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Contact Mics – Four Channels Good to Go

Posted by Tom Benedict on 05/11/2017

I finally finished installing the other two channels in the impedance-matching preamp box I built back in July for plugging contact mics into my recorder. The amplifiers I used came from Stompville in the UK. They’re compact, low noise, and let plenty of the low frequencies through.

Contact Mic Preamp Box

Side note: I just checked the Stompville web site and they’re running out of the JFETs they used in this design. They’re going to offer a new product using a new JFET, so if you build something like this you may want to order all of yours at once rather than piecemeal the way I did. Otherwise there’s no guarantee of a match between channels.

I also finally packaged my contact mics so they’re not just copper tape wrapped piezo discs like the one I used for my kite line recording. (I eventually broke this contact mic through rough handling.)

Contact Mic on Line

Coming up with a housing for the mics took some thought. You don’t want to do anything that changes the sensitivity or frequency response (an impossible task, but you try to minimize those effects) but you also want a housing that works with how you intend to use the mics.

Most of the time I’ve attached my contacts using double-sided tape or Blue Tak (thanks to Tim Prebble and his excellent post, The First Rule of Contact Mic Club for that tip!) Other times I’ve used big honkin’ magnets (thanks to Richard M and his post on his Megalithia site for that tip!) But sometimes the easiest way to attach a contact mic to a thing is to clamp it.

In all the testing I’ve done with piezo contact mics, the biggest gotcha I’ve seen is that they really don’t like to have pressure applied directly to the top of the piezo element. It introduces a big DC offset that scales with pressure. What this means is that clamping directly to the center of the disc is not just bad for the piezo disc. It’s also bad for anything it’s plugged into. So clamping directly to the disc is a big no-no. (That’s also how I cracked my tape-wrapped contact mic!)

I wanted to find a housing for my contact mics that didn’t change the characteristics of the piezo more than strictly necessary, was made of something a magnet would stick to in case I wanted to use a magnet mount, and let me use spring clamps to attach the mic without applying any pressure to the piezo disc. Here’s what I made:

I started with the piezo disc recommended on the Stompville site, the Murata 7BB-35-3. I got mine from Mouser, but you can get these from a number of sources. They’re unfortunately quite large, with the outside diameter a whopping 35mm.

Murata 7BB-35-3 Size

After a couple of months of poking around and not coming up with much, I started searching on metal cans just to see what was out there. I found that the 1/2 oz screw-top tins used for lip balm were a perfect fit. Rather than caking on the lip balm for a couple of months to get empties, I found a supplier who sold them in bulk for DIY folks like me. (Though I think they meant them for people who make their own lip balm rather than people trying to cram electronics in them.) The Murata piezos fit fine.

It Fits!

One of the things I like about the Stompville preamp is that it uses three wires for the piezo: positive, negative, and ground. The ground is intended to be used as a shield. Since the lip balm tins are actually made of steel, this provided a nice way to provide a continuous shield all the way from the piezo element to the recorder. This seriously helps minimize RF interference. The only catch is that the tins are coated to keep them from rusting when used for lip balm, so I had to sand some clear patches for soldering the shield as well as for making good contact between the container and the lid.

The good thing about this coating is that it keeps the brass disc from shorting out against the shield. No special treatment is required. I glued the discs using the same E6000 silicone I use for mounting microphone capsules to their posts. This probably loses me some high frequencies, and epoxy would probably be a better choice, but I kept the layer as thin as possible so the effect should be minimal.

As it turns out, the same servo grommets I used for providing isolation on the shock-mounted capsule posts for the Alice mics are a really good fit on Mogami lavalier cable. I drilled a hole in the side of each tin to take the cable, installed the grommet, and glued it in place with E6000 as well.

Contact Mic in a Tin

With everything installed, all that was left was to solder all the wires and close it up.

Four Contact Mics Ready to Run

Not quite true. All that was left was to solder all the wires, close it up, and go play.

So why four channels?

Initially I knew I wanted four because my recorder has four inputs. I hate having inputs I can’t use simply because I didn’t plan ahead. But after playing with the two channel version for a while I realized something about contact mics: Depending on the medium, sound may not travel all that far. Having widely spaced contact mics may mean that an activator traveling across the object will move out of the range of one mic before it really starts getting picked up by another. With four contact mics you have all the same choices available that you would setting up the mics for a concert or for field recording. In this case, setting up a mic for hard left, center left, center right, and hard right can fill in the gaps on an object that naturally damps sound, like a chalk board.

Or you can use them to pick up four parts of an object simultaneously, like a coffee maker or a vacuum pump or a helium compressor.

Or maybe I just had four inputs and wanted to stuff them all full of contact mics. Who knows? The cost was minimal, and now I have the option to record four channels of contact mic.

One last thing: In addition to being good mics to have in your arsenal, contact mics are useful for other projects as well. June last year, I wrote an article about a shock mount for my SASS I’d made from a re-purposed multi-rotor anti-vibration mount. I used contact mics to characterize the shock mount and arrive at the -21dB of attenuation figure for that mount.

More recently I’ve been trying to solve a handling noise issue on the Alice mics I’ve been building. Having four channels to work with may finally let me tackle that in a more systematic way, or at least characterize the work I’ve already done.

But I may need some smaller discs to work with. The big Murata discs I got for this project are beasts.


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