Why your DIY contact mic needs a preamp even if you don't want to amplify the sound
May 5 2023
Introduction
The internet is flooded with tutorials about how to make DIY 'contact mics'. The overwhelming majority of these tell you to solder a piezo disc directly onto a guitar cable, and voila, you are done. Etsy is flooded with people trying to sell you these at a premium. While a kludge like this does respond to sound and could be used, for instance, as strike sensor, the sound quality is exceedingly poor and I would barely classify these as microphones. There are at least three major issues with the use of this type of device as a microphone.
The disc acts as an antenna that picks up electromagnetic radiation present in the environment that can be heard as a dirty sounding 50 or 60 Hz hum or buzzing in the audio,
It is not very sensitive and will not pick up quiet sounds effectively,
There will be an impedance mismatch between the piezo disc and the mixer or recording interface that you plug it in to. This will remove all of the low frequencies from the audio.
The first point I have covered in a separate blog post. Many people erroneously look the second point and think 'I don't need a really sensitive mic, I don't want to amplify the sound, and therefore a piezo disc on a guitar cable is all I need'. Unfortunately, this thinking is exactly wrong. That is because the third point still guarantees that the sound quality will be poor. The remainder of this post focuses on the third point.
Solution
The solution to the third point is that you need a dedicated preamplifier, sometimes referred to as a 'buffer' amplifier or a 'high-Z' amplifier, that was designed specially for contact mics. The preamplifier needs a higher input impedance than regular microphone preamps. This higher impedance fixes the issue of the low frequencies disappearing, so that you can have nice high fidelity sound even in the bass region. This will work even if the preamp is used at unity gain so that it doesn't actually amplify the sound, it will still give you much better bass response.
Proving it with Data
To demonstrate the bass response issue, I built the apparatus depicted in Figure 1.
Figure 1: Apparatus for demonstrating the effect of a dedicated preamp on a contact mic.
My laptop is used to generate white noise. White noise has the special property that it contains all frequencies in equal amounts. This noise goes into an audio interface, and out into a piezo disc considered as a speaker. The white noise causes this disk to vibrate. It is taped back to back with another disc considered as a microphone, using double-sided Scotch tape. The microphone disc picks up the vibration of the speaker disc. The microphone disc is connected to a Marshmallow DIY Contact Mic Preamp.The preamp is configured to have unity gain, so it is not actually amplifying the sound. The output of the preamp goes back into the input of the audio interface, and into my computer. Using software on my computer, I calculate the spectrum of the audio coming back from the microphone. I repeated this 40 times and the average spectrum is plotted in Figure 2.
Figure 2: The spectrum of sound captured by a contact microphone connected to a preamp.
This should not be understood to be the frequency response of the microphone - it is rather the combined frequency response of the microphone, speaker, and audio interface. Note that the plot is approximately flat below 1000 Hz. This indicates that these frequencies are transmitted through the apparatus equally.
Then, for comparison, I very carefully removed the preamp from the circuit. This was done without disturbing the position of the discs or anything else. The resulting apparatus is shown in Figure 3.
Figure 3: Apparatus for measuring the sound transmission of a piezo disc soldered onto a guitar cable with no preamp.
I then repeated the experiment described above, playing white noise through the apparatus, calculating the spectrum, repeating 40 times, and plotting the average. The result is shown in Figure 4.
Figure 4: The spectrum of sound captured by a contact microphone not connected to a preamp.
The two plots are overlaid at the top of this article. Above 1000 Hz, this plot is virtually identical to the previous one. However, below 1000 Hz the plot drops off sharply. This demonstrates that the low frequency sounds are not being transmitted through the apparatus as effectively as before, with lower frequencies being more strongly attenuated. Because the only difference here is that the preamp has been removed, it can be concluded that the presence of the preamp is what allows these sounds to be transmitted.
Notice that 1000 Hz is actually a relatively high pitch; it is roughly 2 octaves above Middle C. Pitches below this are not picked up very well without a dedicated contact mic preamplifier, and lower bass frequencies are picked up very poorly.
Conclusion
If you plan on building a DIY contact mic, you should use a dedicated preamplifier, even if you do not need to amplify the sound. This will prevent your low frequencies from disappearing. If you don't want to bother with the details of building the preamp yourself, Metal Marshmallow now sells an inexpensive, easy to use, DIY contact mic preamp on a circuit board, optionally with a piezo disc and other components to simplify the process.
