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Chatter: Craig Anderton - High Tech Tactics: Taming the Wild Piezo

June 4, 2013
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 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 bump.

 
 Fig. 2—Sonar X2’s Prochannel EQ is using five EQ bands to tame the raw piezo output.

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 a difference.

 
 Fig. 3—Adding multiple notches and a bit of brightness can help tighten up/brighten the final sound.
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