Photo 1 — A dual-source acoustic guitar system comprising an under-saddle piezo pickup, a bridge-plate mounted boundary layer mic, and a signal-mixing preamp. This technology can be readily adapted to acoustic bass guitar.
Photo courtesy of L.R. Baggs.
Now that we’ve explored how acoustic bass guitarists can use piezo pickups to amplify their instruments [“Amplifying an Acoustic Bass Guitar” and “Positioning Piezo Pickups on an Acoustic Bass Guitar”], it’s time to investigate the microphone. A mic can deliver excellent results with an acoustic bass, but to get the most from this technology, there’s a lot we need to understand.
For most musicians, the world of microphones remains complicated and mysterious. When choosing a mic, even vocalists typically rely on recommendations or trial-and-error instead of wading through technical data. It’s the rare instrumentalist who digs deeply into dynamic, condenser, and ribbon models to study their directional characteristics, diaphragm size, pickup orientation, and frequency response. So the mic remains the domain of recording and sound engineers.
Microphone pickup solutions—and here we’re only talking about microphones permanently attached to an instrument—are most often marketed to upright bassists, not those playing flattop bass guitar. One reason for this is the acoustic bass guitar market is small. Perhaps more importantly, upright players make better candidates for a mic-only system because the upright offers a higher output volume than acoustic bass guitar, and it tends to be used in quieter music styles. Acoustic bass guitarists often prefer to use an onboard mic as a secondary or complementary pickup.
Although a mic is more prone to feedback and crosstalk than a piezo or magnetic pickup, it’s capable of delivering a premium acoustic image, so it’s worth investigating. That said, there are very few systems targeted directly at acoustic bass guitarists, so most players either opt for a custom solution or end up adapting a device designed for upright bass or acoustic guitar.
To maximize volume on an acoustic bass guitar, a mic needs to be very close to the soundhole or better yet, located inside the body. But once inside, a mic is at much greater risk of generating low-frequency, boxy feedback. The most common way to fight this is with a high-pass filter, which essentially allows you to cut the lows. This means the higher the stage volume, the more you’ll end up with a beautifully atmospheric upper end of the frequency spectrum. But that’s not quite what you’re after as a bassist, right?
Here, our typical under-saddle piezo pickup—so often criticized for its harsh upper end—comes to mind. Why not mix the sweet airy highs of a mic with the solid lows of a piezo? This kind of thinking brings us systems designed to blend signals from two sources. Using dual-source technology, we’re able to create a composite signal that emphasizes the fortes of each approach.
Photo 2 — A PZM mic designed for acoustic guitar can also work with a flattop bass. Photo courtesy of L.R. Baggs.
Of course, a dual-source system is more expensive than an under-saddle pickup, and whether it’s worth the extra cost to buy and install depends on how often you play your acoustic bass guitar and in what context. One could argue that it’s not worth the trouble for onstage use, and it’s unnecessary in the studio, because there you’ll have access to excellent external microphones. On the other hand, showing up with a good, pre-installed mic can save a lot of expensive studio and soundcheck time.
As with piezo systems, instrument mics vary, both in their construction and how they’re deployed. Putting aside the mic-technology itself for a moment, instrument mics can be categorized as contact, interfacial, or free-floating. A typical free-floating mic can be mounted on a gooseneck and will pick up less mechanical noise, as it’s acoustically decoupled from the body, which—as its name implies—is not the case with a contact mic.
The interfacial version might appear to simply be a mix of the other two categories, but in fact, it uses an interesting acoustic effect that happens at boundary surfaces. Placing a microphone directly on or in very close proximity to a sound-reflecting surface maximizes the sonic pressure, while also reducing inner body resonances. These devices are known as “boundary layer” or “pressure zone” microphones (PZM), and if you’re inclined to dig deeper, the science behind them makes for a very interesting read. L.R. Baggs has two systems that make use of the boundary or pressure zone effect: the Anthem (Photo 1) and Lyric (Photo 2). And as with many electronics these days, the Lyric employs some “black box” electronic filters to avoid natural resonances and feedback.That’s only the tip of the iceberg when it comes to the world of microphone pickups systems. Stay tuned, low-enders.