Multiple Mics For Guitar Amp Tone Sculpting
The audio files available on this page accompany my article about recording the band Impossible Colours in SOS August 2013. The filenames are hopefully fairly self-explanatory, but the descriptions below should help you understand a little more about what you're hearing. Bear in mind that no EQ or other processing was applied to the mic signals during recording, so example files you're hearing came directly out of the mics in question.
In addition to these demonstration files, you can also download the complete raw multitrack files for one of the songs (including all additional overdubs) from the 'Mixing Secrets' Free Multitrack Download Library at www.cambridge-mt.com/ms-mtk.htm#ImpossibleColours.
GtrMultiMiking01_ShureSM7B
My snare-drum mic on this Impossible Colours session was a Shure SM7B dynamic model, which I then re-used for the guitar overdubs. It was placed around two feet from the amp, and despite this mic's well-deserved reputation for generally understated spectral contour, you can hear that it's quite boxy and aggressive-sounding in this case, on account of my switching in both its frequency contouring options: a bass cut below 200Hz and a broad 4dB presence boost centred at roughly 4kHz.
GtrMultiMiking02_ShureSM57
I'd used a Shure SM57 dynamic mic for the drumkit's top rack tom, and I moved this to a position roughly 15 inches away from the guitar cab for overdubbing purposes. Although the mic's characteristic presence peak is clearly audible here, the combination of a slightly off-axis placement and the mic's own inherent proximity bass boost gave it a surprisingly warm sound — a warmth that I actually ended up using to cancel out (via phase interaction) some of the low end on the AKG C414B XLS for a several of the sounds we recorded.
GtrMultiMiking03_BlueKickball
I also pressed the drumkit's kick-drum mic (Blue's phantom-powered Kickball dynamic mic) into service, positioning it around 15 inches from the cab. With its internal LF contouring switch in its central position, this managed a very nice balance between meaty low end and upper-mid crunch, with smoothed-off highs on account of the mic's inherent HF rolloff.
GtrMultiMiking04_ElectrovoiceRE20
This is the sound of an Electrovoice RE20 dynamic mic I set up about two feet from the amp's grille-cloth, towards the edge of the cabinet but angled a little inwards. I left the low-frequency roll-off switch disengaged, which allowed the mic to capture lots of warm low-mid range. This gives the microphone a rather unattractive woolliness on its own, but it proved to be very useful to fill out the low-mid range in combination with some of the other harder-sounding mic signals, such as those from the SM7B and C414B XLS.
GtrMultiMiking05_AKGC414BXLS
An AKG C414B XLS large-diaphragm condenser mic was the most distant of the five close mics, set back from the amplifier about three feet. However, this signal isn't significantly more ambient-sounding than the SM7B or RE20 because I switched the mic's polar pattern to hypercardioid, which rejects reverberation better than the cardioid patterns of all the other mics. I also engaged the low-cut filter on this mic, as it normally has a very extended low end response, and I was concerned not to overload the mix as a whole with low end.
GtrMultiMiking06_ShureKSM141Pair
In addition to the close mics you've heard so far, I also had a wide-spaced stereo mic array for the room. This was roughly 3m away from the cabinet and around 1.8m above the floor, with both mics in cardioid mode pointing directly towards the cabinet. A pair of reflective screens were set up to either side behind the mics to bounce a few more early reflections into them. Because the mics were spread apart by around 1m, the stereo image is extremely wide, so they act as a widening agent in the mix without impacting a great deal on the guitars basic tone.
GtrSubmix01_C414BXLS
This series of audio examples demonstrates the process I went through to reach the sound I used for the first guitar overdub of the Impossible Colours project. The primary microphone I chose to base the sound around was the AKG C414B XLS, which you can hear in this example.
GtrSubmix02_SM57
In search of some 'growl' in the 700-800Hz region I tried mixing a little of the Shure SM57 in with the C414B signal, but this actually did the opposite of what I wanted, hollowing out the very region I wanted to enhance, leaving me with overblown the low end end and fizzy highs.
GtrSubmix03_SM57Flip
Inverting the polarity of the Shure SM57 signal immediately improved the combination, however, adding exactly the kind of midrange character I was hoping for. However, in the process I it seemed that phase-cancellation between the mics was sucking out too much of the low midrange, so I searched through the other mics to find another signal that might help fill this out better.
GtrSubmix04_RE20
The Electrovoice RE20 signal was very muffled-sounding on its own (as you can hear in the GtrMultiMiking04_ElectrovoiceRE20 file above), but I figured it might still be useful to reinstate some warmth to the C414B+SM57 combination. However, when I mixed it in, it did the opposite of what I wanted, thinning the low midrange even further — a sure sign that it wanted its signal polarity inverting.
GtrSubmix05_RE20Flip
With its signal polarity inverted, the RE20 signal combined with the C414B and SM57 channels in a much more complementary manner, filling out the low midrange nicely.
GtrSubmix06_KSM141s
Adding in the Shure KSM141 stereo room mics allowed me to widen the apparent image of the guitar, so that it could take up a little more space in the mix.
GtrSubmix07_KSM141sFlip
Although the combination of the room mics with three close mics didn't cause a huge tonal change, I nonetheless checked to see if I preferred the composite sound better with the polarity of the room mics inverted — and I did!
GtrSubmix08_CloseMicPanning
As a final touch, I panned the SM57 and RE20 mic signals 30 percent left and right respectively to spread the close-mic image a little too, but if you switch the sound — a subtle but appreciable effect which didn't contribute any unwanted mono-incompatibility.