DV Info Net - Motor sound?

Removing hum and noise from audio tracks
How many times have you shot that perfect clip only to notice that you hear a computer whirr or refrigerator fan in the background? Or even worse, during some very introspective and moody dialog, you're distracted by a 60- (or 50-) cycle hum from improperly grounded equipment that made it onto tape. It's a well-accepted maxim that even the most professionally shot video can be ruined by poor audio, so let's look at some ways you can address noisy hums and buzzes in your audio tracks.

Ducking levels
If there's little dialog or required audio, then you can often get by with ducking the levels (aka hiding the nasty bits) during periods of silence.

Use the mixer and automation to simply lower the track volume between words and speakers, and then raise it again when there's dialog. Since the noise you're ducking will still be audible during the dialog—you're just taking attention way from it during periods of silence—it could sound strange to hear the noisy hum return when people are talking. In these cases, try leaving a little bit of the noise in the silent/non-speaking parts of the clip.

Alternatively, if you can record clean audio with another device from another, quieter location, you can mix that into the silent parts to help take attention away from the ducking.

Parametric equalizers
The audio spectrum, from the lowest bass frequencies to the highest treble frequencies, is represented in Hertz (Hz). The human voice tends to fall between 85Hz for a low bass and 255Hz for sopranos, while music instruments fall both below and well above this range.

Equalizers are simply tools that allow you either to raise or lower the volume of specific frequency ranges while not affecting others, exaggerating—or removing entirely—certain ranges of frequencies. Most audio applications include some form of parametric equalizer—as an audio effect, plug-in, or feature. But if you're not an audio engineer or well versed in sound reinforcement, they're not very intuitive. Fortunately, there's not much to them at their roots.

Basic parametric equalizer in Adobe Audition

A parametric equalizer simply lets you specify a range of frequencies and either raise or lower the volume of tones that fall within that range. In Adobe Audition® the controls of the parametric equalizer are as follows:

Frequency: the target frequency
Amplitude (or gain): the amount you'll cut or boost the selected frequency
Width (or Q): the width of frequencies that will be affected around your target
Note: Although the terminology used in your software or hardware unit of choice may be different, these three functions are essentially what define a parametric equalizer. Some cheaper/lower-quality parametric equalizers do not include a manual width control, however.

In Adobe Premiere® Pro, the Parametric Equalizer plug-in is actually a group of parametric equalizers that you can turn on or off selectively, to affect not just one particular frequency but up to five frequencies at the same time.

In this case, we want to lower the volume of a single frequency, say the whirr of a computer's cooling fan:

Enable the Parametric Equalizer and, next to the Frequency Bands, click the Band 1 check box (make sure the others are unchecked).
Find both a spot in your audio track that's mostly silence and a prominent amount of the noise that you want to remove.
Set the Parametric Equalizer controls as follows:
Q/Width control to 100 (so the equalizer will focus on a very narrow range of frequencies)
Amplitude control to roughly 75% of its highest value
Click the Preview button to play the audio file, and then sweep the Frequency Band 1 slider repeatedly from its highest to lowest range and back again. As you do this, at some point you'll notice the fan noise that you're trying to remove jumping out in the mix and getting quite loud. This means you've found the target frequency.

Pull the Band 1 Amplitude slider down carefully until the fan noise stops offending you. Watch the main graph closely, too; if you push the amplitude up or down too far, you'll notice it has a bell-curve effect on frequencies around it. Make sure to keep that spike as narrow as possible so that you remove only the tones you don't want.

You can fine-tune things by slightly adjusting the Q/Width control, to make sure you're removing only the tones you don't want, and by slightly adjusting the Frequency control, to make sure you're dead-center on the offending fan noise. With Audition's five-band parametric, you can work on up to five different problem frequencies. Just watch the graph and make sure that closely adjacent equalizer peaks don't collide and affect surrounding tones more than necessary.

Always keep in mind that you're cutting out frequencies with this technique, so be sparing with both the Amplitude and Width/Q controls—and be prepared to make some compromises to get the best overall results. There's no substitute for cleanly recorded audio, and any trick like this that attempts to clean things up after the fact will be inherently destructive in nature. Use it cautiously, and let your ear be the final judge.