Reaper’s bundled ReaEQ has a tick box to display the phase‑shift resulting from the frequency boosts/cuts you dial in. Here, two instances are used to apply a low shelving cut of the same amount at the same frequency. But they have a different slope (the bandwidth setting is different). The resulting phase shifts are displayed as orange curves.
When processing a stereo drum bus, does it matter what slope I set a low shelf EQ to? I have both 6 and 12 dB/octave options available.
Mike P via email
SOS Reviews Editor Matt Houghton replies:Well, obviously the different slope will have a different sonic effect, since it means you’re boosting or cutting different amounts at different frequencies. For high‑/low‑pass filters in particular, a steeper slope can result in an audible ‘bump’ — a resonant peak (think of it like the Q or bandwidth setting of a regular bell EQ band) that may or may not be helpful. For example, it can be used to add weight to a kick drum. In some EQs you can adjust the bandwidth of a shelf for a similar effect.
But other than its direct effect on the source, the choice of slope for any shelving EQ or low‑/high‑pass filter shouldn’t have a huge impact unless you have bleed from other instruments on the recording, such as when working with a multi‑miked drum kit. Let’s assume we’re talking analogue‑style minimum phase EQ (linear‑phase ones present a different set of pros and cons). The steeper the slope, the greater the phase‑shift the EQ introduces, although the amount of boost/cut will cause greater differences to the phase. Whatever the cause, if you change the phase of the bleed relative to the same source captured in other mics, the multiple tracks involved can interact differently when they’re mixed.
For instance, if you have dedicated spot mics for individual cymbals as well as a more general overhead pair of mics, then EQ’ing the spot mics or overheads can change how the cymbals sound when the signals are all summed. However, any issues you encounter are more likely to be problematic when EQ’ing the low end, because of the longer wavelengths (slight misalignments can cause more prolonged interference); a low‑shelf EQ or high‑pass filter on the overheads can easily cause a noticeable change in the kick’s tonality.
Don’t just solo the kick and EQ that, but EQ it while also listening to the rest of the kit.
EQ’ing a stereo drum bus is rarely problematic, as you’re applying an identical phase‑shift to all the mics. That said, if you’ve captured a full band in a single‑take recording, with vocals, bass, guitar etc leaking onto the drum mics, it could impact on those instruments’ tonality. It’s hugely important, then, that you audition your EQ moves in the context of all the sounds involved — don’t just solo the kick and EQ that, but EQ it while also listening to the rest of the kit.
In short, there’s no inherent right or wrong choice. Gentler slopes are less likely to cause significant problems, and are thus worth trying if you encounter a problem. So too is changing the corner frequency of the filter — in the overhead example, rolling the overheads filter up just a touch more might well restore the desired tonality. If you can no longer hear a problem, it doesn’t exist!
Finally, since I mentioned linear‑phase EQs above, these can also be worth trying when you encounter problems — their whole raison d’être is to boost or cut frequencies without changing the phase. There is a trade off, though, which is known as ‘pre‑ringing’. It’s a short echo that happens before the wanted sound. It’s only rarely an audible problem in my experience, but when it is, you’ll often perceive it as a slight smearing of transients that can rob, say, a snare drum of a little of its snap and impact.