Real World EQ
by Jeff Towne
Are your audio tracks not sounding quite right? There are several different types of EQ and each kind, shelving, parametric, bandpass, can help you in different ways. If you want to shape the tone in broad strokes, if things just seem too bright or too boomy, or conversely, too dull or thin: try boosting or cutting a few dB with a low or high shelf. It’s kind of like messing with the bass and treble controls on your stereo, although most shelving EQs will give you more control over which frequencies you are adjusting. If there’s a particular frequency bump that’s bugging you, try cutting it back with a parametric. And if there are entire ranges of sound that you’d like to remove, try high-pass or low-pass filters. Some combination of these will likely get you closer to what you’d like to hear. But be careful, too much EQ can degrade the sound, introducing phasey artifacts or metallic ringing. And first, try cutting a few dB rather than boosting, you’ll often get where you need to be faster, and you’ll have less chance of overdriving the sound. Remember: adding EQ can dramatically change your overall level, so be careful that the changes you make don’t cause the sound to overmodulate the track level or the master mix.
Here are a few places to start: if there’s low rumble interfering with the intelligibility of your interview, perhaps from traffic, machinery, wind, mic-handling noise or vibrations, try a high-pass filter. The name is a bit counterintuitive, because we’re adjusting the low frequencies, but remember, we’re letting the high frequencies pass (it might be clearer to call it a “low-cut” filter, but engineers made the rules and we’ll have to play by them.)
The offending low-frequency element of the sound could start to sound very weird at extreme settings, so try to find a balance between clarifying your sound and retaining a natural timbre.
A low-pass filter can go a long way toward reducing the effect of wind or plosive distortion from P-pops. It won’t remove them completely, but the sound can be improved by reducing the rumble that results. Instead of filtering the entire soundfile by inserting the EQ on the channel, try selecting only the offending segments and running the high-pass on the rumbles only, as a destructive pass (in ProTools, select a process from the Audiosuite menu.) If your editing program allows it, try processing a bit more than you need to before and after the target, and crossfade in and out of the processed sound.
Low-Pass filtering can be very effective in reducing tape-hiss or other high-frequency irritations. Just as with High Pass filtering, the frequency one selects isn’t a brick wall, rather the point at which the roll-off begins. One can often start rolling off high-end at 8-10 khz without destroying clarity. But be careful: the consonants so important to intelligibility have information up high, so don’t be too brutal. You’ll find Low-Pass filters much more effective at reducing hiss than cutting with a high shelf, but sometimes too severe. If the resulting sound seems a bit dull, try putting a little edge back in with a fairly narrow parametric EQ at 3-4 khz. The two filters will be fighting each other, but you can punch up the “s” and “t” sounds a bit with the parametric while getting the general high-end reduction you need. If the sound is still too dull, raise the cutoff frequency, or make the slope of the curve more gentle.
While it won’t make a hum disappear, sometimes a high-pass filter can make
it less obvious. Start with a roll-off at about 150 hz and adjust up and
down to find a balance where you get hum reduction without thinning your
sound excessively. Great advances have been made in noise reduction software
that can “learn” a steady-state noise and remove it from a soundfile. These
processes can often be more effective than standard EQ, although artifacts
resulting from these calculations can be unpleasant as well.
Phone transmissions have almost all of their energy in the range between
400hz and 3,500 hz, and so any signal above or below those frequencies is
likely to be noise. Try using high and low-pass filters to eliminate
rumbles down low or hiss up high. The larger problem is the signature
shrillness of phone audio. In order to optimize voice intelligibility,
phones are designed to hype the frequencies from about 1,000-3,000 hz, and
so reducing those frequencies a bit can make phone audio less harsh. Each
phone and voice will be a bit different, but try applying a parametric EQ
and cutting a few dB at about 1,000-2,000 hz. Sweep the center frequency up
and down until you find the best spot to reduce. But remember that phones
will still sound like phones, large ranges of frequencies have been thrown
away in order to have efficient transmission down a cable, so no amount of
EQ will make it sound as if your subject is speaking into a studio
microphone. That bright EQ peak was designed to make voices easier to
understand, so don’t reduce too much, but rolling back some of those
high-mids can reduce the harshness.
Wider parametrics (low Q values) can be very useful in boosting or cutting a range of frequencies: if your sound feels a bit thin in the middle, give it a couple dB at 400 hz; if it seems a little honky, pull out a few dB at 2 khz; if there’s a bass resonance from a small room, cut a little around 100 hz. Sweep the center frequency around a bit to find the right spot, and be gentle, a couple dB will do a lot.
And keep in mind that applying EQ is kind of like laying cinderblocks on a waterbed: cutting one frequency will make the surrounding frequencies seem louder, boosting one band will make the others seem to drop. Sometimes the best way to make something brighter is to cut some bass rather than adding treble. Rather than boosting several frequencies, try cutting in the area you were not going to boost, and raising the volume overall. It’s easy to start piling on multiple EQs and end up with something even murkier that what you started with, so try to be as gentle as possible, and remember that all the frequencies are interacting, so what you do in one range will affect the others as well.
~ Jeff Towne
Transom Tools Editor