I'm going to look for something on ebay probably, just wondering what to look for.

 

 

ideally, i would think that you would want something as flimsy as possible. the goal is to get the speaker to move like a microphone. microphones aren't made with stiff diaphragms.

 

also it doesnt necessarilly have to be the cone that is fllimsy. you can get a cheap pioneer speaker with a butle rubber surround that are quite flmisy. you can tell a weak "or flimsy" surround when you push in on the cone (not the dust cap, the cone itself) and the surround starts to crease.

 

cheap home speakers would be your best bet. not heavy duty pa speakers.

[TheDarkBrown]

 

Originally posted by tedcrilly Sorry mr. school of earth wind and fire you are completely wrong.

 

 

thanks for the completely useless post.

 

care to elaborate, or do you just like calling me mr earth wind and fire?

 

did you register just to be able to tell me this? wow.

[where02190]

I've had good luck with some celestian 12's, but also good results with a god knows what 8" I had kicking around. My fav is the original 15" I took out of an Ampeg B-15-N circa 1962. Small magnet, about a 1' voice coil, lots of throw. Yields some major league sub lows.