SPKR MiK: How to make a microphone from a speaker.

I removed it from a broken multimedia speaker system. It may have been Altec Lansing, or Cambridge Audio, I don't quite remember.

Try searching for 6.5" dvc woofer
google products search turned up a few inexpensive hits.

or try:
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=290-352

Unfortunately there's not really a straight answer.
(sorry- probably more info than you wanted)

most microphones have diaphragms 2" and less, but we're not after "normal" here. The larger diaphragm of a speaker works better to pick up the very low frequencies we're after, but the higer mass, and slower response mean that it also rejects higher frequencies. It is my guess that a cheapo would work better, but still pick one sufficiently large. anything from 4" up to 12" should work, but I'm guessing that a better choice will be a 6.5" or 8" cheapo sub that has a fairly small voice coil -usually given away by the smaller magnet. (think radioshack cheap.)

My best suggestion would be to try a few out and listen to them first before building them into a nice drum shell.
make up something with a mic stand, rubber bands, and paperclips to temporarily hang each speaker in front of your kick or bass cab. Then pick a sacrificial guitar cable, and strip one end to wire to each of the speakers, and plug it into a DI box, to your mixer.
If you have any single cabs to sacrifice, then you could just plug the speaker cab (not the amp!) into the DI or mixer.

hope this helps - and good luck on your project. :)

the preamp should only see the impedance of it's side of the transformer. - so if you use a 600ohm transformer, it should see 600 ohms.
so just pick a transformer with an impedance that matches what your preamp wants to see.

the transformer is only to create a balanced signal. It should be ok without the rest of it.

sorry this reply took so long - I only saw this today. - didn't mean to leave you hanging.

You can easily adjust the impedance and level going into the transformer by adjusting the T-pad. You can have even more control by using an H-pad instead.

and yes, quality transformers do matter. I pulled an audio transformer from a RS ground loop isolator to keep costs down, but still have something that passes for audio quality. - My next version will use a transformer I pulled from sound mixing console.

Usually with this mic I filter out all the high and midrange frequencies, and only use the low end where the standard mic rolls off. this eliminates much of the noise. (oddly, I used the same trick with a noisy piezo pickup on a standup bass, and it worked beautifully.)

I'm planning on writing a very simple "coffee can mic" instructable, with a minimalist approach. I would use materials that I could find around my house as much as possible. (Although the things I can find around my house somewhat resembles the inventory of a fry's electronics, or a Radioshack parts section.)

Here's a schematic I've whipped up in mspaint. (you may need to download it to zoom in to read the text.)
This is the simplest way to connect a speaker to a professional microphone jack. Just connect the speaker leads to pins 2 and 3 of the XLR jack. Pin 1 should be left open, so you don't short the 48V phantom supply coming from the preamps.
For a 1/4" mic, just wire the speaker to the two conductors of a male 1/4" TS (mono) jack.

I hope this helps :)

@ herringgull: you are completely correct. To phase correct the simplified diagram above, the polarity should be reversed.

My original (complex) schematic includes a phase inversion switch, to help correct phase. You may also change the distance between the microphone and the source (e.g. drum head) to adjust phase.

If you connect pin 1 to the chassis in the original circuit, you will need to insulate the 1/4in. jacks from the chassis with grommets, or use jacks with insulated bushings. If you don't, when the phase switch is inverted, the signal will be grounded, and you will get no sound.

-Thanks! I'm quite pleased that this instructable lives on.

the phase switch is between the speaker and the transformer.

I know that switch is way expensive if you buy it new, by itself. I stole mine from a computer printer switch. - read more:

If there is a good thrift store nearby, pick up a used one of these 4 way parallel printer switches - you can usually find them for US$3 to $5 - they have way more poles than you need, but you can just ignore the extras. Just look at the way the wires go from the DB25 connectors to get an idea of the pinout ( i.e., pin 1 on the common plug, and pin 1 on A through D connectors.)

A quick search found this new one:
http://www.buyextras.com/bbimadasw4pc.html

You could also use a 9 pin serial switch, if there are enough poles.

If the local thrift store/ second hand store doesn't have them, perhaps there is a computer recycling store near you, or on eBay?

Both of these are valid points. You would get a little more signal by rigidly mounting the speaker, and a rigid baffle would change the pickup pattern.

I did keep these in mind.
The elastic bungee cords are quite a bit thicker and more sturdy than rubber bands. The mass of the magnet and metal frame of the speaker allow the diaphragm to move nicely. If the speaker coil was very stiff, such as in a high power speaker, a rigid mount would make more sense. However the low power speakers that seem to make better microphones work just fine with a shock mount.
The primary purpose of the shock mount was to reject vibrations picked up through the floor and stand. If you don't worry about these, a rigid mount [i]is[/i] much simpler to implement.

I think I've seen a version with a solid baffle, and they work fine. The reflections and reverb in the room are a natural thing, so I didn't worry about rejecting them. They also tend to be much quieter, and this mic doesn't seem to pick those up. The cone in the front serves to catch the sound, and the magnet and frame in the back seem to reject some rear noise.
If you were using this for live reinforcement, I'd suggest installing some baffle, foam, or other device to reject noise from the crowd, and reduce the chance of feedback. For studio use however, you pick up more room reverb with standard microphones. (like the overheads.)
Many professional mics have vents to allow sound to enter the sides or back of the mic. Some are designed for bidirectional pickup patterns, but others use the length of the ports, with the science of phase cancellation, to shape the pickup pattern into the well known cardioid and hypercardioid patterns.

If you wanted, you could eliminate the drum shell all together. I've seen some mics that are little more than a microphone stand adapter epoxy-glued to the back of the speaker's magnet.

In short, the choice is yours. There are pros and cons to each method. Thanks for pointing them out, I just didn't cover them in this instructable.

Interesting problem. I would guess that the two may be related, and both issues are likely from the same problem.

My first thought is the connection between the XLR ground, and the 1/4" jacks' ground.
The Metal XLRs often connect the shell to ground (pin 1). If you are also using metal 1/4" connectors, then the unbalanced stray signal from the shell is being transferred to the reference of the XLR. (1) Then S2 may be shorting out the signal (2).

The Test: Unscrew either the XLR connector, or both of the 1/4" connectors, and letting them hang (in or out) by their wires, isolate their metal parts from the drum shell.
I'd un-mount the two 1/4" jacks, and let them hang out in a plastic baggie (still connected ) while you test again for the problems.

If this fixes the problem, then I would suggest isolating the 1/4" jacks by using rubber grommets between the 1/4" jacks and the drum shell. I think 3/8" grommets work for Switchcraft style 1/4" jacks. Or you could (dare I say it) use 1/4" jacks that have plastic bushings.

If the test did not fix the problem, my second guess would be the transformer- specifically, make sure that the pin out / connections are correct. A transformer pin out rotated 90 degrees could yield the same results, if the coils are simply conducting the signal across instead of isolating it.

I don't remember where I got mine - I think I pulled them out of some other thing, or got them as part of a "grab bag."

I hope this helps though:

Normalizing jacks are also called "closed circuit" jacks, since it closes the circuit when you remove the plug. I've also seen them called "shunt tip" jacks.

Try Effects Connection , they seem to have odds and ends for this kind of thing.

DigiKey.com part numbers:
SC1089-ND
SC1104-ND
SC1316-ND
-check the website- some part numbers are non-stock, and some have high minimum quantities

Mouser.com has part numbers:
502-12A (switchcraft 12A)
502-112AX (Switchcraft 112AX)
161-0085-E
161-MJ160M-EX

*Switchcraft partnumbers are 12A for open-frame style, and M112A for closed-style (the B versions are stereo/TRS, but you can ignore the other contacts.)
*Neutrik has normalling jacks in stereo/TRS only, part no. NYS232 or NYS234
*Radioshack's part number should be 274-255 - but do check carefully in the store - they're always mixing them up. - I tend to avoid their connectors, they are usually poorly made.

The short answer is: Yes. It would be cheaper.

However, when you want a microphone for a specific purpose and/or of better quality, one must pay to play. Lower price may also mean lower quality.

Sounds like a good project. I've done a whole project on microphones too.
Well, the main types of microphones are Dynamic, Condenser, Carbon, and crystal. (I consider the ribbon mic a type of dynamic mic.) There are others, but not so common: laser, liquid, hot-wire, and possibly others.

The speaker microphone is a dynamic mic.
Your graphite mic (I'd have to look at the design) is probably a carbon mic.
You could also make a crystal microphone from a piezo disk, that is used as a speaker in many cheap toys and buzzers

Wikipedia can give you some ideas, but usually cannot be used as an academic source. You'll want to back up the information with real books.

take a look at the Sure Educational Articles

I hope this helps, let me know how it goes.

Your 8" should work great. I'd say go for it.
The 6.5" that I used was also designed as a subwoofer.

If you're not sure, then test it first... wire it up (since you are doing the simple version) and hang it from a short microphone boom stand with wire or rubberbands, (or just use whatever is handy) and test it out before you mount it in the drum.

The larger diaphragm (cone) of the 8", and its heavier coil will take more air to move, and reject more of the high frequencies. But thats acceptable because we only want the bass frequencies anyway.

The output should still be more than enough for the preamp (If it is too hot, you can wire the speaker to a 1/4" mono plug, and plug it into a regular DI box that has a -20dB pad.)

As I understand it, a woofer is just a speaker that is not a tweeter, so a subwoofer is a woofer that is made for bass response in a multi-speaker system. (for a three-way system, bass to the subwoofer, mids to the midrange woofers, and highs to the tweeters)

So let us know how it turns out. :) I'd be interested in hearing a sample.

You can do a test with some home recording software.

You'll want some software to generate a tone sweep, from low to high, say 5Hz to 20khz. It should last 30seconds to 1 minute (longer is more accurate.)
Alternatively, you could use a hardware function generator, or oscillator, and record a sweep.

Set up a multi track recording session, and put the tone sweep on track 1. Put a good quality speaker in front of your test mic, and make sure you've got enough volume. (test volume level with music or a 1kHz tone)

Set up track 2 to record from the mic.

A sound-proof room is best, or else you can use a quiet living room when everyone else is out of the house. -try not to wake the neighbors.

This should play the tone from track 1 through the speaker, picked up by the mic, and then recorded to track 2.

If your software has a 'Normalize" function, you may want to normalize both tracks individually to the same maximum level (like -3dB or something)

To analyze, find a RTA (real time analyzer) function in the software, and select the entire length of both tracks, this should compare the levels of each track at each frequency.

I've got Adobe Audition - and I know that it does all of these functions, or you may need to use a couple of different programs, like Audacity and Pro Tools, or Cubase, and combine The tone generator of Audacity with the multi track recording of another. (I'm not sure which others have a nice RTA)

hope this helps :)

simply, yes.
You'll want to start with the gain, or trim pot turned all the way down at first, since the signal coming out may be hot.

alternatively, you can solder the speaker directly to a mono (TS) 1/4 inch instrument cable, and plug this into a DI box.

VicFirth writes:
What is the (wattage) rating on the transformers and resistors? (Is there) anything else that has to do with current going through them(?) Does the speaker have to be dual (voice) coil?

Thanks for the question VicFirth.

If you are using it primarily as a microphone, currents will only be in the milliwatts, and low wattage components is acceptable (this is what I used)
The basic 1/4 watt resistors are acceptable, and the transformer specified in the parts list is designed for -10dBV line level signals, (1Volt rms; which is 1mW at 1k ohm load).
If you will be running higher power into the input (i.e. speaker level from an amplifier) for longer periods of time, you may wish to use larger resistors of 1/2 watt or 1 watt capacity.
You will probably not want to have the input signal very high to begin with, as this mic tends to run hot to begin with.
Note: My Whirlwind brand Director DI box uses 1/2 watt resistors in the pad circuit, for speaker level input.
However, if you want to use it mostly as a speaker cab with built-in "power brake", then you may want to start from scratch and build a really beefy circuit.

The speaker does not have to be a dual-coil, but with a single coil speaker, you'll also want to omit the selector switch and damping pot. , and just run the speaker connection straight to the input side of the DI circuit.

CWeeks25 writes:
Yet another question
Sorry to bombard you with questions, but I do have a question about the isolation transformer, as I have never used one of these.
I bought a new one that has a red and black input and red and black out put (with RCA male and female respectively). Can I cut off the the RCA in/out and use the pre-existing wiring or do I need to take it apart and go from there. Thanks yet again. I can't wait to get started.

If you've got your hands on the same isolation transformer I used (Radioshack part 270-054) Then you've got options.
This radioshack part has two transformers inside the big black shrinkwrapped can. One for the red connectors, one for the white connectors. You can take it all apart if you like, to desolder one of the transformers from the little circuit board, or you can use cut the wires, and use it as-is.

The simple way is to cut the wires. Separate the red and white wires (the insulation is stuck together) by just pulling them apart. You'll only need one color/channel, let's pick the white ones. We can bundle up and ignore the red ones.

Cut the white connectors off. You may want to cut the cable shorter too, - I know they come kinda long on one side of the transformer. Just leave enough wire to work with, 6 to 8 inches should be enough.
Strip the insulation back about an inch on both ends, or however far you need. You should find two wires inside the cable. One is the shield, the other we'll call the "hot" wire.

one end is input, the other is output (either one can be in or out. it doesn't matter.)

The input hot wire connects to the ouput center tab of the 20db pad switch in the circuit. The shield of the input wire goes to the ground lift switch, and the ground side of the 10k ohm resistor of the pad.
The ouput hot goes to pin 2 on the XLR mic jack, the ouput shield goes to pin 3 on the XLR mic jack.
The circuit shows a connection to the center, or core, of the transformer. You can just leave that out.

You can secure the transformer can inside with double sided tape, or a ziptie, or however you like.

If you want to take it all apart, you can cut the wrapper off of the can holding the transformers, separate the two halves of the can, remove the circuit board, remove the electrical tape, and desolder one of the transformers. (like I did) -but it's a little more work.

I'm not sure I understand exactly what you are asking, but I'll try.

Usually T and S on a 1/4 jack refer to Tip and Sleeve contacts for a two-conductor, unbalanced jack. The kind used in this project.
The Sleeve is usually the shield, which is almost always the ground.
The Tip is usually the wire that carries the signal, or the "hot".

For reference, a three conductor 1/4 jack is called a TRS jack for Tip, Ring, Sleeve. It is used for balanced signals (hot, cold, and ground) or for Stereo signals, like in headphones (left hot, right hot, and common ground).
Some effects inserts on sound mixing consoles use TRS to send and recieve signal on the tip and ring, while saving space on the connector panel by only using one jack.

CWeeks25 writes:
Another thing...does it really matter the quality of sub used, like will my $11 sub work as well as so a $30? Thanks

A cheap sub ought to work just as well. It may even work better.
More expensive subs are designed to be more rugged and handle the signal from a more powerful amplifier (more watts), so they will have a larger magnet, larger coil, more rigid suspension (from the surround, thicker cone, etc.

When you use it as a microphone, you are trying to pick up sound, so a more expensive woofer with more mass will reject more of the sound- especially high frequencies, because it takes more sound energy to move the speaker cone and coil. The output generated will only be a few tenths of a volt at most, and probably less than a watt, so the large coil will not have much more output than a smaller coil.

You may wish to use a more expensive sub if you want to reject more high frequencies, and have a lower output signal.
A cheaper sub will give a hotter signal and slightly better frequency response. any unwanted frequencies can be cut with the EQ on the sound console, or edited out with an EQ in the recording software. Remember that this microphone is meant to pick up more low frequencies.

The short answer: I would go with the cheaper one. I just used a sub I took out of a broken multimedia subwoofer (the amplifier inside had quit) so it didn't cost me much.