Using more than one mic on a source can expand your sonic horizons — but it can also lead to problems. We explain when multi‑miking is and isn’t a good idea.
It’s easy to understand why multi‑miking is such a hot topic amongst project‑studio recordists. Not only can it offer enormous tone‑sculpting power while retaining lots of mixdown flexibility, but it also provides a welcome justification for buying lots of shiny new microphones. However, this technique can also backfire easily, too, because using extra mics introduces more opportunities for mistakes and complications. Indeed, having mixed hundreds of project‑studio multitracks myself (including plenty for Sound On Sound’s own Mix Rescue column), I can attest that small‑studio engineers frequently sabotage themselves with their multi‑miking methods, slowing down the production process and compromising the quality of the final mixdown.
So in this article, I’d like to suggest how to avoid some of the most common pitfalls, so you can get the best out of multi‑miking.
What Else Does The Sound Need?
I’ll be talking specifically about multi‑miking for tone, as opposed to stereo miking (which necessarily uses multiple mics). And probably the most common misconception about this sort of multi‑miking is that it’s some kind of panacea for getting good recorded tones. When you put up just one mic it’s pretty obvious that you have to pay attention to how it sounds, but somehow the very presence of more than one mic seems to lull far too many people into a false sense of security, so that they switch off their ears under the assumption that they can leave the sonic decisions until mixdown.
But the reality is that there are many more bad microphone positions than good ones, especially when you’re working with budget mics and in untreated acoustic environments. I’d estimate that roughly three quarters of the initial microphone choices and/or positions on any project‑studio session usually need adjusting in some way if you’re going to achieve genuinely workable recorded results, so having two or three mics on the go certainly doesn’t guarantee that any of those mics will actually sound any good! For example, one of the most challenging Mix Rescue remixes I ever tackled featured a triple‑miked acoustic guitar recording where all the mics sounded pretty awful, which made it a nightmare to salvage anything usable. And I’ve lost count of the number of drum kit recordings where I’ve immediately thrown away the additional ‘subkick’ mic (where you basically record the output of a speaker cone hanging in front of the kit) at mixdown because it offered nothing but completely inappropriate low‑end flab.
No matter how many mics you put up at once, it’s paramount that you listen to each one of them critically to ensure they’re actually contributing something useful.
So, no matter how many mics you put up at once, it’s paramount that you listen to each one of them critically to ensure they’re actually contributing something useful. A great way to discipline yourself to do this is first to put up just one mic, and to concentrate on trying to capture the whole sound through that. Only once you’ve given that a proper shot should you then ask yourself: “What else does the sound need?” You may be surprised how often the answer is, “Er... nothing, really!”, in which case it’s perfectly OK not to put up any more mics! But if you do identify some element of the sound that needs extra support, then you’ve immediately got a head start in terms of deciding what mic to add next and where to put it.
Let’s say you’ve already got a dynamic mic right over your guitar cab’s speaker cone delivering plenty of upper‑midrange bite, but you want more solidity or warmth to the tone. It stands to reason that you’ll probably want your second mic to be smoother‑sounding (perhaps a neutrally voiced large‑diaphragm capacitor or a ribbon), and you might want to place it off‑centre to the cone, or a couple of feet away — or even around the back of the cab!
This basic principle is probably most frequently forgotten when multi‑miking drum kits. I often hear overhead mics covered in thrashy hi‑hat, but with dull‑sounding snare ambience. Then, when I investigate the other drum channels, I find there’s a completely unnecessary mic on the hi‑hat, while the snare has only a close mic on the top head that goes ‘donk’! Where the hell is the final snare sound supposed to be coming from? Just listening to the overheads and asking, “What else does the sound need?” could have headed off this problem, perhaps by repurposing the hi‑hat close mic under the snare.
However many mics you use, it’s important to listen to the sound of each one so you can evaluate its contribution to the bigger picture.Photo: Richard Ecclestone
Listen For Contrast
Another excellent rule of thumb when multi‑miking is that you should endeavour to make the mics sound appreciably different from each other. In a sense, this is an extension of asking yourself, “What else does the sound need?”, because “more of what I already have” seems like a pretty lame answer to that question! But the other big reason for contrasting your multi‑mics is to provide greater sound‑shaping power from the control room. The more individual each mic sounds, the wider the range of timbres you’ll get as you fade between them. This is why well‑known producers so often talk about combining different mic types when multi‑miking, for instance, such as dynamics versus ribbons versus capacitor mics, or bright mics versus dull mics.
Contrasting your multi‑mics isn’t just about increasing flexibility at mixdown, either, because the biggest appeal of multi‑miking in my opinion is how much it can accelerate sound‑hunting during the tracking process, particularly when overdubbing electric guitar layers. After all, it’s a whole lot quicker to adapt your recorded sound for each guitar part by waggling a few faders than it is by repeatedly shuffling mics around — especially if you don’t have an assistant to do the legwork, so have to dash back and forth between live room and control room to evaluate each new option for yourself.
Multi‑miking can only provide decent scope for sound‑shaping if the individual mics are sonically contrasted with each other. One way of achieving this is to use a variety of different microphone designs — like the ribbon, large‑diaphragm capacitor, and dynamic models close‑miking the guitar amp here.Photo: Daniel Plappert
Managing Polarity & Phase
That said, the fader level of each of your multi‑mics isn’t the only thing you need to consider, because multi‑mic signals will usually interact with each other in complex ways on account of the polarity and phase relationships between their signals. Recording a guitar cab both from in front and from behind, for instance, will cause the back mic’s signal to be polarity‑inverted compared with the front mic, and if you don’t compensate for that by flipping the back mic’s polarity while recording, you’ll likely discover that the phase cancellation that occurs when you mix it with the front mic actually thins the combined tone, rather than filling it out — in other words, it’ll do the opposite of what you’d expect from listening to each mic on its own! Likewise, if you’re multi‑miking a snare drum from both above and below, or a kick drum from both the resonant‑head and batter‑head sides, then it’s common studio practice to polarity‑invert one of the mic signals to achieve a fuller mixed tone.
But even when you’re only multi‑miking an instrument from one side, if the sound waves reach the mics at different times, you’ll get timing offsets between their recorded signals, which can also cause unexpected (and sometimes quite drastic!) frequency interactions between then when mixed together — an effect usually called comb filtering. Fortunately, you don’t need to know all about the physics of comb filtering to stay out of trouble when tracking, because there are a few straightforward practical ways you can either minimise it or else turn it to your advantage.
If you’re multi‑miking any instrument from different sides like this, then you’ll usually find that inverting the polarity of one of the mic signals generates a fuller sound when the signals are mixed together.
The first option is just to place all your multi‑mics the same distance from the sound source, such that combining their signals will naturally generate minimal comb‑filtering. You have to be quite accurate here, though, because a miking‑distance difference of just half a centimetre can still cause noticeable comb‑filtering effects, and it can sometimes be quite difficult to tell from just eyeballing a mic exactly where its diaphragm is located. For instance, both the Shure SM7B and Electrovoice RE20 have their diaphgragms set back much further from the grille than the Neumann KM184 or Rode NT5 do, but that’s far from obvious without dismantling them. Mind you, it’s nevertheless possible to phase‑optimise your microphone positions by ear. Just mix the mic signals at roughly equal level; polarity invert one of them; hunt for the mic alignment that gives the weakest‑sounding result; and finally remove the polarity inversion.
...mix the mic signals at roughly equal level; polarity invert one of them; hunt for the mic alignment that gives the weakest‑sounding result; and finally remove the polarity inversion.
Comb As You Are
Alternatively, if you don’t want to put all your multi‑mics equidistant from an instrument, you have to accept that mixing their signals together will inevitably incur some comb filtering, so adjusting the polarity/phase relationships between the mics by ear becomes an integral part of the miking process. The simplest tool at your disposal here is the polarity switch, and it should become second nature to always try flipping the polarity of each channel in any multi‑mic setup while listening for the most appealing combination. There’s no ‘right’ setting here, because every setting will still involve some comb filtering — your aim is just to choose the best‑sounding variant! You can also refine phase relationships by introducing small delays into some of the channels, or even by using special phase‑rotation devices such as the Little Labs IBP or Radial Phazer, but to be honest I think those are overkill for most project‑studio tracking sessions as long as you take full advantage of your polarity switches.
...there are plenty of mass‑market playback systems that will sum your mix to mono, at which point the phase mismatch between your multi‑mics could transform your monster guitar riff into something like a ferret blowing raspberries!
You also need to keep your wits about you if you pan some of your multi‑mics during tracking, because separating those mic signals in stereo will also reduce the degree of comb filtering you hear between them. This might seem like great news on the face of it, but you have to remember that there are plenty of mass‑market playback systems that will sum your mix to mono, at which point the phase mismatch between your multi‑mics could transform your monster guitar riff into something like a ferret blowing raspberries! By all means pan your multi‑mics if you want, but never overlook checking the sound in mono too.
Decisions, Decisions...
Although one of multi‑miking’s headline advantages is extra control over the final sound at mixdown, I reckon it’s a bit of a poisoned chalice, because it encourages people to defer their sonic decisions, and that’s probably the best way to waste time in any studio! If you use multi‑miking as an excuse not to commit to a definite sound while tracking your first rhythm‑guitar part, say, then how are you supposed to decide on appropriate sounds for any other later overdubs in relation to that? And how will you know when to stop adding new parts, if you don’t know how much mix real‑estate you have left to fill? I’ve seen plenty of home‑studio multitrack projects where they’ve clearly tried to get around this problem by multi‑miking everything, but that’s no solution either, because the lack of any sonic vision for each part usually means that those multi‑mics aren’t suitably placed — not to mention that the resultant track‑count explosion adds unwelcome logistical complications to all your comping, editing and mixing tasks.
I reckon multi-miking is a bit of a poisoned chalice, because it encourages people to defer their sonic decisions, and that’s probably the best way to waste time in any studio!
So, if you value your sanity, try to commit to a concrete sound whenever you’re multi‑miking, rather than leaving that decision until mixdown. By all means record the microphone signals on seperate tracks as a safety measure, but always ask yourself whether you’re actually hearing the tone you want while you’re still tracking — and if you’re not, then change something! Personally, I’ll often deliberately mix multi‑mics to a single recorder track to force me not to sit on the fence sonically. Now I realise that might seem a little cavalier. After all, how can I really be sure I’ve chosen the right sound for the final mix when the production’s not yet complete? Well, it’s actually a lot less risky than it appears, because you naturally adapt any subsequent overdubs to fit with the multi‑miked sounds you’ve already baked in, effectively transforming your initial sonic guesses into self‑fulfilling prophecies!
When One Mic Is Enough
Multi‑miking is better suited to some applications than others. For example, it’s rarely a good idea to use more than one close mic on any sound source that naturally moves during performance — as a lot of string, wind and brass instruments do. The problem here is that the path lengths to the different mics (and hence the phase relationship between their output signals) will vary with the instrument’s movements, so any comb filtering between the mics will modulate unpredictably and it’ll feel like some maniac is randomly twiddling your channel EQ the whole time! Because of this, it’s little surprise that the most common targets of multi‑miking are things like drum kits, guitar/bass cabs and pianos — although things like acoustic guitar and upright bass are also regularly multi‑miked, and you can still get decent results there if the player doesn’t get too carried away.
Using spaced multi‑miking techniques like this on acoustic guitar can be risky if the performer moves much while performing.
If you insist on multi‑miking moving sources, though, at least try to mitigate any potential phase problems by positioning the mics as close to each other as possible and by phase‑aligning the signals by ear. Even then, though, I’d almost never recommend recording lead vocals this way, because no matter how well phase‑aligned your mic diaphragms, you’ll still incur some comb filtering at high frequencies if the singer moves at all off‑axis to the multi‑mic rig, and that’ll usually rob some of the ‘air’ from your vocal tone.
Another situation where multi‑miking can cause more problems than it solves is when you’re recording ensembles. You see, if you set up multiple mics on one instrument such that their polarity/phase relationship benefits that instrument’s sound, you’ll often find that spill from other ensemble instruments starts sounding horrible. Those instruments will have different path lengths to the mics, and hence will incur different flavours of comb filtering. Again, if you really feel you want to multi‑mike, then I’d favour coincident mic placements.