There are two main ways to record an acoustic guitar: point one or more microphones at it, or use its built-in piezo pickup. The piezo option is very convenient, because you can patch the output directly to a computer’s audio interface.
That’s the good news. The bad news is that by themselves, piezos don’t really sound all that wonderful—but we’ll fix that.
After falling in love with the sound of a J-35, I selected it for recording a track and patched the output into an audio interface (TASCAM’s US-366, which has a high impedance “guitar” input suitable for typical piezo outputs). Unfortunately the warm, rich guitar tone I heard in the room didn’t match what I heard from the piezo—yet I wanted a more “direct” sound than what a mic would produce.
Off to the Lab
I compared the piezo’s response to the response of recording the guitar through a quality condenser mic. Without veering off too far into geek-land, there were four main piezo issues:
The body’s rich low end got lost in translation
An emphasis in the lower midrange created a “muddy” sound
Another emphasis in the mids gave a “honking” quality
The highs sometimes sounded a bit brittle
Equalization to the Rescue
Suitable equalization was the solution. I used the ProChannel QuadCurve EQ (included with Cakewalk SONAR X3 Studio and SONAR X3 Producer), but similar techniques work with other high-quality hardware or software equalizers. (I prefer the QuadCurve because it offers four different EQ “characters,” and some are more suited to particular guitar sounds than others. There’s also a “flyout” spectrum analyzer that can speed up pinpointing specific frequency response problems.)
Fixing the Midrange
First up: Cutting the midrange “honk,” which is the most common complaint about piezos. The easiest way to find where to cut is to set up a stage of equalization with a fairly narrow bandwidth (also called Q) and a significant boost. As you sweep from low to high frequencies, at some frequency the sound “jumps out” and honks excessively. This is where you want to cut.
Sometimes there are additional resonances, which you also need to find and reduce (Fig. 1).
Fig. 1: Equalizing a piezo output often requires cutting at certain frequencies.
The main midrange emphasis was around 700 Hz (the green curve); reducing this cut the honk, while reducing another resonance around 240 Hz (the yellow curve) reduced the mud. Another tweak was offsetting a little bit of emphasis around 100 Hz (the red curve)
The white line shows the overall response curve. Note how reducing the midrange automatically emphasizes the low and high frequencies.
A Tweak for the Lows
In addition to four bands of parametric EQ, the QuadCurve EQ also has a separate highpass filter stage. As the name implies, this passes high frequencies. Setting it to the lowest possible frequency and cutting reduced the bass response below the guitar’s range. This minimized boominess and handling noise (i.e., thumps).
The Final Fix: The Highs
Setting the QuadCurve’s lowpass (LP) filter for a very steep, high frequency rolloff gave a more natural sound by reducing the extreme highs, which a piezo tends to emphasize. However this left the guitar sounding a little duller than I wanted, so a small high-frequency boost around 5 kHz brought out the picking noise and “zing” of the strings.
Note that this uses the QuadCurve’s “E-Type” EQ character (selected by the row of switches toward the left) that emulates a famous analog console’s EQ design. It’s easy enough to try all four characters; one will usually match a particular musical application better than the others.
Variations on a Theme
The equalization curve in Fig. 1 sounded the most natural to me, but one of the beauties of the recording process is that you can create larger-than-life sounds, or customize the tone to fit with music in a particular way. So, I created two more presets in anticipation of future projects.
The “Big and Bright” preset uses a shelving response for the high and low frequencies. A high shelf starts boosting (or cutting) at a particular frequency, and maintains that same amount of boost or cut for higher frequencies; a low shelf works in reverse (Fig. 2). The moderate midrange scoop is not enough to reduce the sound’s “size,” but leaves more room for vocals. This preset uses the QuadCurve’s Hybrid EQ mode, which tends to smooth out the EQ curve somewhat.
Fig. 2: The high-frequency shelf adds brightness, and emphasizes the upper midrange so notes can cut through a crowded track more easily.
A third preset, “Rhythm Articulated,” emphasizes the upper midrange and highs to bring out the sound of finger-picking and note attacks (Fig. 3).
Fig. 3: The “Rhythm Articulated” preset.
This still leaves a little bit of a midrange “hole” for the vocals, but the emphasis is clearly on the high end.
A Touch of Gloss
Note that all three presets enable the Gloss button toward the right. This is a very subtle effect that adds a bit of “air,” and gives the highs a sweeter character when they’re emphasized.
Fun with Downloads!
There are three companion audio examples so you can hear how these presets affect the sound. The first half of each example is the “raw” piezo sound, while the second half features the equalized sound. Also, SONAR users whose program includes the ProChannel can download the three ProChannel EQ presets used to make these tweaks.