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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
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
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. One easy way to correct an inverted phase situation is
simply to turn one of the piezos over so that its other
side presses on the soundboard. 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 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.
Bart Hopkin, http://windworld.com
hello,just a comment ,i have found that when using 1 piezo in a guitar tends to be very sensative,is there any way of making them less sensative,or could using more than 1 be the answer,thanks for your time,from peedyboy
p.s-where is the best place to position the piezo in the cigar box?
hello,wes.thanks for your reply,i have used a piece of foam on my piezo,which i wrapped round it,and then added some electricians tape round the foam to secure it,this actually works well to de-sensitize the piezo,it also gives the sound a more bassy feel,rather than the tinny sound you might get,try it and tell me what you think,thanks alot.
p.s i mounted the piezo just under the bridge on the box lid. peedyboy.