Fig.1—Three spectra from an acoustic guitar. The top is the miked sound, the middle is the piezo sound, and the bottom is the piezo sound after being processed by EQ.
THERE ARE TWO COMMON ways to capture a mono acoustic
guitar sound. You can use a
mic positioned about 6" to 12"
directly in front of the 14th fret,
and angled so it’s pointed toward
the fretboard between the body’s
edge and the soundhole. Or you
can grab an acoustic-electric outfitted
with a piezo pickup, and
route the signal directly to your
recorder. But you’ve probably
noticed that what comes out of
a piezo doesn’t sound like a
miked guitar. The reason is that,
in some ways, a piezo is too accurate.
However, some strategic
EQ can give the piezo option
more of a miked sound.
In Fig. 1, the upper plot shows
the frequency spectrum of a
miked Gibson J-45 acoustic, while
the middle plot shows the piezo’s
spectrum. The miked output has
a major boost around 165Hz
that corresponds to the body’s
“acoustic filtering.” You’ll find
a characteristic low-frequency
bump when recording virtually
any acoustic guitar, and capturing
that bump is part of the
sound. There’s also a slightly
higher frequency dip above this
The piezo not only misses
the bump’s peak, but the frequency
response extends much
lower, producing a boomy sound.
Also, the piezo’s high frequencies
are more pronounced
because piezos tend to have a
natural brightness. Finally, in
the miked version, there’s a bit
more energy in the upper mids.
These differences are why a
miked guitar often sits better
in a track than one recorded
with a piezo, as the miked version
occupies a narrower part
of the frequency spectrum.
While a piezo can’t be made
to sound exactly like a miked
guitar, Fig. 2 shows how EQ can
tailor a piezo’s sound. The highpass
filter uses a steep, 30dB/
octave slope to roll off lows starting
at around 116Hz, and the
lowpass filter reduces highs starting
at around 9.3kHz with a gentler,
18dB/octave slope. The Low
parametric stage boosts at 161Hz,
the Lo Mid cuts at 460Hz, and
the high section lifts the upper
mids a bit around 3.1kHz. (Note
that in Fig. 1, the EQ’ed piezo
plot at the bottom is much closer
to the miked sound.)
To tighten up the sound further,
try adding multiple notches
to help simulate the effect of
sound bouncing around in the
guitar’s body and all around
the room. For a little more sparkle,
add a subtle high-frequency
boost (Fig. 3).
The results are impressive,
but hearing is believing. Check
out the web example (More
Online), which plays the miked
sound, piezo sound, piezo with
EQ, piezo with EQ and notches,
and, finally, piezo with EQ,
notches, and stereo emulation.
Compare the final version to the
raw piezo, and you’ll hear quite
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