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October 13, 2011 / Randy Coppinger

Cascaded Low Rolloffs

When we previously looked at a few techniques for applying rolloffs I suggested that cascading a series of several might be helpful. I use a specific strategy for sounds I want to really sculpt. But for comparison sake, let’s start with an ordinary method.

I like to use low rolloffs (high pass filters) to remove low frequency information I don’t need. I will often start by applying a simple 6dB per octave rolloff and move the corner frequency higher and higher until I hear it start to cut stuff I want to keep. I’ll adjust back down until it’s just below the spectrum I want to hear. But I often find that it doesn’t remove enough below the corner frequency. Most of the time I’ll simply increase the slope of the filter (12dB, 18dB, or 24dB per octave) until I get the desired result.

But for featured tracks — such as lead vocals — I like to keep the gentle 6dB slope and simply use it two more times below the first filter. Here’s how it works: I’ll set that first rolloff. Let’s say for this example I decide to stop at 80 Hz. Then I’ll add another rolloff in series after the first, set for half of the value, which would be 40 Hz in this example. Then I’ll add a third rolloff in series after the first two, set for 10 Hz below the second, 30 Hz in this case. Notice how the volume at the corner frequency changes very little but the lower left area under the white line trims back. The filter is quite steep at that third corner frequency: 30 Hz.

The combined shape of the three filters seems to cut significantly more than a single rolloff with a 6dB slope, but with smoother results than simply increasing the slope. This is especially helpful for filters that have a reciprocal rise above the corner frequency as the slope increases. If all you want to do is cut, and you want a gentle transition, these progressively lower corners on three cascaded low rolloffs work for me. Give it a try and let me know how it sounds to you.

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