Tech Tip:Understanding Acoustic Amplification
By Gary Allen and Dan Halberg
Amplifying the acoustic guitar has been an ongoing dilemma for both manufacturers and producers and players alike. In live situations, you may sacrifice a bit of sound quality for tonal consistency. However, in recording situations you may rely on microphones that reproduce the guitar with outstanding fidelity, but are prone to feedback and ambient noise. In this lesson, we review your options and help you decide which is the best option for your needs.
Piezo pickups are the most popular for acoustic-electric guitars. These are also sometimes referred to as "under saddle transducers" since these pickups are typically installed in the bridge, usually directly below the bridge. These pickups use crystals that resonate in response to string vibration. These kinds of pickups are typically ideal for stage situations because of their resistance to feedback problems that manifest in uncontrolled environments (such as a live gig). Although makers such as Fishman, Dean Markley, and L.R. Baggs have been innovative in developing great-sounding piezo pickups, it is still widely viewed that these pickups do not give a clear representation of a guitar's natural sound. Specifically, piezo pickups are often cited as sounding "quacky," which references a characteristic sound resulting from frequency compression. Although great for the stage, it is more likely that a good sound engineer will forego the piezo pickup for a microphone (or several) during a recording session.
Magnetic pickups have been the mainstay for electric and semi-hollowbody guitars from the beginning. However, although some makers experimented with magnetic pickups over the years (e.g., John Lennon's famous slope-shoulder 1962 Gibson J-160E used in Beatles recordings from 1962-64), they never became widely available for acoustic guitars. Nevertheless, there has been a trend over the last decade during which magnetic pickups are becoming a more recognized option for acoustic guitar amplification. This can be largely attributed to the release of the ProMag soundhole pickup line from Dean Markley as well as other manufacturers following Dean Markley's lead. Magnetic pickups are advantageous in that they do not suffer from the "quack" associated with piezo pickups and they tend to offer superior reproduction of midrange tones. Since these pickups are resistant to feedback problems they are ideal for stage situations. Magnetic pickups also typically offer one of the easiest options for retrofitting a pickup to an acoustic guitar that does not already have one installed since most are simply attached to the soundhole. Unfortunately, like piezo pickups, magnetic pickups are not typically used in recording situations because they tend to introduce a pesky 60-cycle hum. This unwanted noise is typically inaudible in a stage mix, but can be much more pronounced in a controlled studio environment. Because magnetic pickups sense the vibration of metal strings, they do not work with nylon strings.
Contact pickups have probably been the most revolutionary development in acoustic amplification techniques. The most vocal proponents of this pickup style are L.R. Baggs and B-Band. These pickups are typically directly attached to multiple points on the underside of the guitar's top, allowing for a more complex, multitonal sound. Evolving from both piezo and microphone technology, contact pickups are highly acclaimed for their ability to reproduce and amplify the acoustic guitar sound while faithfully maintaining its natural timbre. This, of course, offers tremendous advantages in recording situations. Due to their direct contact with the guitar top, contact pickups often register more frequencies in the bass range. Contact pickups usually are not preferred in live gig situations because of their potential for feedback. However, this disadvantage has been mitigated by newer designs and by feedback reduction units.
The condenser microphone is widely considered to be the holy grail of acoustic amplification in professional studios around the world. Many popular models are made by Shure and AKG; however, many quality brands have made inroads into this market. They offer without a doubt the best representation of the guitar's natural complex sound; even when compared to contact pickups. Although some types of condenser microphones can be built directly into the guitar as an internal pickup option, the most commonly used method is to externally record the sound emanating from the guitar using one or more condenser microphones. Debates rage among sound engineers and producers as to the best placement of these microphones to collect both direct and reflected tones. The downside to condenser microphones is their inherent tendency to cause feedback in an uncontrolled environment. Furthermore, they are also a hassle to set up and place, suffer from sound-quality variations in the guitarist's orientation to the microphone, and can magnify sound check difficulties during live situations. Many well-known artists do continue to use condenser microphones in live shows, but they have the benefit of a capable and competent sound crew. For the rest of us, the piezo and magnetic pickups are probably still the easiest option.
As a final note, the most recent trends seem to be moving toward combinations of two or more kinds of pickups, either separately or manufactured into one pickup system. This approach tends to take advantage of each pickup type's best properties while minimizing its worst. Nevertheless, the combinations may yield types that are still best for heavy gigging situations and those that are best for recording and playing smaller venues.
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