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Recording Studio Tips: Minimizing Mic Leakage
Joe Albano on Sun, April 9th | 0 comments
Interference in your mic recordings can come from a bewildering array of different places. Joe Albano explains the most common culprits, and how to avoid them.

Commercial recording studios often invest considerable time and money ensuring that the spaces they use for live recording are thoroughly isolated from unwanted sound of all kinds. Small noises from inside the recording space, as well as loud intrusive ones from the world beyond the studio walls, can’t be allowed to leak into the microphones - especially vocal mics - potentially ruining that perfect, unrepeatable take.

Even though audio repair software can often work wonders after the fact, it’s still always better to shoot for the cleanest recordings possible from the get-go. Of course, nowadays a lot more recording is done in much smaller, less formal locations - everything from customized, tricked-out personal studios to budding artists’ bedrooms can serve as a recording space, primarily for vocals. These spaces often have little or none of the big-ticket isolation employed by larger rooms, and, in most cases, probably little or no budget to devote to this issue. But there are still steps that can be taken to help isolate your studio mics from unwanted leakage. Here are a few typical issues and suggestions for dealing with them, for smaller-studio operators. 

Inside job

Besides the obvious noise sources like planes, trains, and automobiles, one source of potentially problematic leakage, from right within the recording space itself, is...you! OK, maybe not you personally, but anyone in the studio who’s in the room with the mic during a take, from the engineer and producer to the artists themselves, can often generate enough sound to create problems and headaches for the editor later on. Typical studio condenser mics - even the relatively affordable ones - are much more sensitive to small sounds than your average stage mic, and you may be surprised at what they’ll pick up. 

 

Some typical (sensitive) studio condensers, ranging from a few hundred to a few thousand $$$$

Some typical (sensitive) studio condensers, ranging from a few hundred to a few thousand $$$$

After the recording is done and compression is added in the mix, small sounds that would normally go unnoticed in the room at the time are often suddenly emphasized to a surprising degree, somehow managing to stick out even in a busy arrangement, distracting from the music and requiring time and effort away from more creative tasks to deal with. 

Be on the lookout for quiet sounds that might be enhanced to the point of distraction later. I’m not even going to mention recording-related issues like headphone leakage, which we all know to avoid: this time I’m just going to focus on sounds that are extraneous to the actual recording process. Things like creaky floorboards, lightly rattling or jangling jewelry, and whirring motors (in computer gear or even nearby appliances) are all likely culprits.

I even had one voiceover session where the talent had skipped breakfast, and during a very soft voiceover performance, the barely perceptible sound of her stomach rumbling managed to elicit a host of “what is that sound?” complaints from the producers during playback - we had to feed her a healthy breakfast to be able to continue with the session (I guess it really was the most important meal of that day). It’s always worth the time to go out onto the live room and listen carefully for any potential problems, and put on headphones and spend a few minutes examining what the mic is picking up, before getting the session rolling full steam. A few minutes' attention to the little things can forestall a big editing headache later on.

Extraneous sounds often end up leaking into recordings simply because the engineer didn’t pick up on them ahead of time. Many of these small noises can be dealt with with just a little forethought. Have the artist remove any jangly jewelry (especially if he/she moves a lot during takes); make sure everyone in the vicinity of the live mic has turned off their phones and such before hitting record; map out and avoid creaky floorboards; put any computer or peripheral gear with audibly whirring drives or motors in a closet of cubby, and watch out for sounds that emanate from the performer while they’re performing, like rustling clothes or unconscious foot-tapping. It’s easy enough to avoid lots of these nuisance noises, by just anticipating a little beforehand, and then keeping one ear open during takes for any subtle sounds that don’t belong.

Rattle & hum

More obvious (and often persistent) in-the-room sounds may be more easily noticeable, but often harder to deal with. Room air conditioners, and even blowers in split systems, can be a major source of unhappiness. Obviously, the effort (and money) put onto securing a quiet HVAC system initially is a very worthwhile investment, as is the advice to have a pro install it, but even then you still need to make sure they know just how quiet “quiet” needs to be for a studio space, even a part-time one. One of the many small ductless split systems could be a good choice—the noisy compressor outside, coupled with a low-velocity blower, which minimizes the sound of rushing air.

If you’re adventurous enough to design and install an HVAC system yourself, watch out for ductwork (especially metal ductwork) that can transmit sound like a foghorn. Make sure to insulate the ducts from noise transmission—build/buy and install duct silencers—these are bends in the ducts that break up unwanted sound transmission, while still permitting airflow.

A window AC (L) vs a typical small split system (R); silencers for HCAV ductwork (bottom)

A window AC (L) vs a typical small split system (R); silencers for HCAV ductwork (bottom)

If you’re at the session, and stuck with what’s there (like a typical window AC unit in the room), you may have little choice but to shut it down during takes, manually running it imbetween, to keep the environment livable. And don’t count on audio repair software to “learn” and eliminate the sound afterward - even if that can get the job done, it’s extra time and effort, which can be a drag on session momentum.  

Outside agitators

Of course, in all but the most well-isolated studios, it can be hard to keep sound from the outside world from intruding, creeping into the studio mics, typically at the most inopportune moments. The obvious villains are traffic, (especially trucks), sirens (which seem to go on forever when you’re waiting to resume a take), and in my urban paradise, helicopters (especially those news choppers that hover overhead whenever there’s a public event nearby). Naturally, any openings can be trouble spots. When it comes to outside windows, double-paned designs provide better isolation then older, cheaper single-paned ones. Make sure closed windows seal well enough, and if not, try applying some temporary sealing to the cracks - even the tiniest openings can admit a surprising amount of sound. If there’s an AC unit installed, even if it’s not running, that can be a big source of leakage. Those thermal covers you use during the winter can also help with sound, at least a little. Watch out for the sound of rain on the outside part of a window AC unit: that random pitter-patter can be hard to edit out later; try putting a blanket or towel over the exposed (outside) metal surface of the AC which should help reduce that particular noise sufficiently. 

Doorways can be even worse, even doorways to adjacent, noisy interior spaces. Hollow-core doors should be replaced with solid-core doors, and pay attention to the airspace at the bottom. If necessary, stuff towels or blankets into that area (just pretend you’re back home as a kid, or smoking in your dorm room!).  

The mic is your friend

Another obvious way to at least minimize mic leakage is to utilize the mic’s directionality to reject unwanted sound. Most studio mics have at least one directional pickup pattern, usually cardioid, and that in conjunction with other isolation steps can often help quite a bit. Get to know the directional pattern of your mic, and rather than angling the mic arbitrarily (so the performer has an “inspiring view”, or whatever), set up the artist and mic so that its rejection is pointing in the direction of whatever unwanted sound you’d like to avoid. 

Various mic patterns (omni, bi, cardioids)

Various mic patterns (omni, bi, cardioids)

Remember, cardioid mics have maximum rejection at 180° to the rear, while supercardioid and hypercardioid mics have maximum rejection at little to the side of that. Bi-directional mics may have even better rejection at the sides (90° and 170°), but don’t forget that they have full-strength pickup both front and back (180°). Sometimes a Figure-8 pattern will let a mic be angled so that it does a better job than a cardioid of rejecting sounds generated by the artist, like the rustling of sheet music, or a singer/guitarist’s vocal mic picking up the voice while rejecting more of the acoustic guitar. For that application, position the mic sideways, angle the front at the singer’s mouth, and one side at the guitar - just be mindful of what the rear (180°) is pointing at, like a overly reflective surface or a poorly-sealed doorway.  

Blankets, & baffles

Putting up a baffle, even a makeshift one, around the artist/mic can sometimes help quite a bit, especially with higher-frequency sounds. A thick blanket (like a packing blanket, if available) hung on a wire, or a couple of old mattresses propped up around the artist/mic is the traditional, if lowbrow solution, and it can help a little. A more elegant approach may be those commercial mic baffles that have become popular.  

Some commercial microphone baffles

Some commercial microphone baffles

They’re really more for controlling sound in the room than isolating it, but they can do a pretty decent job in some cases. These solutions may work decently with high and high-mid-frequency noises, but lower mids and lows, with their longer wavelengths, easily travel around smaller boundaries. Larger baffles - referred to as “gobos”, can be quite effective. Taytrix offers a line of lightweight portable, stackable gobos that offer a surprising degree of isolation when assembled into a makeshift “room”. 

Portable baffles (gobos) from Taytrix, arranged into a makeshift iso-booth

Portable baffles (gobos) from Taytrix, arranged into a makeshift iso-booth

But for the most effective overall isolation, the final option is to bite the bullet and get a room!

Think inside the box

If all else fails, there’s always the option of buying or building a full iso-booth. Commercial designs are common, though fairly pricey (at least for anyone on a tight budget, and who isn’t!) at a few thousand or so.  Finished designs range from tight phone-booth-size boxes (4x4) to larger, roomier models, and most include (at least options for) wiring, a window, and some kind of ventilation system.

Some commercial iso-booths (top); a home-made vocal booth (bottom)

Some commercial iso-booths (top); a home-made vocal booth (bottom)

You can save significant $$$ if you build your own, which is easy enough to do with standard power tools and building materials (studs, drywall, insulation). There are plenty of books available with plans and instructions, and the savings can be substantial - just be prepared for it to take longer (typically a lot longer) than you planned. 

Wrap it up

I’m out of space, so I’ll end with one last reminder. The best way to avoid big or small leakage issues is by doing a little due diligence beforehand, and keeping one ear open during sessions. Better yet, if possible have an assistant or a trusted intern worry about noise and leakage, so you can focus on technical and musical concerns, without compromising the integrity of the recordings. 

Learn more about recording and other audio concepts in the Ask Audio Academy here. 
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