This article is an excerpt from the following book: The Home Studio Guide To Microphones.
The acoustic guitar puts out a surprisingly broad range of frequencies, from under 100Hz to harmonics extending beyond the human hearing range. Low frequencies come from the resonance of the largest strings, while the plucking or picking action itself generates the shorter-lived high frequencies. Most sound radiates off the face of the guitar, with the soundhole contributing to the bass frequencies.
Over the years, engineers have established many common miking techniques for this popular instrument. Both condenser and dynamic mics will work on acoustic guitar, though the brightest, most detailed sound comes from the condenser mic. Both large-and small-diaphragm condensers will capture great sound from acoustic guitar. When picked hard, the acoustic guitar puts out a sharp transient considerably louder than the sustain portion of the waveform. This percussive sound may not record easily or sit well in the mix. Try recording picked acoustic guitar lines (such as leads) with a dynamic mic to reduce these transients for a more even, consistent sound.
Mic placement depends on the instrument itself, the recording space, whether the guitar is being picked or strummed and the musical style. Perhaps the most versatile placement for one mic is one to two feet from the guitar, pointing at a spot midway between the soundhole and where the neck meets the body. With a directional mic, adjust distance to strike a balance between room ambience and proximity effect. With an omni mic, you should be able to place the mic closer and still achieve a balanced sound.
Placing a mic (especially a directional one) close to and directly in front of the soundhole often creates a dark, boomy sound. Likewise, placement further up the neck can produce a thin, tinny sound. The area around the bridge often produces a very strong upper-mid component, which may be objectionable on some guitars.
Other possible miking setups for acoustic guitar are too numerous to count. Sometimes the sound of a more distant mic is appropriate for the song and instrument. Some engineers have great luck miking the guitar from close to the player's head or even from behind the instrument. As with any aspect of recording, experimentation is key.
Stereo miking an acoustic guitar can effectively add space and apparent size to the instrument, especially on picked guitar parts. Directional or omnidirectional mics are appropriate for this application.
The most common miking setup is much like a spaced pair arrangement. It places one mic near the joint between the neck and body and the other closer to the bridge. Starting 8 to 12 inches away, try different distances between mics and guitar to achieve the sound you're after. Another possibility is a spaced pair of mics roughly 12 inches apart, placed several feet from the guitar. This picks up a softer, more distant sound that may be more appropriate for a strummed part. Some engineers have captured pleasing guitar recordings with a M/S stereo miking arrangement a few feet from the guitar.
Another way to use two mics to capture a fuller guitar sound is to place one close to the guitar and one more distant. The distant mic will pick up more reverb and room ambience, which can add space around the sound. This is only effective in rooms with a pleasing reverb tone.
Miking techniques similar to those used for acoustic guitar should deliver good results on classical (nylon string) guitar, dobro, banjo and mandolin. Condenser mics will best capture the silky air of classical guitar and mandolin, while dobro and banjo may benefit from the slower transient response of a dynamic mic. For more information, see The Home Studio Guide To Microphones.
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