Working with a sound's magnitude instead of amplitude I'm working on a project, where we're recording sound with a piezo-disc which looks a little something like this:

Now, unless we're doing something horribly horribly wrong, I've discovered that we're only recording the magnitude of sound, since a piezo-disc emits a response to pressure, regardless of direction.
This is a sound-recognition project and previously, we'd been working with smartphone microphones. Back then, we could MFCC to perform feature extraction, yet now it looks like some things will break:

I fear that the pre-emphasis step in particular might lead to faulty results.
What kind of an approach could we follow?

Is there a way to &quot;guesstimate&quot; the amplitude from the magnitude?
Assuming that I do skip pre-emphasis, I don't expect the Fourier transform of the magnitude over time to differ from that of the Fourier transform of the amplitude over time. But am I better of skipping it, or fine-tuning it?
I should just ignore all of these concerns and just continue to take MFCC's as usual.

Any other ideas?
There is no reason why your piezo shouldn't be able to produce a bipolar output, if you use proper biasing and/or preamp. See for example https://www.homemade-circuits.com/diy-contact-mic-circuit/
Working with the magnitude only is probably a non-starter. $y = |x|$ is a highly non-linear operation and will dramatically change your spectrum. For example a sine wave at $f_0$ has just a single spectral line. The spectrum of the magnitude has a strong DC bias, lots of even harmonics $2f_0, 4f_0 ...$ but no content at $f_0$ at all anymore