This is my step-by-step guide to DIY contact microphones. There are many other ways to do it, but my goals with this method are simplicity and durability.
What you need:
- Shielded audio cable & plug (Hint: Choose your plug based on what preamp you plan to use. Save some soldering by buying a cable with the plug already attached. )
- 35mm piezo disc “bender” (Hint: Get one with wires attached so you don’t vaporize the plating on the piezo ceramic. I use Murata 7BB-35-3L0.)
- 2x 1.5″ felt discs intended to protect floors from furniture legs. I used two different colors, just for show.
- Soldering iron & solder
- Hot Glue Gun
- Knife / Scissors / Wire-Cutters
Cut a little pizza slice from one disc (and save it for later). Remove a narrow canyon from the other (no need to save).
Stick the pizza to the piezo disc. The missing slice should reveal the solder points and allow the wires to escape.
Solder the cable to the piezo wires:
Center of piezo = center conductor of cable.
Rim of piezo = shield of cable.
Hot-glue the cable to the top of the pizza. Groom the wires so they run along the cable, and return the missing slice. (It helps keep the wires from accidentally touching.)
Fill the canyon with hot glue. It’s a good idea to seal the edges and build up some glue around the cable exit as a strain-relief.
Use a little dot of reusable poster adhesive to stick the mic onto things. (The exposed brass side should always touch the object.)
There are other methods of attaching the mic (clamps, heavy rocks, magnets). The general rule of thumb is more pressure = more loudness & bass.
You Need a Preamp
Contact mics have extremely high electrical impedance, which is not well-matched to normal mic inputs. If you plug these mics directly into a mixer / recorder you’ll get OK signal levels but very little bass, and an exaggerated “screechy” treble. The solution is a buffer preamplifier between the mic and the recorder:
“Alex Rice” preamplifier (DIY balanced 48v phantom powered circuit, with links to commercial derivatives)
Using Contact Mics Right – Richard Mudhar’s DIY preamp circuits
Inexpensive preamp for acoustic guitars (since they frequently use piezo pickups)
Mods and Upgrades
These simple mics have a very high output, but they possess a mid-range “honk” that tends to color all of your recordings. The felt discs will damp some of this natural resonance, but it’s still there.
- Cut the edges off the piezo with metal shears (careful not to crack too much of the ceramic). This will help marginally reduce resonance.
- Glue a plastic disc to the bottom (or try other hard materials). This dramatically flattens the frequency response at the expense of sensitivity. (No free lunch folks!)
- Use a different transducer with less inherent resonance (see below).
The recording above is a comparison of three mics clamped to a piece of metal as I draw circles with a pencil on the surface:
- 35mm bender as shown on this page: Severely lacking in high frequency detail
- Upgraded with acrylic disc attached: Biased toward high frequencies at the expense of lower midrange / bass.
- Humidifier disc: Pretty balanced, although the output is lower than the benders.
Your contact mic will probably pick up stray electromagnetic fields. To prevent it, insulate the top ceramic of your piezo disc with electrical tape. Then add a layer of copper foil tape on top, making sure that the foil touches the brass outer ring. (Solder the foil to the brass in a few places.) This will trap the ghosts before they get into your signal.
Dip the finished mic in Plasti-Dip (or similar liquid rubber coating) to waterproof it. The soft rubber has less effect on the frequency response than I expected, but it will reduce the treble. (It probably won’t make a good hydrophone. You need slightly different construction for efficient underwater recording.)
Interested in recording bridges, buses, or light poles? Embed a strong magnet inside the mic so it sticks to ferrous surfaces without clamps.
The cheap piezo “benders” that I specify are the defacto standard for DIY contact mics, but other options are also worth pursuing:
Not as sensitive, but no resonance to speak of. They’re cheap too, and come with leads attached so you won’t hurt them during soldering. I highly recommend them.
Good inexpensive “turn-key” option, although difficult to mount since they’re so skinny. Inside you’ll find one piezo wafer for each string, mounted in a shielded channel. I tried to shorten mine by removing wafers, but it’s easier to buy a smaller one (for a uke or violin?).
Expensive but sounds better than the simple piezo benders. You can buy it with a preamp (4000 system) or just the pickup (4000PI). After spending the money I don’t think it’s a huge improvement over a humidifier disc for most sources.
They are flexible and non-resonant, but have very low output. Avoid for all but the loudest sources!
I was hoping that these small multi-layered piezo speakers would make convenient contact mics. They’re non-resonant and come with an adhesive back so they’re easy to stick to things. Unfortunately the output is way too low to be useful.
Tim Prebble’s The First Rule of Contact Mic Club
Tim Chilina’s Building Contact Microphones From Piezo Discs