Recording - August 2013 - 64

Confusing? Of course it is, and thus most folks prefer bandwidth. Q settings are
not intuitive, and knowledge of how to calculate quality is practically useless in the
recording studio. This said, I have actually needed to calculate Q a few times in
my life for various tasks. Luckily, a quick Internet search will find you an online calculator to perform the math in a jiffy. (We see “Q” a lot on mixer channels and eq
knobs because it’s easier to write than “Bandwidth”. It doesn’t matter what it’s
called as long as you know what the knob will do when you twist it—whether turning it “up” will give you a broader or narrower peak.)
Unfortunately, not all parametrics employ
the octave system in their bandwidth controls.
Instead, many eqs are labeled in Q, or quality. Q is the “tightness” of a filter, and a higher number refers to a narrower bandwidth. In
this manner, quality values are the inverse of
octave-labeled bandwidth. The following two
equations translate Q from cutoff frequencies
and/or from bandwidth in octaves:

Figure 6. Highpass filter example: –6 dB
per octave HPF at 300 Hz

Figure 7. Lowpass filter example: –6 dB
per octave LPF at 3 kHz

Figure 8. A two octave band pass filter
created with a –18 dB/octave HPF @ 500
Hz and –18 dB/octave LPF @ 2 kHz

Figure 9. Notch filter with a Q of 1 at 1 kHz
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RECORDING August 2013

Graphic eq
Another common example of peaking filters is graphic eq. Graphics contain
many sliders, with each tied to a separate filter and fixed in center frequency and
bandwidth. (See Figure 5). Graphic eq gets its name because its approximate
response curve is graphically visible through the position of its sliders. Available
in a variety of configurations ranging from three to thirty-one bands, graphics are
powerful tools.
Interestingly, graphic eq use is generally avoided in the recording studio. With
so many bands available yet no control over bandwidth, multiple sliders can be
needed to achieve a particular curve. With use of so many controls required,
overuse of the effect, especially by inexperienced engineers, is common. The shunning by the studio world aside, graphics are standard in the sound reinforcement
industry for removing feedback and tailoring system response to room acoustics.
Misuse is commonplace, though, and moderation should be practiced whenever
graphic eq is employed.
Low- & highpass filters
While shelving and peaking equalizers are technically “filters,” the word also
refers to a particular form of eq. Simple in construction, low- and highpass filters
are common in audio electronics. Figure 6 demonstrates a highpass (lowcut) filter,
which progressively reduces the amplitude of all frequencies lower than the cutoff
frequency. Conversely, Figure 7 shows a lowpass (highcut) filter, which increasingly
attenuates all frequencies above the cutoff frequency. Some basic highpass filters
are fixed in cutoff frequency and offer only an on/off switch for control. Often used
to remove low-frequency rumble, these simple circuits are typically found in mic preamps. Slightly more advanced filters offer adjustment of the cutoff frequency.
Common in both highpass and lowpass varieties, these filters are popular for a
variety of corrective and creative applications.
Cutoff frequency is technically defined as the point on the frequency spectrum in
which the filter has reduced amplitude by 3 dB. In more advanced circuits, control
over the slope, or steepness of a filter to and beyond the cutoff frequency, is possible. Slope, which can sometimes be labeled in order or pole, is measured in
amount of reduction in dB per octave. First-order or single-pole filters progressively
reduce energy at a rate of 6 dB per octave. Second-order or two-pole filters
increase the amount of attenuation to 12 dB per octave, and each successive
increase in pole/order steepens the filter by an additional 6 dB per octave.
2-pole (12 dB/octave) and 4-pole (24 dB/octave) filters are common on synthesizers. Slopes up to 48 dB/octave are commonplace in modern digital eqs, and
offer a variety of utilitarian and creative uses. Steeper filters are generally avoided
in recording applications, though, because they exhibit negative side effects such
as ripple, ringing, and increased phase shift. Without going into detail about these
downsides, it’s safe to say that if you like what a filter does in a mix, then you are
doing the right thing! As long as you do not engage processing without using your
ears, and you audibly compare all available filters and settings, you’ll know you’re
achieving the best sound possible.
Band-pass & reject filters
The last filters to mention are bandpass and band reject. Essentially a combination circuit of a high- and lowpass filter, bandpass filters (Figure 8) eliminate all
energy on either side of a center frequency. Creative use of bandpass filters is common on synthesizers and in mixing when lo-fi “telephone effects” are required.
A band-reject filter is the inverse of a bandpass, and deeply cuts a swath of frequencies around a center frequency. Essentially a peaking eq with extremely narrow (high Q) bandwidth, band-reject filters are also known as notch filters.
Primarily corrective tools, band-reject filters are great for removing acoustic feedback, power-line hum, snare ringing, and other distracting frequencies. An example of a band reject filter can be seen in Figure 9.
With the various eq and filter varieties and parameters now introduced,
Fundamentals will continue on next month with a look at eq applications, myths,
tips, and tricks. See you there!

Recording - August 2013

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