Making Contact Microphones for Sonic Experiments!
Have you ever had a birthday card that played a bleepy tune when you opened it? Chances are if you did the “speaker” that made the nose was a piezo transducer. These little brass discs with a couple of wires attached can be found in all manner of objects that make simple beeps and you can buy “piezo discs” online very affordably. Rather wonderfully piezo disks can operate both as a speaker but also can act as a microphone. It’s essentially the same process either way the piezo disc converts vibrations into an electrical signal when it’s a microphone and converts an electrical signal into vibrations (sound waves) when it’s acting as a speaker. They aren’t quite like a regular microphone however and they are often referred to as “contact microphones” as they are better at picking up the vibration of objects or instruments rather than human voices.
It’s really simple to make a piezo contact microphone and it can be done with or without using any soldering. For our first contact microphone we are going to simply use a piezo disc, some crocodile clips and a 1/4” mono jack plug (the sort of plug a guitar lead might have). The piezo disc has a black and a red wire and these are attached to the disc. The discs are a little bit fragile and certainly the joint where the wires connect are prone to breaking, so the first thing we are going to do is to reinforce those joints by putting a blob of hot glue over them to make them more secure.
Next let’s get a crocodile clip and connect it to the black wire and then connect this “ground” connection to the part of the jack plug that connects to the outside of the plug and not the tip.
Next let’s connect the red wire with a crocodile clip to the tip connection of the jack plug. Now this contact microphone is ready to go! To keep things safe we aren’t going to connect our microphones to any mains powered audio equipment and we are going to plug our microphone into a tiny battery powered guitar amp. Start with the volume low and increase it slowly whilst tapping the contact microphone and you should get some sounds! As a more interesting experiment use a small clip to connect the piezo disc to some object of your choosing, springs can make a fabulous science fiction laser shot sound!
Of course this set up isn’t very robust or portable. A good next step can be to remake this microphone but use a guitar lead and remove one of the jack plugs making a note of which of the two internal wires connects to the tip and the sleeve. We can then make the connections using an electrical screw terminal block.
If you have a spare piece of screw terminal block they can be used to make really interesting little “instruments” for contact microphones. Find some stiff wire or some thin metal strips and attach them to the screw terminals and then clip the contact microphone to it. The brilliant thing about these scrap built “Kalimba” is you can alter the tuning of each metal key by shortening or lengthening them using the screw terminals. You can find free apps for smartphones that act as tuners and as such you can make an experimental instrument that plays in tune with other instruments.
We love experimenting with recording sounds from our contact microphones and a good way to collect sound samples is to simply record the output of your battery powered guitar amp using the microphone on your smartphone. However we went a step further and we’ve soldered 2 piezo discs to a stereo mini jack socket to create a pair of contact microphones that we can record using the microphone input on our small dictaphone type digital recorder. You could also do this by adapting the wires to an old or broken set of headphones taking care to connect the red wires of the disc to the separate tips of the headphone jack and you could again use screw terminal blocks if needed.
Having a portable way to use these contact microphones really opens up a weird world of listening! We’ve used ours to record the sounds of stones being slid across a frozen lake or recording spooky creaking gates we found out on a walk.
Learn More
Making a contact microphone
https://www.youtube.com/watch?v=MpjbvSwBUvs
Make a contact mic
https://www.instructables.com/Make-a-Contact-Microphone/
What is a contact microphone
https://en.wikipedia.org/wiki/Contact_microphone
