Can you attach multiple piezo discs to a single contact mic preamplifier?

Introduction

Several people have asked recently whether it is possible to connect several piezo discs in parallel to a single Marshmallow DIY contact mic preamp, as is depicted in the image at the top of this post. Before I get into the details, note that

• doing so will not give you stereo or multichannel audio. All of the discs will be combined into a single mono audio channel. If you want multichannel audio, you will need one preamp per channel.

Having said that, multiple discs mixed to mono could be useful, for instance, if you are trying to record a large instrument; spreading discs out over the instrument might help you capture the sound more evenly. That leads me to the second point:

• multiple discs could lead to phase issues. If the same sound arrives at two discs in-phase or out-of-phase, there could be constructive or destructive interference that could preferentially amplify or attenuate certain frequencies.

However, this is less likely to be an issue than it is with regular microphones, since the speed of sound is typically so much faster in solids than in air, the affected frequencies will be much higher (potentially ultrasonic).

Does it affect the sound?

The more general question is whether the discs will interfere with one another, or with the circuitry, so as to ruin the sound quality. To test this, I started by connecting a single piezo disc to a Marshmallow DIY preamp. The exact circuit I am using is described in a previous post. I used poster putty to stick that piezo disc, considered as a microphone, to another piezo disc considered as a speaker. A photo of this setup is shown in Figure 1(a), and a diagram of it is in another previous blog post. To avoid confusion henceforth, I will refer to the speaker disc simply as a speaker, and when I refer to piezo discs, I am referring to the microphone disc(s) only.

I played white noise out of the speaker and recorded it with the microphone, and plotted the spectrum of that recording. I then connected a second, third, and fourth piezo disc to the preamplifier, in parallel to the first one. The additional discs dangled off the side of the circuit where they would not pick up any sound. The circuit with four discs total is shown in Figure 1(b). For each additional disc, I again played white noise out of the speaker and recorded it only with the first disc. Each time, I plotted the spectrum. All the spectra are shown plotted together in Figure 2.

It can be seen that the shape of each plot is the same, meaning additional discs do not change the overall sound of the first disc. However, each subsequent plot is lower down, meaning that increasing the number of discs does reduces the sensitivity of that first disc.

Does the sensitivity really decrease?

I calculated the average value of each plot between 1 kHz and 10 kHz. The results are in the following table.

N Discs	Avg dB
1	+9.31
2	+3.21
3	-0.509
4	-2.50

Fitting a logarithmic curve to this data shows that the relationship is

G_dB = -20 * log₁₀(N)

where N is the total number of discs and G_dB is the amount of gain in dB by which the output of the first disc will be reduced as a consequence of adding more discs. This equation is interesting, because if we consider the gain in V/V (Volts per Volt) instead of dB, the relationship is

G_V/V = 1/N

This means that in a circuit with 4 discs, each disc's amplitude will be 1/4 of what it would be if it were in the circuit by itself. However, there are also 4 discs whose amplitudes get added together. This means that, ignoring phase issues, the total output of 4 discs in parallel is the same as the output of one disc alone.

What does it sound like?

I took the circuit with 4 discs connected to one preamp, and stuck all of the discs to my guitar, as is shown in Figure 3(a). I strummed the open strings of the guitar twice.

I then carefully disconnected three of the discs from the circuit, as is shown in Figure 3(b). Note that the discs are no longer connected to the preamp even through they are still stuck to the guitar, so the sound is only coming from one disc. I then strummed the guitar two more times. The resulting recording is in Figure 4.

I personally do not hear any difference in sound between the first two strums and the second two.

Conclusion

Yes, multiple piezo discs can be connected to a single preamp. The sensitivity of the discs will self-adjust so that the total sensitivity will be the same regardless of how many discs are attached. There does not appear to be any real downside to attaching multiple discs. Nor does there appear to be any practical limit on how many discs can be attached. I did not mention it, but due to Marshmallow DIY's design, in theory the frequency response overall will actually improve for very low frequencies below say 0.1 Hz when attaching multiple discs. I'm not sure whether there are any other hidden side effects. I will also note that how a given preamp responds to multiple discs will depend on the design of the preamp. These tests were done with Marshmallow DIY. If you are using something else, your mileage may vary.