Spill is not always the enemy, and for some musical and recording styles it should be embraced as an essential part of the sound and mix. A mix devoid of any spill sounds ultra-clean, and possibly too sterile for some projects. A mix which embraces good sounding spill can sound organic, wholesome, and united.
4.4 EQ FILTERS
An EQ circuit is known as a filter. Different filter types affect different frequency ranges in different ways. Several types of filters are commonly found on mixing consoles, outboard gear, and as plug-ins.
SHELVING FILTERS
A low shelf filter, as shown in Figure 4.1, allows frequencies below a specified cut-off frequency to be boosted or attenuated by up to ±15 to ±18 dB or more. The maximum amount of boost or attenuation depends upon the specific circuit or plug-in being used. The amount of boost or cut can be anywhere between maximum cut, through 0 dB (no change) to maximum boost.
Fi g u r e 4 .1 The straight line at “0 dB” on the y-axis represents “flat,” or “no change” – the filter has no effect on the sound, and nothing is boosted or attenuated. The largest “uphill” plot as you read to the left, represents the maximum boost of +15 dB, below the cut-off frequency of 100 Hz in this example.
The largest “downhill” plot as you read to the left, represents the maximum attenuation of −15 dB below the cut-off frequency. The other plots represent some possible intermediate settings of +6 dB and −9 dB – but remember it is possible to boost or cut by any amount between no change and the maximum boost/cut values.
f (Hz)
25 50 100 200 500 IK 2K 5K 10K 20K +15
+12 +9
+6
+3
0
-3
-6
-9 -12 -15
" f l a t "
dB
“−3 dB” cut-off point, which is 100 Hz.
SLOPES
It is impossible to build a filter that operates like a brick wall – doing nothing to frequencies above 100 Hz, but immediately being maximally effective below 100 Hz.
That type of filter would be musically unusable anyway! All filters have a slope – a range of frequencies over which they become increasingly effective. Even though the filter in Figure 4.1 is described as having a cut-off point of 100 Hz, its slope means that it does affect a broad range of frequencies well above 100 Hz.
A high shelf filter, as shown in Figure 4.2, allows frequencies above a specified cut-off point to be boosted or attenuated by anywhere between minimum and maximum values.
Shelving filters are often switchable between a couple of different cut-off frequencies, or they are sweepable – meaning that the cut-off frequency can be smoothly moved up and down over a range of frequencies. This allows a low shelf filter to boost or cut below a cut-off point of 60 Hz in order to focus on the weight and thump of a kick drum or bass guitar, or that same filter might be swept up to 250 Hz in order to attenuate low and low-mid frequency proximity effect build-up.
Fi g u r e 4 . 2 A high shelf filter. The maximum boost of +15 dB, above the cut-off point of 10 KHz is represented by the largest “uphill” plot as you read to the right. Note that the –3 dB point, the cut-off point, is at 10 KHz and the filter’s slope affects frequencies well below this. The largest “downhill” plot as you read to the right, represents maximum attenuation. Any amount of boost or cut between these minimum and maximum values is possible.
f (Hz)
25 50 100 200 500 IK 2K 5K 10K 20K
+15 +12 +9
+6
+3
0
-3
-6
-9 -12 -15
dB " f l a t "
If a shelving filter has a switchable or sweepable cut-off frequency, it will usually have two controls:
• A gain knob to dial in the amount of the boost or attenuation.
• The switch to select, or knob to sweep the cut-off frequency.
Low shelf filters are the default low frequency (LF) EQ circuit, and high shelf filters the default high frequency (HF) EQ circuit on most traditional equipment and EQ plug-ins.
PEAK/NOTCH FILTERS
As pictured in Figure 4.3, peak/notch filters allow a range of frequencies around a center frequency to be boosted or attenuated by any amount up to ±15 dB or ±18 dB or more, depend-ing upon the equipment or plug-in. Mid frequency (MF) equalization filters are always peak/
notch filters.
Peak/notch filters may also be sweepable – which allows them to really focus on an issue.
Additionally, they may have a bandwidth control, allowing them to affect a wider range of frequencies for broader, more “musical” tonal changes, or a very narrow range of frequencies for more surgical problem solving. The bandwidth controls on many hardware EQ sections often use icons to represent narrow and wide filter widths. Some plug-ins use Q or Quality Factor indications of bandwidth.
BANDWIDTH, OR “Q”
Q, or Quality Factor, is an indication of a peak/notch filter’s musical bandwidth. It is calculated using the following equation:
Q = center frequency / bandwidth
Figure 4.3 A sweepable and bandwidth adjustable peak/notch filter, showing two of an infinite number of possible settings – a wide +15 dB boost around 1 Khz, and a narrow -12 dB attenuation around 2 KHz.
➠
linear, exponential scale, and higher octaves are made up of a much larger numerical frequency range, so to retain a Q factor of 1 when centered around 1 KHz, the same filter must have a bandwidth of 1000 Hz.
• Higher Q factor numbers mean that the filter is narrower, and affects a smaller fre-quency range.
• Lower Q factor numbers mean that the filter is wider, affecting a broader frequency range.
If the bandwidth of the filter above stayed at a constant 100 Hz, it would have a Q factor of 10 if it was centered around 1000 Hz. This would be a constant bandwidth filter – but they are musically useless, so not found on audio equipment.
LOW CUT FILTERS
Also known as high pass filters or HPFs, low cut filters are sometimes found as part of an EQ section or plug-in, or as part of the pre-amp section of a hardware device. In the recording studio they are more of a technical problem solving filter and less of an artistic or musical filter than shelving or peak/notch filters. As shown in Figure 4.4, low cut filters roll off, or progressively attenuate frequencies below the cut-off frequency, by a fixed amount.
Many HPFs only have an in/out button to turn them on and off. Some are sweepable, with an additional frequency control. Some, particularly plug-ins, offer adjustable slopes.
Fig u r e 4 .4 This low cut filter severely attenuates below a cut-off point of about 80 Hz when engaged.
No boost is possible. The filter is either “in” or “out” of circuit – either fully effective, or “off.”
25 50 100 200 500 IK 2K 5K 10K 20K f (Hz)
OFF = "flat"
ON
-6 0
-12 -18 -24 -30 -36 -42 -48 -54 dB