In any multiple-mic setup, sound from an instrument travels to the nearest mic, and also leaks into the mics intended for other instruments. It could be the snare sound picked up by a floor-tom mic, or the drum set sound picked up by a scratch-vocal mic. Leakage also comes from sound reflecting off room surfaces. Leakage becomes a big problem when you record loud instruments and quiet instruments simultaneously. Because you typically need more gain on the quiet instruments’ mics, you hear a lot of leakage or “off-mic” sounds in their signals.
For certain types of audio productions, it’s very important to minimize leakage, and ensure each mic picks up only its intended instrument. Ideally, you want each mic to pick up only what it is aimed at. Maximizing isolation among mics has several benefits. For one, the drum mix becomes tighter and clearer. Also, if you need to punch-in to correct a mistake, good isolation will reduce ghost drum notes that appear on other tracks due to leakage. Another benefit of isolation is that it prevents the coloration caused by phase interference between mics. Here are ten ways to keep leakage under control.
Mic each instrument closely so that the sound level at each mic is high. The farther a mic is from an instrument, the more ambience, leakage, and background noise it picks up. Close-miking positions can help reject these unwanted sounds. Be aware that close miking can color the tone of a recorded instrument, and this can’t always be fixed with EQ.
Do you hear drums in the acoustic guitar track? It might help to mic the guitar a few inches from the soundhole. The tone at that position is very bassy, so cut low frequencies on your mixer EQ until the sound is natural. This way, you can reduce leakage and improve tone quality at the same time. Also, cut a few dB around 3kHz to reduce the harshness often caused by close miking.
Is the scratch-vocal mic picking up a lot of drums? Ask vocalists to sing with their lips touching the microphone grille (or its foam pop filter). Turn down excess bass—and minimize plosives—by cutting 6dB around 100Hz or 200Hz.
Overdub One Instrument at a Time
This is the most effective way to prevent leakage. You might track all the loud instruments first (drums, bass, electric guitars), and then overdub quiet instruments such as acoustic guitar, vocals, etc.). Overdubbing lets you mic sources from farther away if you wish for a more natural sound. A drawback of this method is that you lose the interaction between musicians that occurs when they all play together.
Try recording acoustic guitar with a pickup, which provides much more isolation than a mic. Also, consider playing electronic drums instead of acoustic drums.
Filter out frequencies above and below the spectral range of each instrument. For instance, remove the highs above 9kHz on the kick drum to reduce leakage from cymbals. Cut frequencies below 75Hz on instruments and vocals (except for bass and kick drum). You might roll off the lows below 500Hz on cymbals if the overhead mics are intended to pick up only the cymbals, and not the entire set. If your mixer or DAW has a sweepable low-cut or high-pass filter, you can reduce a fair amount of low-frequency leakage in the mix. Solo each instrument. Start with the filter frequency very low, and then gradually turn it up until the sound starts to become thin. Then, back off a bit.
Assign a noise gate to each drum track, and set the gating parameters so that you hear each drum hit clearly, but do not hear other drums or cymbals. Make the gate attack time short enough to preserve the attack of the drum, and make the decay time long enough to let the toms ring.
Narrow Your Focus
Use directional mics, instead of omnidirectional mics. An omni mic pattern picks up sound equally well from all directions, so it also picks up a lot of leakage. In contrast, a directional mic pattern—cardioid, supercardioid, or hypercardioid—tends to focus on the sound source at which it’s aimed, and partly rejects sounds to the side and rear of the mic. Aim the “dead” side of the mic at instruments you don’t want to pick up. To reduce hi-hat leakage into the snare mic, for example, mic the snare closely, and then aim the snare mic away from the hi-hat. (You might even use a de-esser on the snare, or choose to overdub the hi-hat.) Here are the null points of each polar pattern, where sound is rejected the most (Figure 2):
- Cardioid: 180 degrees (the rear of the mic).
- Supercardioid: 125 degrees.
- Hypercardioid: 110 degrees.
- Bidirectional: 90 degrees (the side of the mic).
To reduce drum set leakage into a scratch-vocal mic, aim the null point of the vocal mic at the drums. For piano, remove the lid, and mic with two bidirectional mics with their side nulls aiming at the drums.
Record in a large, fairly dead studio. In such a room, leakage reflected from the walls and ceiling is weakened by the sound’s attenuation via its distance, and by sound-absorbent treatments on the room surfaces.
Enclose & Cover
Put portable walls (gobos) between instruments, or use isolation booths. Clear Plexiglas panels or thick wooden panels around the drums are options. Use gobos as a last resort, however, because low-frequency sounds bend or diffract around gobo edges, causing muddy-sounding leakage. If your piano mics are picking up the drums, close the piano lid, and cover it with a moving blanket to keep sound out. The closed cavity in the piano makes the sound boomy, so compensate with EQ by cutting 1dB or 2dB around 250Hz.
Place the instruments closer together, so that the leakage does not sound so distant. This seems counter-intuitive, but reducing the distance between the musicians often can decrease apparent leakage. That’s because the leakage sounds clean, due to the lack of room echoes in the signal bleed.
Have the drummer play softer—or, at least, at the same volume level as the other musicians. You can also suggest that drummers play with brushes instead of sticks when it’s musically appropriate.
All that being said, sometimes a little leakage is a good thing. It can add a live, airy feel to your mixes. If you want this effect, simply place your mics a little farther away. To prevent ghost notes caused by leakage, don’t punch in. Instead, record several performances, and edit together the best parts of the takes.