Microphone Techniques For Guitar
Often times, in our quest for the ultimate tone, we guitarists are so focused on finding the perfect guitar, amp, or pedal that we forget one of the most important parts of the signal chain - the microphone. Whether you’re rocking a live show or recording an album in the studio, you’ll be depending on a properly placed microphone to faithfully translate your “ultimate tone” to the audience. In this article we will focus on the “properly placed” part of this equation and cover some tried-and-true mic techniques you can use to get great guitar tones.
Distance & Depth
Perhaps the most crucial element of good mic technique is finding the optimum distance from the mic to the sound source. Your wall of sound will crumble quickly without an understanding of the profound affect distance has on your microphone’s bass response and clarity, as well as the sense of depth heard in the final product.
Mics with directional polar patterns (cardioid or figure-8) will show a marked increase in bass response the closer they are to the sound source. The technical term for this phenomenon is “proximity effect”, and all directional microphones are subject to it in varying degrees. A good example is the Shure SM57, easily the most common cardioid mic used on guitar cabinets. In common use the ’57 will often be shoved right up onto the grill of a speaker cabinet to maximize the proximity effect. This technique works well with the SM57 because it complements the ‘57’s natural frequency response, which exhibits a sharp bass roll-off below 150Hz and a sharp increase in mid and treble frequencies above 3000Hz. The natural frequency curve of the Shure SM57, with added low-end from proximity effect, results in a guitar sound that exhibits both good clarity and respectable low-end.
Microphones with flatter frequency response and deeper bass, like the Sennheiser MD421, or the Royer R121 ribbon mic that we use here at the PGS studios, often need to be placed further away from the speaker to avoid the low-end mud and distortion that can result from a build up of proximity effect. For example, we usually place our Royer figure-8 ribbon microphone 6-8 inches from the speaker of our Fender Deluxe. In addition to achieving a more balanced overall tone, moving the mic back further from the amp also gives the sound space to “breathe” and interact with the room before it hits the microphone diaphragm, making for a lively, organic guitar tone that sounds more like what your ears hear in the room.
On or Off-Axis?
Microphone off axis, photo from recordingbible.com
The most common way to mic a guitar amp is to simply aim the diaphragm of the microphone in a direct line at the center of the speaker cone. This can often work splendidly with the right mic and guitar amp, but it can also result in harsh treble and a constipated, overly “cone-y” tonal quality. One solution to this problem is to use what is referred to as an “off-axis” mic technique.
Off-axis means that the microphone is not aimed with the diaphragm pointed straight at the speaker cone, but angled slightly. Directional microphones will respond quite differently when aimed off-axis, often exhibiting a high end roll-off that can be very helpful in smoothing out treble response for a warmer tone. The slight angle can also allow more indirect room sound to come into play, which can do wonders to liven up a sterile close- mic’ed tone.
A variation on this theme is to keep the diaphragm of the microphone on-axis, but point it just off the edge of the speaker cone, rather than right in the center. The end result is similar to turning the microphone off-axis, in that it will attenuate high end somewhat, making this technique excellent for rounding off a slightly-too-aggressive distorted tone.
If you have the time and resources, using two (or more!) microphones on a single amplifier can add depth, width, and detail to your guitar sound that just isn’t possible with a single mic. The idea with multi-mic’ing is that one microphone should capture the direct sound from the speaker, while the second microphone captures the ambient sound from further back in the room. The two signals can then be panned in various ways in the mix stage for a very realistic guitar sound that mimics the way your ears would hear the amp in the room.
Image from uaudio.com
Here at the PGS studios we are constantly striving to achieve the most realistic recorded guitar tones we can in an effort to represent the gear we demo accurately. With that in mind we generally place our Royer R121 ribbon mic about 6-8 inches away from our Deluxe for our main close mic’ed sound, and a Shure KSM27 condenser mic several feet back, usually in a direct line behind the Royer. At mix time we pan the Royer slightly off-center, with the signal from the Shure panned further left or right.
Another useful multi-mic technique is to place your second microphone at the rear of your speaker, directly in line with the mic at the front of the speaker, and then reverse the phase on one of the microphone channels. The mic at the rear of the speaker will only pick up low end, due to the omnidirectional quality of bass frequencies, making this technique perfect for adding low-end “thud” to a small combo amp during a recording session.
When using a multi-mic setup it’s very important to be aware of possible phase cancellation issues between microphones. This is possible whenever multiple mics are picking up the same sound source from different locations. Symptoms to watch out for are phase-y, hollow sounds, or a very thin sound when the two mic signals are combined in the mix. If you suspect phase cancellation, simply move your distant mic slightly and check again. Often moving the microphone a few inches to one side or the other will do the trick. You might also try reversing the phase one on channel of your mixer or audio recording software.
Hopefully this article enlightened you with regards to this crucial, oft-overlooked final element in your guitar’s signal chain. We have covered just a few of many possible ways to mic a guitar amp, so we would encourage you to explore the subject further. Information abounds! As with anything else, mastering these techniques requires practice and experience with your available microphones, amps, and other gear. So get out there, throw a mic or three on your amp, and let your ears lead the way.