For the first 8 years of my career as a live sound engineer I thought that the more high-pass filters I used, the better protected I would be. More is better, right?
I would put a high-pass filter on my console outputs, on my DSP, and on the speakers if they had built in processing.
I didn’t realize it at the time, but the result was a super steep roll-off into a gap crossover. All I knew was that it sounded strange, but at least I was playing it safe.
Why is this happening?
The filters you insert at each point in the signal chain get burned into the signal. They are permanent. Once you reduce a portion of the frequency response so low that it goes into the noise floor, you can’t get it back.
Witness the effect of a high-pass filter at 120Hz.
Then when I try to reverse the effects with a low shelf.
Here’s the result of two matching high-pass filters in line with each other.
And now three high-pass filters in a row.
And what if they are asymmetrical, as is probably the case between your console, DSP, and speaker from different brands?
Check out the result of a 24dB/oct Linkwitz-Riley into a 12dB/oct Butterworth into a 48dB/oct Chebyshev.
And that’s when they all have matching frequency settings. Many times in the past I had thought, for one reason or another, that one filter should be lower and another higher.
For this reason it’s best to leave system calibration filters until the end of the signal chain, right before the speaker. That way you can get the job done without a bunch of unnecessary steepness and phase delay.
- Input channels. I should make it clear here that I’m focusing on output channels. When it comes to input channels I am much more liberal with the HPF. Put’em on every channel and crank’em up as high as possible.
- Either you don’t have access to the DSP or the filters are baked into the speaker (as they often are), but you want to change the filters. You can’t put sound back that’s been taken away, but you can make the filters steeper and/or raise the frequency in the example of a HPF. Proceed with caution and measure the acoustic results.
- Either you are very experienced or you know what you’re doing, as with any rule.
My friend David uses a common setup: DSP into a powered speaker with more onboard DSP. In this case it is the DBX Driverack into a Turbo IQ12 and Yamaha DXS15XLF.
Here’s how he has the crossover filters set up:
- Out1: HPF 110Hz LR48
- IQ12: HPF 120Hz LR24
- Out2: LPF 100Hz LR48
- DXS15: LPF 110Hz LR24
I didn’t measure each step in the chain individually. Let’s look at the result electrically instead.
If a gap crossover is your goal, you acheived it, but in my case I was often doing this on accident.
I know the acoustical result is really what we care about here, which is what you saw at the beginning of this article.
After David and I had a chat about what he wanted to achieve he decided to remove some filters and move their crossover frequencies. Here’s the result. There’s still a gap, but now it’s less violent.
For more on how to identify crossover slopes, please read Spot Crossover Slopes in Smaart® and Avoid Falling Sharply.