Guitar Amp Recording Part 4
Written by Mike Senior, SOS
Although a lot of engineers prefer to mic up the single, best-sounding speaker cone of a multi-speaker cab, some blend the sounds of more than one. Steve Churchyard: "If I'm using a 4x12 cabinet, I find two of the best-sounding speakers, and I'll put an SM57 right on axis and right on the cone of both those guys. Then I'll mix them in the control room, combine the two together. It seems a little different than just using one mic. It's not twice as good, but it's just mixing the character of two different speakers." While recording AC/DC's Back In Black, Tony Platt used a pair of condenser mics to pick up different speaker cones and give a wider sound to each guitar: "I developed a technique for recording guitars with two microphones roughly pointing at different speakers, which can be spread out in the stereo mix so it's not just a series of mono point sources. It makes for a more open-sounding guitar. That sound suited their particular technique, which involved Angus and Malcolm playing the same chords but with different inversions to get a very big unison guitar sound." Hugh Padgham adopted a similar tactic for recording Andy Summers' Roland JC120 when working with the Police: "The chorus [was] always switched on in order to produce the slightly out-of-tune guitar sound that was all the rage during the early '80s. The amp's two 12-inch speakers would each be close-miked with a Sennheiser MD421, panned left and right one speaker would produce a straight signal while the other would be chorused, and these would sometimes be double-tracked the other way around in order to produce an especially wide stereo picture." Of course, no-one says you have to use the same mic on each speaker cone. For example the SOS interview with Toby Wright shows an SM57 and an MD421 on separate speakers, and Don Smith mentioned using an SM57 and an AKG C451 on separate speakers when recording Keith Richards. Sylvia Massy Shivy also uses the SM57+MD421 combination, but sounds a note of caution when deciding on the exact positioning of the mics: "You have to be very careful with phase, just check it until the signal is the strongest."
Coincident dual-mic techniques are commonly used by professional recording engineers and producers. Not only does the close positioning of the mics help to minimise problems with phase: the mix engineer is also left with plenty of control over the texture of the sound without the need for radical processing.
The 'phase EQ' technique: three mics (two close mics and another further from the amp) are positioned to form a triangle. The faders on the desk (or in the DAW) can then be raised or lowered for each signal. This alters the phase relationship between them and provides an interesting and, according to some producers, less intrusive alternative to conventional EQ. SM57Moving KSM137Stationary SM57KSM137Phasing
In response to Bill Price's comments about matching the phase of different close mics, I lined up a Shure SM57 dynamic mic and a small-diaphragm Shure KSM137 condenser mic on axis, over the centre of the speaker cone. With the KSM137 around six inches away from the grille, I started off with the SM57 an inch further away and then moved it (while recording) through a distance of around two inches, ending up with it an inch closer to the grille than the KSM137. The third audio example combines the SM57 and KSM137 mic signals at equal levels to demonstrate the tonal changes created by phase-cancellation between the two mics. PhaseEQSM57 PhaseEQMD421 PhaseEQC414BXLS PhaseEQCombinations
The first three audio examples here are recordings from three mics set up in the 'phase EQ' configuration that Jack Douglas describes in the main article. For the final example I've tried to demonstrate something of the range of sounds available using only the fader and phase-invert button of each mic channel.
Adding Ambient Mics
"I've always thought that most people mic amps too closely," comments Alan Parsons. "They supposedly make up with an ambient mic, but I much prefer to find a mic position that works and process that, rather than mix in too much ambience." Despite Parsons' disapproval, though, a lot of the engineers I researched divulged that they use additional ambient mics to capture more of the sound of the room in which the guitar cab was recorded. For example, Al Schmitt starts with the traditional SM57 close mic, on axis but a little off the centre of the cone. "Then I'll put a really good mic up maybe a Neumann U67 or an M50 for the room... It could be anywhere from 15 to 20 feet away."
It's worth noting that the M50 is an omni microphone and, although the omni polar pattern is only very occasionally mentioned for close-miking, it makes a much more sensible choice for capturing natural room ambience. Tony Visconti is also into using ambient mics: "I'm very much a fan of the room sound, too. I always record it if it's a real heavy rock guitar with power chords and crunches and all. I'll go around the room and clap my hands and I say, 'Put the mics there, that's it.' Quite often, I'll turn the room mic towards the studio window, and you'll get a reflection of the guitar sound not directly facing it, because you're looking for reflections." Although he states in the same interview that he'll try to use a pair of U87s for ambience if possible, he's also mentioned elsewhere using PZM mics as an alternative. Perhaps the most dramatic of ambient mic techniques, though, comes courtesy of Chris Tsangarides. His 'Vortex' involves using studio screens to build 30-foot-long walls along each side of the guitar cabinet, creating a flare shape (apparently inspired by the shape of a bass bin). Within this flare, he places a close condenser mic and typically another couple of condenser mics with different distant positionings, perhaps at 15 and 30 feet away. "I walk around while the guy's playing and find a sweet spot and put the mic there", says Chris. By panning the distant mics to the opposite side of the mix from the close mic, you can create interesting panning effects for solos. "If it's a rhythm part, you get this huge sound because the whole thing is spread across the stereo spectrum."
When double-tracking lead or rhythm parts, a useful trick is to reverse the panning of the direct and distant mics. "If there were two guitarists in a band, I would record them like that, so you got a wall of sound that had a transparency that would allow the drums and bass to come through." While experimenting with the Vortex for this article, I was impressed by quite how well the ambient mics seemed to turn a close-miked guitar sound into something that sounded like it was on a record, but the downside of this approach for most home recordists will be that the Vortex is not easy to recreate in a smaller studio so I thought I'd pass on some ways I found to make it more manageable on a smaller scale. One problem most small studios have is that they don't have large numbers of screens, but in practice I found that I was able to get decent results by putting the guitar cab in the corner of the room and using one or both of the room boundaries in place of the screens. Visconti's trick of aiming ambient mics at the studio glass also turned out to be handy to increase the apparent distance of the farther ambient mic.
Combination Techniques & The Phase EQ
Let's look at how the professionals go about combining the close and ambient techniques we've looked at so far, in order to create specific custom setups for different recordings. Joe Barresi, for example, relies heavily on the trusty SM57 and MD421 combination, but he'll choose from a variety of other mics to give character to particular sounds. "The two microphones I use most for recording electric guitars are the Shure SM57 and the Sennheiser MD421, often both, close up, placed at the edge of the speaker, where the speaker centre meets the cone, or, if I'm looking for a more bright sound, dead centre. When I want more low end, I may have an AKG C414 on there, and when I'm after a little more personality, a Neumann U87, backed up a foot, or a ribbon mic, like the Royer 122, or an RCA BK5 or 77." Eddie Kramer has a slightly different approach, working from a familiar setup of favourite close and ambient mics (including the Beyerdynamic M160 ribbon mic) and then mixing them together to taste. "I use a three-mic technique: an SM57, an MD421 and an M160, all in a very tight pattern. Then I can pick and choose the tone quality, because each mic is totally different. I combine these together, and then I put a U67 away from the amp to get the ambience." For his work on Supernatural, Glenn Kolotkin turned to elaborate multi-miking as a way of managing Carlos Santana's complicated setup. "I used multiple microphones on Carlos' guitars: Electrovoice RE20s close, Neumann U47s further away, an SM56, U87s. He was playing through an assortment of amplifiers at the same time, and by using multiple microphones I was able to get just the right blend."
Complex though some of these techniques are, probably the most powerful use of multi-miking I've encountered during my investigations comes courtesy of Jack Douglas, who makes creative use of phase cancellation between microphones. "For guitar overdubs, the best EQ in the world is the phase EQ, which you get by using multiple mics on a speaker. For example, take a Shure SM57, a Sennheiser MD421 and your favourite condenser, and set them up in a triangle with the two dynamics at an angle up against the grille, but off axis. Then take your favourite condenser mic, put a 10dB pad on it, and place it about a foot away, facing the speaker, on axis. "Bring up one mic at a time and get it to optimum level on your board. To check that they're all in phase, make sure the signal is adding and not subtracting as you add in the other mics. If not... reverse the phase. Then start to put up each mic, one at a time... as you move the faders back and forth, you'll hear the greatest EQ, because of the phase relationship... Then if you flip the phase on one of the mics, you can really have some fun it'll act like a filter." Having tried out this technique, I have to say that it's something of a revelation to hear the enormous range of radically different sounds it makes available. When you start inverting the phase of a mic, it sounds like the most extreme EQ you've ever heard, which means that you can substantially reinvent guitar sounds at mixdown without using any heavy processing. For even more sonic mileage, you can also take a leaf out of John Leckie's book and process each of the three mic signals independently. Jack Douglas also points out a beneficial side-effect, in his experience, of recordings made using the phase EQ approach: "When you build a mix I don't care if it's four tracks, eight tracks, or 96 the real nightmare is when you put something up and the only way you can hear it is by blasting it. There's nothing worse than putting up something you're excited about, and it's gone. If you [record guitars] like this, I guarantee that as soon as you put the sound in the mix it will be there. Not only that, it won't wipe out everything else in the mix, because it will have such a separate and distinct character."
Too Many Mics?
Being by nature rather sceptical, I have to admit to initially dismissing many of the recording methods in this article as 'studio snake oil', and because there was usually too little time during my own sessions to experiment with new ideas, I'd usually end up with an SM57 glued to the speaker grille by default. Taking the time out to trial the above techniques in the studio showed me quite how much I had been missing not only much better raw recordings, but also tremendous extra flexibility at mixdown. But don't take it from me listen to the audio examples for yourself and make up your own mind. If they don't expand your recording horizons, I'll eat my SM57...
Many thanks to www.soundonsound.com for this article