Got a mic and an interface? You can record professional-sounding acoustic guitar tracks from your bedroom by closely following these simple steps. Get started!
By Craig Anderton
There’s nothing quite like a strummed acoustic rhythm guitar part to drive a song, or the expressive nuances of a finger-picked solo. However, to take full advantage of what acoustic guitars can do, you need to record and mix them properly—so let’s get started.
The easiest way to record guitar is by taking a direct output from its onboard electronics (e.g., a piezo pickup mounted under the bridge). Patch this output to your audio interface’s instrument input—done! The main disadvantage is that this approach doesn’t capture the full sound of the guitar body.
A more complex option is using one or more mics to pick up the guitar’s strings, body, and interaction with an acoustic space. This approach requires mic stands with booms, so you can attach the mics securely, and position them in relation to the guitar. But before you can place your mics in position and start recording, you need to decide what kind of mic to use.
1. Microphone Types
The three main microphone technologies used for recording guitar are:
- Dynamic mics. These are rugged, require no power, have no internal preamps to generate hiss and are popular for live use. They can sound less “bright” than condenser mics (see next), and don’t have the same level of detail. They’re often used for recording guitar amps.
- Condenser mics. These are the go-to mic type for recording acoustic guitar. They require power, which is typically +48V. The voltage from an audio interface or mixer travels up the mic’s cable to the mic. This power source is called phantom power, because there’s no visible power supply—just the cable that makes the audio connection. (If a condenser mic doesn’t seem to be working, make sure the input to which the mic connects has phantom power turned on.) Compared to dynamic mics, condenser mics can be more sensitive to nuances, have a more open sound, and are somewhat more fragile.
- Ribbon mics. These have a warm midrange, and aren’t as bright as condenser mics. Unlike older, fragile ribbon mics, modern versions are more robust. Despite their expense, they’ve become increasingly popular for recording acoustic guitar, especially for stereo.
Condenser and dynamic mics are further differentiated by the size of their diaphragm (the thin membrane that moves in response to sound waves). To generalize, large-diaphragm mics are more sensitive than small-diaphragm mics, but the tone is somewhat less bright. You might choose a large-diaphragm mic on nylon-string guitar or ukulele, and a small-diaphragm mic on steel-string acoustic guitar.
2. Microphone Pickup Patterns
A mic’s pickup pattern (fig. 1) defines how the mic responds to sounds coming from different directions:
- An omnidirectional mic picks up sound equally from all directions. It’s rarely used live, but in the studio, is useful for picking up a combination of acoustic guitar and room sound. High-quality omnidirectional mics have a natural sound, and flat overall response.
- Cardioid microphones are directional, and used both live and in the studio. They pick up sound at the mic’s front, while rejecting sound from the rear and sides. This rejection isn’t perfect, so sounds arriving from the back and sides have more coloration.
- The figure-8 (also called bi-directional) pickup pattern is most common with ribbon mics. This picks up sounds from opposite sides of the mic, while providing almost total rejection at the front and back. One guitar application is placing the mic between two guitarists doing a duet, because they’re picked up equally well. Ribbon mics are also popular for stereo miking.
Furthermore, directional mics (e.g., cardioid and figure-8 pickup patterns) exhibit a proximity effect—moving the mic closer to the sound source accentuates the bass response. This can impart a fuller acoustic guitar sound, but moving too close makes the sound bass-heavy and muddy.
3. Using Direct and Miked Sounds
Combining a direct feed and a miked sound allows blending the direct string sound with the body. However, sound takes time to travel through air (about 1 millisecond per foot), so the miked sound will be delayed slightly compared to the direct sound. For the fullest sound quality, it’s good practice to “nudge” the miked sound ahead a little bit within your DAW, so that the two tracks line up (fig. 2).
To adjust the delay time, monitor both signals by mixing them together. In your host program, reverse the polarity for one of the signals (i.e., flip it out of phase). Then, nudge the miked sound earlier until you obtain the thinnest sound. This occurs when the direct and miked sounds are lined up as closely as possible. For the fullest sound, return the out-of-phase channel to being in-phase.
4. Miking Options for Acoustic Guitar
There are many (many!) ways to mic an acoustic guitar. The following common techniques don’t require specialized mics (like a matched pair of mics).
A single mic eliminates any potential phase issues caused by phase mismatches between two mics. Position the mic about 6 to 12 inches directly in front of the 14th fret, angled in slightly so it points toward the fingerboard between the body’s edge and the sound hole (fig. 3). Experiment with the distance to dial in the desired midrange response.
For stereo, add a second mic about the same distance away, but placed over the lower bout (the lower curve of the body), or just behind the bridge (fig. 4). Many engineers minimize phase issues by following the “3:1 rule,” which says the second mic should be around 3:1 as far from the first mic as the first mic is from the body. Although that’s a good place to start, experiment with the mic placement until you hear the sound you want.
Another option is an X/Y stereo pair, placed about 12 to 18 inches in front of the guitar. Center this stereo pair so that it’s almost directly in front of the sound hole (fig. 5). Pointing a mic directly at the hole usually isn’t recommended, because the sound can be bass-heavy and muddy. However, with the X/Y pair’s 90° angle, the two mics will point to either “side” of the sound hole instead of directly into it.
5. Processing Acoustic Guitar with EQ
Once you’ve recorded your guitar track, applying EQ can enhance the sound further. It can also minimize the difference between different mic technologies—for example, boost a dynamic mic’s treble to make it brighter. Use the bypass button often to compare unequalized and equalized sounds. This important reality check can prevent you from “chasing your tail”—e.g., the guitar sounds thin, so you boost the bass, but now it’s too bassy, so you boost the highs, but now the midrange seems weak…
a. Tighten up the sound
An acoustic recording may sound “boxy,” or have too much low end. Here are two ways to tighten the sound (fig. 6):
- To minimize audio below the guitar’s range, insert a steep, high-pass filter between 20 and 90 Hz. Note that some engineers don’t like to reduce frequencies below a guitar’s note range, because they believe this removes audio that people may not hear per se, but can feel. Listen, and decide for yourself.
- If a guitar lacks both high-frequency and low-frequency definition, try a shallow cut in the lower midrange (200-350 Hz). This tightens up the high and low frequencies.
- Example 1 – Dry
- Example 2 – EQ
Audio example 1 is dry guitar. Audio example 2 trims the lows and lower mids per fig. 6. The EQed sound is not just tighter but also louder, even though the peaks for both examples are normalized to the same level. This is because having fewer low frequencies opens up more headroom.
b. Tame resonances
Some acoustic guitars have body resonances that can help project better on stage. However, recording has different requirements, where you often want a more even/balanced response. (Note that a piezo pickup, not just the guitar itself, can also have resonances.)
Dynamic range compression or limiting are potential solutions, but they can alter the guitar’s attack and decay characteristics. A more natural-sounding option is to use boost/cut parametric EQ to reduce response at the resonant frequency. To find a problem frequency, which you can then fix with EQ:
- Turn down the monitor volume, because the sound could get really loud.
- Set the EQ for lots of boost (10-12 dB) and a narrow Q.
- As the track plays, sweep the frequency control slowly. Any peaks will jump out due to the boosting and narrow bandwidth. Some peaks may even distort.
- Find the absolute loudest peak, and then cut the EQ until the peak falls into balance with the rest of the guitar’s sound (fig. 7). Widen the bandwidth somewhat with broad peaks, or narrow it for sharp resonances.
- Example 3 – Dry
- Example 4 – EQ
Audio example 3 is dry guitar with a mild resonance. Audio example 4 applies the resonance reduction shown in fig. 7.
Musician/author Craig Anderton is an internationally recognized authority on music and technology. He has played on, produced, or mastered over 20 major label recordings and hundreds of tracks, authored 45 books, toured extensively during the 60s, played Carnegie Hall, worked as a studio musician in the 70s, written over a thousand articles, lectured on technology and the arts (in 10 countries, 38 U.S. states, and three languages), and done sound design and consulting work for numerous music industry companies. He is the current President of the MIDI Association. www.craiganderton.org.
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