The High Pass Filter, Your Best Friend

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The High Pass Filter, Your Best Friend

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 overlooked tool in the sound engineer’s 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.

 

Learn more about the high pass filter and other tips at http://www.prosoundweb.com/article/the_high_pass_filter_your_best_friend

Thanks to John Mills for the great article!

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