building contact microphones from piezo discs

While researching the EM172 capsules, I became intrigued with contact microphones. The concept of a stereo field can be loosely applied to any object that transmits sound, which can change as it propagates thru a solid or liquid object. Thus, the field depends less on physical separation, and more on the nature of the object(s) and the length, path, and nature of propagation. Contact microphones are well suited to capturing this type of sonic information. They are commonly constructed from inexpensive piezo discs, which have a high impedance and require a suitable preamp for an accurate frequency response.

The construction of a microphone from a piezo disc is more or less an art form. The discs are pretty fragile and lack shielding, so are likely not suitable for field work without some modification. However, any construct is closely coupled to the piezo and unquestionably affects the sound it captures. There is lots of information on the internet (videos, blogs, etc) related to building contact mics, but my research more or less produced a haphazard array of materials, layers, methods, and applications. I needed something durable and accurate for field recording. So I decided to build and compare several different options to see just how different they are in terms of the sounds they capture.

I’ve tried various strategies to make the discs more rigid, all of which produced different audio results. Various rigid materials (steel, epoxy, wood) were bonded (epoxy) to a set of disks. For whatever reason, these disks all required more gain from the Scarlett by about 15 – 25 dB (as compared with an unaltered or an unbonded disc). But their signal levels were (mostly) sufficient to produce useful, low-noise recordings. The acoustic properties of the substrate materials (ie the way they “sound” when dropped or struck) do seem to translate to the signal.


 
The steel washer is my favorite material for bonding; it gave the most realistic frequency response (ie what I can hear directly from the object with my ears), with only moderate loss in signal. The piezo with the thick epoxy coating had a nice sound too, but required too much amplification for recording quieter sounds (too much noise); it would be well suited to very loud sounds however, where excessive gain could become an issue. The recordings of the disc with lauan plywood seemed kind of flat to me. I’m not sure why and I may need to make a few more recordings with it; I didn’t test it beyond an initial assessment. On a practical note, the metal washer can be used with a magnet to affix the microphone to a steel object, which can be quite handy when recording a bridge, washing machine, or non-electric fence.

Other discs were either sealed between 2 layers of duct tape or encased in Plasti Dip. While offering less rigidity, this approach still seems to stiffen and protect the discs. Signals from these required very little gain from the Scarlett. Both produced nice recordings that sound slightly different when compared to one another.

There are several different sizes of piezo discs; they also differ by resonant frequency. The mics above were built with the Murata 7BB-35-3. I also tested the 7BB-20-3 and 7BB-41-3, which noticeably differ in frequency response. The 7BB-35-3 is my preferred disc. They can be ordered with or without presoldered leads.

So at this point I plan to include 3 pairs of microphones in my kit. I can use the set with metal washers to record relatively louder sounds, and for general purposes. They’ll likely be my go-to mics for most recording activities. I also plan to have a pair of the duct-tape and Plasti Dip mics for quieter sounds. I’ll have to continue to assess them for a while to see which ones I like better.

The discs are not the only fragile component in the assembly. The lead wires and their solder connections can also be fragile. I used a little hot glue to secure the lead wires to the discs after soldering. A soldering iron with an adjustable temperature is well suited to the piezos. A hot iron can melt thru the ceramic coating, ruining the disc. I set my iron to about 400 C and made nice connections without any problems.

These mics will also require shielding. And even with it, its possible to pick up various interference from radio stations or the touch screen of a mobile device. Disks are coated in Plasti Dip to electrically isolate the piezo from the shield. They are then wrapped in copper tape, and the shield wire from the cable is soldered to the copper. To protect the shielding, the whole thing is coated with Plasti dip one more time.

I’ve included various audio recordings of different objects below. Except where noted, they were all made with the Phantom Piezo Preamp from Stompville. Mono recordings were expanded to 2 channels and normalized in Audacity; they are meant to comprise an initial assessment and allow for qualitative comparison.

Piezo recordings of a blown glass bowl:
with steel washer

with epoxy topcoat

sealed with duct tape

encased in Plasti Dip

with lauan plywood

 
Piezo recordings of a stainless steel bowl:
with steel washer

with epoxy topcoat

sealed with duct tape

encased in Plasti Dip

 
Obligatory recordings of a washing machine:
with steel washer

with epoxy topcoat

sealed with duct tape

encased in Plasti Dip

 
Listening to the trellis on the back deck:
with steel washer

with epoxy topcoat

sealed with duct tape

encased in Plasti Dip

 
How important is a preamp? The Plasti Dip contact mic and the blown glass bowl:
with preamp

without preamp

 
Comparison of piezos of different sizes (no additional modifications):
7BB-41-3 (blown glass bowl)

7BB-35-3 (blown glass bowl)

7BB-20-3 (blown glass bowl)

7BB-41-3 (trellis)

7BB-35-3 (trellis)

 

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