Better Sound from Acoustic Guitar Piezo Pickups
By Anderton |
Better Sound from Acoustic Guitar Piezo Pickups
Don't settle for a less-than-the-best acoustic guitar sound
by Craig Anderton
I just finished a live recording where the player was using an acoustic guitar with a piezo pickup – and every time I hear a piezo pickup, the first thing I want to do is grab a parametric EQ and make it sound like a real guitar! The output of an acoustic piezo pickup doesn’t sound like what you’d hear when listening to a guitar in a room, but at the same time it doesn’t sound like a miked guitar either. In some ways, an acoustic guitar piezo pickup is too accurate because it doesn’t discriminate in what it picks up. Fortunately, properly-applied EQ can tame the piezo pickup’s sound and make it more realistic.
Acoustic Piezo Pickups - Comparing Frequency Responses
The upper plot in Fig. 1 shows a miked acoustic guitar and it’s spectrum, while the middle plot shows the unprocessed piezo pickup’s spectrum.
ig.1: Three spectra from a Gibson J-45 acoustic guitar. The top is the miked sound, the middle is the piezo pickup sound, and the bottom is the piezo pickup sound after being processed by EQ as described later.
In the output of the miked guitar, note the major boost around 165 Hz. This corresponds to the guitar body’s “acoustic filtering.” Virtually any acoustic guitar exhibits a characteristic low-frequency bump, and capturing that bump is part of the sound. You should also note that there is also a slightly higher-frequency dip above this bump.
The acoustic guitar piezo pickup not only misses the bump’s peak, but the frequency response extends much lower, giving a “boomy” sound. Also, the piezo pickup’s high frequencies are more pronounced because piezo pickups tend to have a natural brightness. Finally, in the miked acoustic guitar spectrum, there’s a bit more energy in the upper mids. These differences are why a miked acoustic guitar often “sits” better in a track than one recorded with a piezo pickup. This is because the miked version occupies a narrower part of the frequency spectrum.
You can’t make a piezo pickup sound exactly like a miked acoustic guitar, because the physics of the transducers are so different. However, EQ can tailor the sound of an acoustic guitar’s piezo pickup. (Fig. 2).
Fig. 2: Cakewalk SONAR’s ProChannel EQ is using five EQ bands to tame the raw piezo pickup output.
Here’s what each filter stage is doing:
- Highpass filter: A steep, 30dB/octave slope rolls off lows starting at around 116 Hz.
- Lowpass filter: This reduces highs starting at around 9.3 kHz with a gentler, 18 dB/octave slope.
- Low parametric stage: Boosts at 161 Hz
- Lo Mid parametric stage: Cuts around 460 Hz
- High parametric stage: Lifts the upper mids a bit around 3.1kHz.
Now refer back to Fig. 1, and note how the EQ’ed piezo pickup plot at the bottom is much closer to the miked sound.
VARIATIONS ON A THEME
If you don’t have a miked acoustic guitar sound as a reference for comparison, the EQ settings above are fairly consistent “ballpark” settings. But of course, you don’t have to imitate the miked sound, and you can use EQ to enhance or reduce particular frequencies for specific applications.
As just one example, a guitar might have additional resonances you want to reduce (Fig. 3).
Fig. 3: Note how the Low, Lo Mid, and High Mid settings have been tweaked to affect three specific midrange resonances.
Taken together, these three response dips still reduce the midrange, but do so with more precision.
Another option is wanting a “big” sound to accompany a solo singer, but not overwhelm the vocals (Fig. 4).
Fig. 4: The highs and lows are accented, and the midrange scooped to make space for vocals.
In this curve, the EQ still raises the highs and lows of the acoustic guitar, but doesn’t roll off the highest frequencies to give a bright sound. This also gives a significant low end boost to offer a big, beefy sound. Also note that the high frequency boost extends down into the upper midrange, which makes the highs less brittle by comparison. Meanwhile, the midrange is taken down to carve out additional room for the vocals.
Finally, suppose you want the EQ to support fingerstyle guitar picking and provide a highly articulated sound (Fig. 5).
Fig. 5: This curve provides increased definition.
The major boost in the 2-3 kHz range makes the note articulations really stand out, although there’s still some lower midrange drop to make room for vocals.
Tweaking Acoustic Guitar Piezo Pickups
Hopefully this will inspire you not to accept what comes out of an acoustic guitar piezo pickup, but to tweak it for a more natural sound that’s much more like what we hear from a guitar in a room, or when miked. The results will be much more aesthetically pleasing than the midrangey, “honking” vibe of an unequalized acoustic piezo pickup.
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Craig Anderton is Editorial Director of Harmony Central. He has played on, mixed, or produced over 20 major label releases (as well as mastered over a hundred tracks for various musicians), and written over a thousand articles for magazines like Guitar Player, Keyboard, Sound on Sound (UK), and Sound + Recording (Germany). He has also lectured on technology and the arts in 38 states, 10 countries, and three languages.
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