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speaker as a mic?

basscooker

basscooker

Inactive
Commercial User
Apr 11, 2010
5,727
10,063
Northern KY
Disclosures
Cab fan, hobbyist
#1
what's this about? a guy calls me the other day about the jensen mod 8 speaker i have up on CL. i was away from my phone, so he leaves a voice message. he says he's going to make a kick drum mic with it. i've always known that mics and speakers are similar animals, but this one kinda intrigued me. apparently the speaker's design makes little to do with what it can replicate as a mic, vs its frequency response as a speaker? he said it wouldn't matter that it's a guitar speaker and wasn't rated for the frequencies usually applied to subs. this is where i get a little confused. i would see this working really well within a speaker's operating range. BUT i don't see how just reversing the signal can make it duplicate frequencies lower than it was designed for. can anybody shed some light on this for me?
 
#2
it does work, i had a 8" speaker fitted in the back off and old 4x12" unit, the amp was an old 30 watt WEM and the output of the 8" speaker went to the PA systen we use to all plug into. it worked really well.
again that was back in the early 70s.
 
#4
The OP asks an interesting question I've wondered about too. Why does this work - even with seemingly crap speakers?

I'm entirely unqualified to answer the question, but in my head the difference is that instead of the electrical signal flowing into the electromagnet to move the cone, it's the air in front of the speaker moving the cone and generating the electrical signal, so totally different forces at play.

That's as far as I've gotten before my head starts to hurt and I decide to think about something else.

Sorry for wasting your time - ;)
 
#7
par_SKRM-100_Back.jpg

par_SKRM-100_Application.jpg
 
#8
the concept isn't lost on me. i've known it can be done for a long time. my question is concerning a speaker's ability to provide a signal as a mic that falls below the frequency it can deliver as a speaker. was this guy misinformed about using any speaker for this, or should it be a sub (or at least a bass speaker)? he was going to put it in a tom shell, probably in an attempt to make his own version of what seamonkey posted. will a mid-woofer deliver low thump as a mic, even though it won't as a driver?
 
#9
It is all physics. A coil of wire passed through a magnetic field produces voltage. This is the way dynamic mics like the sure sm58 and sm57 work. Usually the diaphram is thin and light making the mic more sensitive. The speaker will work the same way, but will require much more force to move the diaphram/cone assembly. This is bad for vocals and instruments that do not produce high spl accoustically, but for a kick drum this is great as you are not going to tear a hole or over exert the diaphram letting you capture the high spl and lower freq rezponse. You just need a preamp as the voltage produced is lower than a regular mic.
 
#10
Hi.

basscooker said:
my question is concerning a speaker's ability to provide a signal as a mic that falls below the frequency it can deliver as a speaker.
Click to expand...

Every conventional cone speaker with a magnet+coil motor is able to reproduce everything down to the fraction of a herz.

Try it if You don't believe it, but BE VERY CAREFUL not to over excurse the cone, and also closely monitor the temperature of the voice coil.

Controlled 1Hz looks pretty neat, sort of, but it'll cook the VC in no time if there isn't adequate cooling in and around the gap.


As a reproducer in free air, the main determining factors for a drivers capabilities is the stifness of the cone suspension and the thermal rating of the voice coil.
All else is refining, and one can easily make a driver that can reproduce a AF signal from scrap parts and materials, but making it a good one, that's not that easy ;).

As a transducer, there's not many limits other than the rather erratic and shy-on-low-end frequency response due to the relatively stiff suspension, which also leads to rather modest output.


Regards
Sam
 
#13
Two things the speaker/mic has going for it, the sensitivity is not that great, so it will pickup mostly just the kick drum. Secondly the frequency range is very limited which is OK if all you want is a thump with no tonal qualities.
It will be great to reject high pitch feedback, but bad for L.F. droning if used in a P.A.
 
#14
I have no expertise in this whatsoever, yet i am compelled to post my thoughts: :p

That image that seamonkey posted says it all, i think. It depicts an ideal application of using a speaker vs a mic to record ultra-low frequencies such as a kick drum which produces incredibly high SPLs which may damage the super-fine and delicate innerworkings of a dynamic mic.

So basically, the speaker "mic" is just cruising along no matter how agressively the drummer is playing. It seems it would be virtually impossible to damage the speaker mic as there is no physical coupling with the kick drum head.

Very clever of Yamaha to employ this technology. I havent heard what it sounds like, but Im guessing it works rather well since they got past the prototype stage and actually manufactured the device.
 
#15
George Martin & Geoff Emerick did this with the bass track on Paperback Writer. The irony: I believe Ken Townsend was was giving a disciplinary warning for "mis-matching impedances"!
 
#16
Using a speaker as a mic results on a lot of resonance from the momentum of the large heavy cone. On kick drum in particular, the signal is really less a representation of the sound of the drum and more just an impulse response of the speaker, pretty much the same as if you'd tapped it with your finger. It only reacts to close sounds so the feedback rejection is great, but of course there is no high frequency response. They should be thought of more as an effect than a mic.
 
#18
basscooker said:
what's this about? a guy calls me the other day about the jensen mod 8 speaker i have up on CL. i was away from my phone, so he leaves a voice message. he says he's going to make a kick drum mic with it. i've always known that mics and speakers are similar animals, but this one kinda intrigued me. apparently the speaker's design makes little to do with what it can replicate as a mic, vs its frequency response as a speaker? he said it wouldn't matter that it's a guitar speaker and wasn't rated for the frequencies usually applied to subs. this is where i get a little confused. i would see this working really well within a speaker's operating range. BUT i don't see how just reversing the signal can make it duplicate frequencies lower than it was designed for. can anybody shed some light on this for me?
Click to expand...

A speaker can usually reproduce frequencies far outside the rated range, just not as good as the frequencies inside the rated range. A kick drum will create more of a pulse than lots of frequencies, so you could think of the speaker as a microphone that only have to pick up one frequency (Ok, this is very simplified). You can mix in the signal at that one frequency to sound good with the rest of the mix. How the response of the "mic/speaker" is on other frequencies doesn't really matter as they will never be produces by the drum anyway.
 

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