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Cut ‘Em Off At The Pass: Effective Uses Of High-Pass Filtering

Applications and benefits of one of the most under-utilized features on the console

So there I was, system engineer at a county fair gig. The act of the day was a traveling 1960s reviews with three or four artists who were, shall we say, past their prime. They weren’t carrying engineers, so we got the duty.

Sound check went fine. The artists cruised through their paces and the hired back-up band was surprisingly good. Nothing to do but hit catering and wait for the “white hair, blue hair and no hair” crowd to show up.

Show time. The band started the intro, everything was rocking in an old school sort of way and the emcee/star came out. He was much more animated than he had been at soundcheck – running around the stage, exhorting the crowd to put down their walkers and dance, generally getting them in the mood.

Suddenly I heard a phantom kick drum that was waaaay off the beat. I cued up my cans and began to solo channels.

The offending thump came and went, but I finally put my eyes and ears together and realized that the star, we’ll call him “Frankie” for the sake of this article, was running around clapping his hands while holding his SM58.

At first I tried riding the mute button on his microphone, but I was spending so much time on him I couldn’t mix the rest of the show.

So I reached for the variable high-pass filter knob and ran it up to 300 Hz. It thinned his voice out a bit but I doubt anyone noticed but me.

Problem solved.

Combat The Unwanted
High-pass filters are probably one of the most under-utilized features on the console. The most common use has traditionally been to combat unwanted proximity effect, which is the tendency of directional mics to increase their output at low frequencies as the sound source gets closer to the mic.

Cardioid and hypercardioid mics get their directional characteristics from ports in the mic capsule that allow sound to impinge on the rear of the diaphragm as well as the front. The added length of the ports creates a difference in path length between sounds hitting the front of the diaphragm and the rear.

Pressure differences between the front and rear of the diaphragm are what make it move. These different path lengths cause a difference in pressure because of two factors: phase and amplitude.

The phase component is dominant at higher frequencies. A 20 kHz wave is slightly more than a half-inch long. The path length difference from the front of the diaphragm to the rear is large as a percentage of the wavelength, so almost complete cancellation can occur.

This is one reason why microphone directivity breaks down as frequency decreases, and it is also why the diaphragms of cardioid mics are damped at about 6 dB per octave as the frequency rises. Remember: more pressure difference equals more diaphragm movement.

But the key to proximity effect is the amplitude disparity. The inverse square law tells us that every time we double the distance from the source to the diaphragm, we lose 6 dB. This is very powerful at short distances; for example, the difference between a singer being a quarter-inch from the mic and a half-inch from the mic is 6 dB.

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It also means that the difference in path length from the front of the diaphragm to the rear becomes more and more significant as the source gets closer.

Since phase cancellations are a fixed percentage of amplitude at any given frequency the amplitude factor becomes much more dominant at close distances than the phase factor. The phase part of the equation has less and less effect at longer wavelengths while the amplitude part holds true at all frequencies.

Hence proximity effect.

Proximity effect can go as high in frequency as 500 Hz depending on the mic, although 200-300 Hz is more common. The amplitude gain can be as much as 16 dB! This is probably why high-pass filters were put on mics and into consoles in the first place.

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