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Home » John Mills » The High Pass Filter, your best friend.

The High Pass Filter, your best friend.

By John Mills

What is it?

A high pass filter, or HPF, is exactly as it sounds.  It is a filter we can use on our soundboards that ONLY allows the higher frequencies pass.  It is sometimes referred to as a Low Cut filter for a similar reason. It is also the most over looked tool in the sound engineers arsenal. 

Where it’s found.

Some soundboards only have a High Pass Switch which is fixed at a certain frequency, often 80Hz or 100Hz.  This includes most Mackie, Behringer, Allen & Heath and similarly priced consoles.  Usually, only on higher priced consoles do you find the most amazingly useful type… the coveted golden ticket… the end-all-be-all… the “Variable High Pass Filter.” 

The variable high pass filter is more useful because it allows you to change the frequency where the cut off begins, or more importantly where the lows no longer muddy up the bottom of our mix.  But rest assured, I have a little trick for you folks not yet blessed with a variable HPF.

Why we need it.

Well, simply put, the more low frequencies allowed into a mix, the more muddy or unintelligible a mix usually is.

Let’s take a violin for example.  For the most part the violin is made up of mostly mids and highs.  So if we have 4 mics on our violin section, we are probably picking up a good deal of low frequency content from the timpani, bass guitar, kick drum, and so on. The problem is that the leakage from the other instruments, into our violin mics, is out of time with any of the close mics on the low frequency instruments. 

Let’s take a short trip back to physics class.  Sound is made up of waves, waves take time to move through air, and low frequency waves are longer than high frequency waves. Son if one mic hears two sound sources arriving at the mic at different time, we can say they waves are out of sync.  When waves are out of “sync” with each other we have cancellations and/or additions. 

It is best to not have multiple mics picking up multiple instruments, especially if they have the same frequency content, but are different distances from the source. 

Like I mentioned above.  If the violin mics were picking up the bass guitar, it would be safe to say that the low frequency leakage of the bass into the violin mics is not “in time” with the actual bass input.  Which would result in some of the bass guitar sound being compromised because of the out of time (or out of phase) leakage into the violin mics.

What do I do with it.

If you are lucky enough to have a Variable High Pass Filter the trick is to engage it and while listening to the violins play, sweep their HPFs up until you hear their lower notes change.  At that point, back it off just a little bit, and know that the bass guitar leakage has been eliminated from the violin channels.

Did you follow that?  By making the HPF higher, but not so high it altered the low notes of the violin, we have effectively eliminated any lower frequencies from leaking into those inputs and ultimately into our mix.

What if I don’t have a Variable HPF or I have a fixed frequency one.

So you more moderately priced Mackie, Behringer, and other folks are feeling a bit left out at this point.  I wish we all had unlimited budgets to buy the consoles that had this feature, I realize they are expensive consoles, and sadly I know how that one goes.

Here is the trick for you guys.  Almost all soundboards have at least a HPF switch that can be engaged.  First trick… engage it on all channels except things like Kick, Bass, CD, Video, and anything else that has the potential to make really low notes.

Now since you do not have a variable HPF what else can you do…  well you can use your low EQ to do a similar trick.

The low eq knob on most of the consoles at this price point are what is called a shelving filter.  Which means everything below that frequency is attenuated similarly.  So even though you still can not sweep it up to hear the low notes cut off, you can still clean up a little more low frequency leakage by turning this eq knob down. 
What you do here is similar to the variable folks.  Listen to your instrument, and have them play some of their lower notes.  Turn down your low eq until you hear a substantial change in the sound of the low notes.  Then turn it back up just a notch.  You have now cleaned up any leakage from those mics similarly to how the folks with the variable HPFs were able to.

Why it cleans up the sound.

It’s actually not just about the leakage.  It is also about finding the holes in the mix for instruments.  If an acoustic guitar is to be placed in a contemporary mix with electric guitar, bass, and keys, then the low frequencies of the acoustic are really not necessary.  That is not to say you should make it sound like a swarm of bees, but the bass and electric guitars are certainly more capable of providing low frequencies.  So if the acoustic is also taking up that range in the mix, it is very likely that section of the frequency spectrum will easily get clogged up.

How does it help the amplifier and speakers?

Eliminating any unnecessary low frequency content also helps our amplifiers and speakers.  Amps and speakers pretty much do what we tell them. So if we have a sloppy low and low mid section of our mix, they will reproduce it just as we mix it, but if we clean up our mix by eliminating conflicting and extraneous low and even low mid frequency content, we not only get a cleaner mix, we actually allow our speakers and amplifiers to run more efficiently since we are not asking them to reproduce content that is not necessary.



John is an 20-year veteran of the road and a graduate of the school of hard knocks.  If you are looking for down to earth training for your volunteers why not send John an email.

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