Miking Guitar Amp Cabinets

It's not as simple as just placing a mic up against a speaker

by Craig Anderton

Miking guitar cabinets may seem like a simple process, because all you really need to do is to pick up moving air with a mic. But there are many variables: the mic, its placement the room environment, the cabinet itself, and the amp settings. So, let’s consider some of the most important considerations when miking amp cabinets.

MIC SELECTION

Many guitarists record with the amp cranked to get “that” sound, so under these circumstances it’s important to choose a mic that can handle high sound pressure levels (SPL). Dynamic mics are ideal for these situations, and the inexpensive Shure SM57 is the classic guitar cabinet mic—many engineers choose it even when cost is no object (Fig. 1). Although dynamic mics sometimes seem deficient in terms of brightness, this doesn’t matter much with amp cabinets, which typically start losing response around 5kHz or so. A couple less dB at 17kHz isn’t going to make a lot of difference. That said, there are also more upscale dynamic mics, like the Electrovoice RE20 and Sennheiser MD421, which give excellent results.

Fig. 1: Shure’s SM57 is the go-to cab mic in many pro and project studios.

Condenser mics are often too sensitive for close miking of loud amps, but they can give a more “open” response. They also make good “auxiliary” mics—placing one further back from the amp adds definition to the dynamic primary mic, and picks up room ambience that can add character to the basic amp sound. For condenser mics, AKG’s C414B-ULS is a great, but pricey, choice; their C214 gives similar performance but at a much lower cost. Neumann’s U87 is beyond most budgets, but the more affordable Audio-Technica AT 4051 has a similar character and it’s also great for vocals.

Then there’s the ribbon mic. Although ribbon mics used to be fragile, newer models use more modern construction techniques and are much more rugged. Ribbon mics have an inherently “warm” personality, and a polar pattern that picks up sounds from the front and back—but not the sides. This characteristic is very useful with multi-cab guitar setups; by choosing which sounds to accept or reject based on mic placement, ribbon mics let you do some pretty cool tricks. Royer’s R-121 and R-101 are popular for miking cabs, Beyer’s M160 is a classic ribbon mic that’s been used quite a bit with cabs.

Regardless of what mic you use, check to see whether the mic has a switchable attenuator (called a “pad”) to reduce the mic’s sensitivity. For example, a -10dB pad will make the mic 10dB less sensitive. With loud amps, engage this to avoid distortion.

MIC PLACEMENT

First, remember that while each speaker in a cab should sound the same, that’s not always true. Try miking each speaker in exactly the same place, and listen for any significant differences. Start off with the mic an inch or two back from the cone, perpendicular to the speaker, and about half to two-thirds of the way toward the speaker’s edge. To capture more of the cabinet’s influence on the sound (as well as some room sound), try moving the mic a few inches further back from the speaker.

Moving the mic closer to the speaker’s center tends to give a brighter sound, while angling the mic toward the speaker or moving it further away provides a tighter, warmer sound. Also, the amp interacts with the room: Placing the amp in a corner or against a wall increases bass. Raising it off the floor also changes the sound.

The room’s ambience makes a difference as well. If the room is small and has hard surfaces, the odds are there will be quite a bit of ambient sound making its way into the mic, even if it’s close to the speaker. This isn’t necessarily a bad thing; I’m a fan of ambience, because I find it often adds a more lively feel to the overall sound.

DIRECT VS. MIKED

Some amps offer direct feeds (sometimes with cabinet simulation); combining this with the miked sound can give a “big” sound. However, the miked sound will be delayed compared to the direct sound—about 1ms per foot away from the speaker. This can result in comb filtering, which you can think of as a kind of sonic kryptonite because it weakens the sound. To counteract this, nudge the miked sound earlier in your recording program until the miked and direct sounds line up, and are in-phase (Fig. 2)

Fig. 2: In the top pair of waveforms, the top waveform is the direct sound and the next one down is the miked signal. Note how it’s delayed compared to the direct sound. In the bottom pair, the miked signal (bottom waveform) has been “nudged” forward so it lines up with the direct sound.

THE MIC PLACEMENT “FLIGHT SIMULATOR”

IK Multimedia’s AmpliTube 3 (Fig. 3) lets you move four “virtual mics” around in relation to the virtual amp. The results parallel what you’d hear in the “real world,” and you can learn a lot about how mic placement affects the overall sound by moving these virtuals mics. While this doesn’t substitute for going into the studio, moving mics around various amps, and monitoring the results, it’s a great introduction. Nor is AmpliTube alone; Softibe’s Metal Room offers two cabs and mics (Fig. 4), Overloud’s TH2 has two moveable mics for their cabinets (Fig. 5), and MOTU’s Live Room G plug-in for Digital Performer 8 (Fig. 6) also allows various mic positions for three difference mics.

Fig. 3: IK Multimedia’s AmpliTube offers four mics you can place in various positions.

Fig. 4: Softube’s Metal Room has two cabs, each with two mics you can position as desired.

Fig. 5: Overloud’s TH2 has two mics for covering their cabs.

Fig. 6: MOTU’s Digital Performer 8 has two “live room” plug-is, one for guitar and one for bass, that provides for various miking options.

Craig Anderton is Editor Emeritus 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.