Another Important Consideration for Multiple Piezos

by Bart on March 26, 2010

A couple of months ago I did a posting titled “Wiring Multiple Piezos Together.”  That post has information on how many piezos you can run to a single output before you start to lose signal strength and fidelity, followed by suggestions on how to wire such hook-ups.  The other day it occured to me that I didn’t address one important aspect of the topic.     

What I neglected to discuss was phase cancellation.  In multi-piezo installations where the piezos are on  independent sounding bodies — say, different bars on a marimba – then phase relationships aren’t an issue. But when you have two or more piezos on the same vibrating body — as, for instance, on a soundboard – it’s likely that the signals coming from the piezos will not be perfectly in phase. Then there will be some cancellation when you join them, resulting in a loss of signal strength. Depending on circumstances, the cancellation could be negligible or serious.  

The rest of this post discusses this issue.  To simplify the discussion, I’ll use the case of two piezos mounted on a soundboard as an example.   

There are two ways cancellation can come about when two piezos are mounted on a soundboard: 

1) Phase reversal.  Something about the way the piezos are positioned or wired could result in the piezos capturing opposite phases of the soundboard’s vibration.   This could happen for any of the following reasons: a) one piezo is positioned on the inside of the soundboard; the other on the outside.   b) One piezo is reverse-wired relative to the other. c) One piezo is attached upside-down relative to the other – that is, the two piezos are  mounted with opposite sides against the soundboard.  In any of  these cases, if the piezos are located reasonably close to each other, then their two signals will be close to 180 degrees out of phase, and serious cancellation will occur.

2) Phase offsetting. If the two piezos are somewhat far apart on the soundboard, then each will, at any given instant, be picking up different phases of the waves as they travel through the soundboard.  The phase difference will most likely be much less than 180 degrees, so cancellation will not be severe.  More interestingly, different partials within the sound (with their different wavelengths in the soundboard) will have different and variable phase relationships at the pickup locations.  Keep in mind that this kind of effect occurs naturally in the acoustic sound coming off the soundboard anyway; it’s part of the sound our ears are used to hearing and not necessarily a bad thing.  In spite of being less efficient, with luck it will contribute to a sound that is warmer and more natural than what a single pickup would capture. 

Practically speaking, what can you do to manage these phasing questions in multiple piezo hookups?

First, regarding the 180 degree cancellation described in #1 above: This is normally an undesireable situation and it’s easily remedied by reconfiguring things so that the two piezos are in phase.  Do this by reversing the phase on one of the pickups. One easy way to do that is simply to turn one of the piezos over so that its other side presses on the soundboard.  Another way to invert the phase of the signal from one of the pickups is to reverse the wires coming from the pickup – take the wire that had been the hot lead and wire it to ground, and take the former ground lead and wire it to the hot terminal.  On the other hand, if you’re interested in a weird hollowed-out sort of sound and are willing to accept a serious loss in signal strength, you might enjoy exploring the possibilities of a deliberately out-of-phase hook up.

Regarding the subtler phase interactions described in #2 above:  In theory you could try to analyize oscillation patterns and their interactions as a way of determinining ideal soundboard locations for two or more piezo pickups. In practice this isn’t very realistic, given the almost infinite complexity of possible tones and all their harmonic and inharmonic partials in interaction. Instead, it’s usually worthwhile to spend a little time in trial-and-error mode, testing different locations for the piezos, including varied distances apart, in search of the most attractive sound. In addition, try reversing the phase relationship by inverting of one of the pickups. , Do this either by turning it upside-down or reversing the output wires as described in the previous paragraph.   

In most cases and for most musical tastes you’ll find that having two or more piezos on the same soundboard can yield a warmer and more satisfying sound than a single piezo would.  An exception is the case where you want a particularly bright and edgy tone with a sharp attack. That’s because the cancellation with multiple piezos happens most in the higher frequencies/shorter wavelenghts (as is the case in the natural acoustic sound as well).  A single piezo, being free of these high frequency cancellation effects, tends to sound brighter.  Also, multiple piezos do a better job of capturing the complexity, variablility and non-static nature of the natural sound. This makes for a softer attack,  and contributes to what the our musical ears hear as a warmer and more alive sort of sound. A single piezo, by contrast, will tend to have a slightly more “clinical” feel to its sound.   

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