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In The Studio: Microphone Leakage—How It Sounds & How To Control It

Methods for tightening recorded sound by increasing the isolation between mics (includes audio samples)

By Bruce Bartlett November 7, 2017

The Sound Of Leakage

Here’s the drum mic alone: Listen

Now, the leakage mic alone, 5 feet from the drum mic: Listen

You can hear how the leakage mic sounds muddy and distant compared to the close-up drum mic. That’s because it’s picking up a lot of early reflections from the studio surfaces.

Let’s mix the drum mic and leakage mic that is located 5 feet away: Listen

Figure 2: Leakage test setup.

The mix sounds more distant than the drum mic alone. Play the drum mic alone to hear the difference.

Effect Of Mic Separation

What happens if the leakage mic is 10 feet away instead of 5 feet? Let’s compare.

Here’s the drum mic alone: Listen

Here’s the drum mic mixed with the leakage mic 10 feet away: Listen

Moving the leakage mic farther from the drums did not make a big difference (in this studio at least). That means we can place performers fairly close together and still get a good recording. Sometimes it provides a tighter sound than if the players are far apart, because the leakage has a shorter delay.

Effect Of Close Miking

Now let’s listen to the effect of moving the leakage mic closer to its instrument. If we reduce the miking distance by half, that increases isolation by 6 dB because we can turn down the mic’s signal by 6 dB.

Here’s the drum mic alone: Listen

Here’s the drum mic mixed with the leakage mic 5 feet away: Listen

And here’s the drum mic mixed with the leakage mic turned down 6 dB: Listen

As you can hear, moving the leakage mic closer to its source (and turning down its level) makes the drum sound tighter or drier. In other words, we can make the drums sound tighter by miking everything else more closely.

Effect Of Signal Alignment

Note that the leakage mic’s signal is delayed compared the drum mic’s signal due to the travel time of sound through the air from source to mic.

As an experiment, I aligned the leakage-mic signal with the drum-mic signal. That is, I slid the leakage mic’s track earlier in time so that the drum mic and leakage mic were heard exactly together. Let’s compare:

Here’s the drum mic mixed with the leakage mic without aligning their signals: Listen

Now here’s the drum mic mixed with the leakage mic with their signals aligned: Listen

It didn’t make much of an improvement, if any. Although the two tracks are aligned in time, the leakage mic is still picking up wall sound reflections which make the perceived sound distant.

Signal alignment can work well on multiple drum tracks because the signals are relatively dry.

For example, suppose you’re using two overhead mics on the kit. Place them equidistant from the snare to produce a more coherent, centered image of the snare drum. Or slide the cymbal tracks earlier in time so their snare leakage aligns with the snare-track hits.

Figure 3: Test setup with a gobo in place.

Effect Of A Gobo

Next I set up a 4-foot high, 8-foot wide padded plywood gobo in front of the drum kit (Figure 3). Although low frequencies can diffract around the top of a gobo, the mids and highs are attenuated, reducing leakage.

Here’s the drum mic alone: Listen

Here’s the drum mic mixed with the leakage mic, and with the gobo in place: Listen

The gobo does a good job of isolating the drums, at least at mid-to-high frequencies. Figure 4 shows the frequency response (sound attenuation vs. frequency) of a 4-foot high gobo.

The Sound Of Leakage On Cymbals

I also recorded a cymbal using the drum mic and the leakage mic.

Figure 4: Attenuation vs. frequency of a 4-foot high gobo (approximate).

Here’s the cymbal picked up by just the drum mic: Listen

Here’s the leakage mic 5 feet away: Listen

Here’s the drum mic mixed with the leakage mic at 5 feet: Listen

Here’s the drum mic mixed with the leakage mic at 10 feet: Listen

As you can hear, the leakage mic 5 feet away colors the tone of the cymbal, but the leakage mic 10 feet away has less coloration. That’s because the highs diminish with distance in a room.

I hope that these audio examples have demonstrated the effects of leakage, and demonstrated the effects of various methods to control it. Good luck in your quest to tame leakage.


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About Bruce

Bruce Bartlett
Bruce Bartlett

Recording Engineer
   
AES and SynAudCon member Bruce Bartlett is a recording engineer, audio journalist, and microphone engineer. His latest books are Practical Recording Techniques and Recording Music On Location.
http://www.bartlettaudio.com

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Virgil Segal says

Awesome article. Thank you Bruce!

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