Behind The Mixer

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One button that still looks pristine.  No fingerprints.  No worn finish.  It doesn’t even have a name.

Working in audio, it’s easy finding my way into a foreign sound booth.  It could be it’s my station for an event or it might be that I’ve been invited in by the sound tech on duty.  Over the years, I’ve noticed one button that’s often not used.  It’s the fixed-point high-pass filter button on each channel – those now it’s found as an adjustable point on many consoles.

The single high-pass filter is often labeled as “/80” or “/100.”  Honestly, not the best label choice.  How many new sound tech’s would know what the number represented, let alone would know what happens when they pressed it?  But anyway…

What is a high-pass filter?

A high-pass filter (hpf) is an audio frequency filter that cuts (filters out) frequencies below a set level.  For example, if you set a high-pass filter at 300hz, you’d lose a lot of the bass sound from instruments that have frequencies in that lower range.

A high-pass filter also has a slope associated with it.  Think of it as the rate of reduction of that filter.

Photo provided by wikimedia

In this image you can see that the hpf filters out frequencies by attenuating (cutting) their decibel levels.  In advanced filters, you can control the degree of the slope.

What does “/80” or “/100” mean?

This label indicates a filter (/) and the frequency point for the filter.  In the case of “/80,” it’s saying filter out all frequencies below 80 Hz.  A resulting hpf then might look like the image here.

What about Digital Consoles?

Digital consoles, and a handful of analog consoles allow for a rolling frequency point.  This way you can roll up the filter to whatever level is needed.  For example, one vocalist might have an HPF set at 125 Hz while another has it at 187 Hz.

Why should I use a channel’s high-pass filter?

There are mean nasty ogres that live in the lower frequencies.  They walk around the stage speaking in tones so low…”how low are they?”  They’re so low that they sound like deep earthly rumblings.  And when the hpf isn’t engaged, you get all those rumbling in your sound system.

Wait a minute, that’s not the whole truth!  If they were there all the time, wouldn’t there always be a filter at that frequency?  True indeed.

Frequencies in that lower range can be both good and bad.  Kick drum, bass guitar, Froggy from the Little Rascals, those all work around that range.  However, a tenor’s vocal microphone that picks up the kick drum on the stage…not good.  It’s in these areas you start using the high-pass filter.

Rules to using the high-pass filter

Rules in audio mixing are few and far between.  Guidelines are more the terminology but when it comes down to it, if it sounds good then it’s right.  With that in mind, there are times when I’ll engage the hpf for a guitar and times when I won’t.  There are some simple guidelines you can follow;

  • If the microphone’s source sound doesn’t use those lower frequencies, then engage the hpf.
  • After setting your basic mix, engage and adjust the hpf on channels and listen to the difference.  Pick the best setting.
  • Experiment.  What if you engaged the hpf on a bass guitar and it gave it a unique sound that fit the song?  Try it!

Conclusion

The high-pass filter channel control shouldn’t go untouched.  It’s one more way you can clear up your mix and provide the best sound possible.

Question(s): How do you use the hpf?  What other types of filtering have you used?  

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Comments

  1. Hi Chris,

    I have a question. What if you use active sub woofers in the PA and the subs have a high pass filter. So, for example, when the HPF on the subs is engaged everything above 80hz will go to the top boxes and everything below this frequency will be handled by the subs. Of course this will depend on what frequency the HPF in the sub is pre set at.

    So, in this scenario you are effectively applying a high pass filter to every channel of the mixer. Am I correct? So, if the HPF’s on each channel of the mixer are also set at 80hz they will become redundant or ineffectual when engaged. Is this correct?

    I ask this question because I’ve seen people engaging the HPF on the sub (80hz), but then using the HPF’s on certain channels (80hz) of the mixer. Is this a silly thing to do or am I missing something?

    Best Regards

    Brad

    Reply

    • There should be a crossover that’s used, either as a unit or that’s built into the speakers. That crossover unit then sends the lower frequencies to the subs. It would be possible to use a HPF on the main speakers as long as you ran a low pas filter to the subs as well to eliminate the higher frequencies. A crossover takes care of all of this for you.
      To the question of using a HPF on a sub and channels. If you have a sub that can’t handle really low frequecies or, for whatever reason, they need to be rolled off in that room, yes, you coud use a HPF. Would you then still even need to use the HPF on the mixer channels? I’d say yes. You’ve got monitors and recording devices that are being fed so they would need proper mixing of the low end.

      Reply

  2. I’m a worship pastor that oversees sound in our small church, and I understand the concept of using HP filter (You explained it very well, by the way!), but I get confused on something…when I set it at, say 100 Hz, does it START rolling off there, or does it start up higher? On our digital board (Allen and Heath QU-32), it seems to show on the graph on the screen that it starts higher than that, so it confuses me. Do you know if there is a standard to this?

    Reply

    • There isn’t a standard, in fact, there aren’t many (if any) standards in console design. On one console, there is an ON/OFF button that’s lit when it’s ON while another console has a Mute button and it’s lit when it’s muted. Regarding the HPF, I did a little digging.

      The Mackie Onyx manual states, “…cuts bass frequencies below 100 Hz at a rate of 18dB per octave.”
      The A&H manual for the QU-series states, “The filter has a 12dB slope…”

      This means that when you pick a HPF frequency, the degree of rolloff varies between these two consoles – however, both are a significant amount. As far as when does it take effect, go with what your screen shows.

      Reply

  3. We don’t have a bassist nor a drummer in our tiny church…there’s only a guitarist, keyboardist, and an occasional flutist.

    Sometimes when the guitarist uses the acoustic guitar as a bass guitar, i’ll deactivate the HPF, and there’s a noticeable improvement in sound. Since then, i’ve left it deactivated, since there’s no bass guitar around. Is this a good or bad decision?

    The other thing is, i also left the the HPF deactivated for the digital keyboard by the same reasoning. That too i’m not sure is wise or not.

    For all other channels (vocals), the HPF is always left activated.

    That’s all…hope someone can shed some light on this. Tq n God bless :-)

    Reply

    • You’ve got the right idea. Let the acoustic guitar drop into the low end because you don’t have anything else that does. As far as the keyboard, that can change from song to song and whatever settings the keyboardist is using so just test it to hear the difference then let the better result drive your decision.

      Reply

  6. Yep, second only to getting the input gain right a HPF is the most valuable tool in cleaning up a mix. I use it on almost every live music input channel. Kick drum and Bass guitar I roll off at 40hz. Everything else gets the 80hz HPF.

    Reply

  8. I use the HPF on almost every channel (48). The HPF (especially with variable frequency) is the simplest and often the most expedient way to achieve a more natural sound and eliminate any muddiness in the sound. You are correct in stating this is often one of the most overlooked features and too often many an individual mixing thinks along the lines of adding to the equalization curve rather than eliminating frequencies.

    Reply

  10. I don’t have to deal with a bass drum or bass guitar (although I’d love to). So all my live mikes have HPF on. My “hall” is prone to subsonic feedback and destroy woofers.
    Its done it before.

    Reply