This installment concerns the use of high pass filters in the mixing process. I have found this to be one of those mixing basics that is often misunderstood and neglected, yet one of the most important tools found on the mixing console. For those new to audio engineering (and some not-so-new), the high pass filter is typically found at the top of the audio console input channel strips, or as part of the equalizer section. There are two basic types of high pass filters typically found on mixing boards (both analog and digital). These filters are either the “fixed” type or the “variable” type (the former being a single switch, the latter is a variable pot). The basic function of this feature is to filter out all or part of the low frequency information from passing into the mix buss, monitors, and aux sends. Why would you want to do this? Well, there are some very good reasons why you would, and we’ll explore some of those.

First off, a brief technical explanation of the high pass filter is in order. The Wikipedia definition is as follows: A high-pass filter is a filter that passes high frequencies well, but attenuates (or reduces) frequencies lower than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a low-cut filter; the terms bass-cut filter or rumble filter are also used in audio applications.

Some of the confusion surrounding the high pass filter is the fact that it filters the “low” frequencies, but it’s a called a high pass filter! What this means, in essence, is that it allows the highs “to pass” while restricting the low frequencies. The amount of low frequency information that is “attenuated” is determined by a couple of different factors. For one thing, the actual “crossover” frequency that is chosen determines where the low frequency filtering begins to start rolling off, and then the predetermined filter slope determines how steeply the filter attenuates all the frequencies below the chosen center frequency. Most onboard high pass filters are either a fixed 6 db per octave or 12db per octave filter. This means that the further the frequency is from the crossover (or center) frequency , the more it is attenuated. So if a center frequency of 150hz. is chosen, then all the frequencies below this will attenuate at the rate of 12db per octave (75hz. would be down 12 db, 37.5 hz would be down 24db, etc). A more detailed description (with graphs) can be found online. (google “high pass filter”).

Now on to the good stuff. Why would I want to filter out the lows, you ask. Isn’t low end a good thing? The low frequencies are an extremely important part of the overall sound, but not necessarily on every input, to the same degree. The fact is that nearly every instrument and / or mic. input will have some amount of low frequency information (either directly related to the signal generated by that instrument or as bleed from other adjacent sources). The accumulation of all this competing low frequency information added to the mix can create a significant amount of mud that clouds the mix and contributes to what is commonly called “masking”, and almost guarantees you will have trouble with pesky, low-frequency feedback issues.

Here’s where the high pass comes to the rescue. By selectively tuning out the unneeded low frequency information on a per-channel basis, we can clean up all this low-end smear and open up space in the low frequency spectrum for the instruments that utilize that range the most (ie bass guitar, kick drum, toms, tuba, explosions, cellos, etc.) One of the most important places to use the high pass filter is on the vocal mic. inputs. In most cases, I find that the vocal channel high pass filter can be set between 100hz and 200hz because the 12db per octave filter doesn’t attenuate significantly until it gets to about 1/2 octave below the chosen. Remember to use your ears and not your eyes when adjusting the high pass filter. I typically sweep the filter up to just past the point that it begins to remove too much information, then back off a little from there (listening to how it sits in the mix with the other vocals). For instrument channels I use the same method of sweeping the filter up to the point of “over-filtering”, then I back it off a bit and see how it sits with the other mix elements. This way I effectively clean out all the stuff below the lowest needed frequencies of that particular instrument or voice, leaving plenty of room for low frequency elements to shine with clarity and punch, while reducing low-end feedback issues in the house and monitors at the same time. With most the low end issues now taken care of, I can use the remaining equalizer section for more subtle, creative shaping.

NOTE: Some mixing consoles only provide a fixed high pass filter switch (usually permanently set at around 80hz), that can only be switched on or off. These fixed high pass switches are not variable (sweepable) but are still effective for filtering out subsonic frequencies.

Judicious use of high pass filters is one of the best ways to insure that your mix will be clean and open sounding, with little worry of low frequency feedback. I recommend this as the one of the very first steps in building a great sounding mix.

Happy mixing!

-Michael Grosso, senior audio tech – CCFL

This entry was posted on Thursday, August 6th, 2009 at 2:12 pm and is filed under Gearheads. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.