Beatles record producer George Martin once commented that using any more than a single mic to record an instrument compromises audio quality. Why? Well, it has a lot to do with the multi-directional nature of sound.
Upon reaching a microphone’s diaphragm (the mechanical equivalent of your eardrum) sound waves originating from any given source get summed together with echoes of themselves reflecting off adjacent surfaces. Depending upon the relative strength of the direct sound, the reflections, and the staggered times by which they arrive, each partially reinforces or cancels each other out to varying degrees. As a consequence, even small changes in mic placement can produce an easily discernible difference.
Once a second mic is used in the same vicinity, the direct sound it receives functions as a high amplitude/small-delay echo of the first. In theory, this could all add up for the better – and indeed, stereo mic techniques such as XY, ORTF, Blumlein and so on use these naturally occurring amplitude and ‘phase’ relationships to positive effect – but left to chance, two open mics do not usually sound as good as one.
When one factors in how the electromechanical design of various mics influences their performance, it’s easy to understand why recording engineers place such importance upon microphone choice and placement. Different mics will vary markedly with respect to sensitivity across the audible frequency spectrum, directionality, and reaction time to changes in air pressure. Many mics also feature changeable parameters, allowing their performance to be optomised for different recording situations.
Post-production choices will further impact phase-related issues: For eg., Will each signal emanate from a separate speaker, be blended across the imaginary stereo ‘soundstage’, or sound equally loud out of both (i.e. be summed to ‘mono’)? The interaction between each discreet mic signal will simply be a product of the degree by which each attempts to push or pull any one speaker at any given moment. Start adding further mics (such as when recording a drum kit) and all those individual sonic ‘colours’ may well mix together to produce the audio equivalent of mud.
If you can’t beat ’em, join ’em.
Even for music producers who work primarily ‘in-the-box’, mics are often needed for tracking vocal overdubs. Here, room acoustics, environmental ambient noise and headphone bleed still pose problems. While the first two issues can be addressed using relatively insensitive dynamic cardioid mics, (portable) diffusers and hi-pass filters, headphone spill is somewhat harder to eradicate. But is it really worth the trouble?
A common complaint made by young artists after having made their first recording in a studio is how the end result sounds too ‘clean.’ Many end up preferring their own home-made demos to the remade ‘broadcast-quality’ versions. Engineers and performers, it would seem, so often have competing interests: clarity vs. ‘vibe’. This is understandable, given that engineers don’t want to be misjudged as incompetent and performers don’t want to promote records that aren’t representative of their aesthetic sensibilities.
Noise, distortion, and technical limitations were once inseparable from the studios where the roots of today’s popular music arose. As a result, so many of the recordings indelibly stamped into our collective audio consciousness endure, not in spite of their technical limitations but, in no small way, thanks to them. What was once necessity now informs purely aesthetic choices. Sounds that were once undesirable by-products of makeshift studios and self-trained recordists have come to signify a ‘legitimacy’ and ‘outlaw’ chic, like faded pre-torn denim in a upmarket boutique.
Making mic bleed your friend
A much overlooked aspect of the recordings of yesteryear is ‘overspill.’ Limited available tracks on tape recorders meant that performers had to play the bulk of their music together in close vicinity. Despite the use of baffles, a high degree of mic ‘bleed’ was always present, and consequently gave these records a sound that so often eludes those who would recreate it. Like so many things, however, there is good spill and bad spill.
In the case of recording using a single microphone, judgements can be made relatively easily (given sufficient time). Plug in, move the mic – and/or the performer – around to hear the variety of sounds on offer. Try different rooms, different mics and variations at the source (different amplifier settings, different plectrums, strings or drum sticks). Maybe drape tea towels over drums, or put foam dish-washing pads under bass strings. Ask performers to sing or play softer or louder to change their timbre.
When a second mic is added, it should be judged together with the first mic still open, so their blend can be heard from the get-go. In this way, overspill is simply ‘factored in’ for positive effect rather than eliminated.
If it sounds good, it is good.
If such a scenario seems daunting and has you running back to the safety of your samples collection, then take solace in the knowledge that addressing such issues empirically is a reliable strategy. Even though experience will eventually inform time-saving ‘go-tos’ and a sense of where best to look for sonic bliss, one shouldn’t forget that only hearing is believing.
Oh, and by the way, depending on the type of audio feedback that the performers are receiving – for e.g. headphones, no headphones or live PA-style speakers-in-the-studio (believe it or not, a great many albums have been made using PA-style monitors: listen to Elvis Costello’s ‘Blood & Chocolates‘ or Talking Heads’ ‘More Songs about Buildings and Food‘ as cases-in-point) – not only will bleed be a factor, but their performance, articulation and timbre will vary as a result.
Watch this informative video from ‘Sound On Sound’ magazine to hear how microphone placement, and the number of microphones used, influences recording and performing. Whether you use only one microphone (or one hundred) on your next project the issues raised herein will be pertinent. Enjoy.
Dr Marshall Heiser 