Last month, we discussed the basics of filters and examined in detail the concept and application for high-pass filters (FRONT of HOUSE, Nov. 2011, page 41). This time we’ll check out low-pass, band-pass and band-reject (a.k.a. “notch”) filters.
Low-Pass
As the name implies, a low-pass filter cuts highs. Fig. 1 shows RTA of pink noise in Metric Halo Labs’ SpectraFoo software. You can see that the frequency response is reasonably flat — from 30 Hz to 20 kHz. If we apply a low-pass filter to the signal we get Fig. 2. This particular low-pass filter is set for a corner frequency of 1,000 Hz (note that the 1 kHz band is down 3 dB) and a slope of 18 dB per octave. Changing the slope to 6 dB/octave gives us the curve shown in Fig. 3, which, as you can see, allows more high-frequency content to pass through. (The numbers at the top indicate which band has peaked —ed.)
A low-pass filter (LPF) comes in handy when you want to reduce brightness or remove the shrill character of a sound, like taking out some of the bite on a crunchy guitar. It also comes in handy when you have a vocal mic picking up a lot of leakage from cymbals (which happens all the time). If you’re faced with a drummer who hits hard and a singer who sings softly, or a singer who does not stay on-mic, you can get as much cymbals in the vocal mic as you get vocal in the vocal mic. Careful use of the LPF will help reduce this problem, but beware — the overzealous use of a low-pass filter can make instruments sound dull. It’s an unfortunate fact that most hardware consoles do not provide low-pass filtering, which is one of the reasons I always loved the Midas XL4 console (which does). Many digital consoles’ EQ have a high band that can be switched from high-shelf to low pass filter, which is eminently useful.
Band-Pass
The band-pass filter is not used to weed out bad musicians. (If only…). It’s a filter that simultaneously cuts high and low frequencies, allowing a certain midrange band to pass (see Fig. 4). A band-pass filter is actually a combination of high-pass and low-pass filters. Most mixing consoles for live sound don’t have band pass filters, but you will find them in outboard processors, most often in crossovers for creating low-mid, mid- and high-mid outputs for multi-amped systems. Don’t underestimate the value of a band-pass filter on stage, which can easily be created by using high-pass and low-pass filters simultaneously. I have found them useful in situations where you need to cut highs and low on an instrument and leave the midrange untouched, like on a guitar amp.
Band-pass filtering also comes in handy for the side chain of a gate. By filtering out the low and high frequency content of the signal that triggers the gate, you can reduce the tendency for kick drum, toms and cymbals to open the gate. Ditto for the toms, where you can allow only the low-mid band to pass, thereby reducing the tendency of the tom gate(s) to open on snare hits. Another very useful application for band-pass is on a bottom snare mic, where you definitely want the low end removed, but also want to make sure that the high hat does not bleed into the mic.
Band-Reject and Notch Filters
The band-reject filter (no wise cracks about your peers, please…) leaves high and low frequencies untouched, cutting a certain range of mid frequencies (Fig. 5). Band-reject filters are seldom seen on mixing desks, but a severe form of band-reject known as a notch filter can be found in many drive rack processors or EQs intended for use on monitors. Fig. 6 shows a notch filter centered at 1 kHz. Notice the notch in the graph at 1 kHz. Also notice how the adjacent frequencies (800 Hz and 1.25 kHz) are barely touched, and how, by the time we move down to 630 Hz or up to 1.6 kHz, the filter is barely affecting the audio. That’s because notch filters have very narrow bandwidth (bandwidth is how wide a range of frequencies are being adjusted). Most notch filters give you control over frequency and bandwidth. They are generally intended for use in controlling feedback, the idea being to notch out the frequency that is ringing and leave the rest of the audio band untouched. In fact, most feedback eliminators feature “floating” notch filters that are automatically assigned by the device to offending frequencies, thus removing the ring. Notch filters can also be used to slice out a small range of offending frequencies — for example, the high-mids on a female vocalist that can sometimes become shrill.
