CUT ‘EM OFF AT THE PASS
So there I was, system engineer at a county fair gig. The act of the day was one of those traveling 60’s reviews with three or four artists who were, shall we say, past their prime. They weren’t carrying any engineers so we got the duty. Sound check went fine. The “names” 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 sound check. He was 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 waaaaay 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 mic 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 300Hz. It thinned his voice out a bit but I doubt if anyone noticed but me. Problem solved.
High pass filters are probably one of the most underutilized features on the console. The most common use has traditionally been to combat unwanted proximity effect. Proximity effect is the tendency of directional microphones 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. This is also why the diaphragms of cardioid mics are damped at about 6dB 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 6dB. This is very powerful at short distances re: the difference between the singer being .25 inches from the mic and .5 inches from the mic is 6dB. This 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 effects can go as high in frequency as 500 Hz depending on the microphone 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 microphones and into consoles in the first place.
sweepable high pass filters can also be used to help you clean up your overall mix.
One of the things we learn from audiology is that low frequency sounds obscure higher frequency sounds but not the other way around. This is one of the principles that makes sound masking work. It’s useful in sound masking systems but in a live performance situation, not so much. Many live mixers react to this unconsciously when they reach for the house graphic and hack away at 125 and 160Hz. True, many rooms react poorly in that frequency range but the room is only one part of the problem. Let’s think about the physics of low frequency sound waves.
A 100Hz wavelength is 11.3 feet long (at sea level, at 72 degrees Fahrenheit etc. etc.). This is typically above the crossover point for your subs so this is probably being reproduced by your main arrays. In order to provide good directivity at any frequency the array must be larger than the wavelength. If the array is not larger than the frequency of interest the sound waves wrap around the array and it behaves as an omnidirectional source. Even if you are using a fairly long line array you only get the directivity benefits in the vertical axis. Chances are your array is four feet wide (or less). This means that in the horizontal plane pattern control starts to break down at around 250-300Hz. What is in close proximity to the array on the horizontal axis? The stage. And the microphones on the stage.
Even if you are running your subs off an aux send on the console (which I highly recommend) there is still energy from the sources that are routed to the subs finding its way back into the stage mics. Since the same laws of physics hold true for the stage sources as for the main arrays, your mics are picking up the desired musical content in these frequency ranges plus the adjacent instruments and floor wedges plus the room resonances plus the wraparound from the FOH system in the longer wavelength frequencies below about 300Hz. This is even if we don’t consider the artist clapping with a mic in his hands or tapping his foot on the mic stand base. And to compound the problem, the cardioid pattern of your microphones breaks down in the lower frequencies as well. The inverse square law (minus channel compression) is your only friend at this point!
So, what’s a poor sound engineer to do? We have already made gains cleaning up the stage sound in some cases with tools like in-ear monitoring and amplifiers located off stage in isolation cabinets. While these techniques are incrementally helpful there is another tool at our disposal. The console channel’s variable high pass filter.
The earlier we can deal with these issues in the signal chain, the better. If your microphones have a shelving filter, try that first. If it doesn’t degrade the instrument sound leave it switched in. Next, at sound check start your equalization process for each mic by sweeping the high filter up until you hear it affect the sound. Obviously, there are some inputs that might be left out of this process like the kick drum, bass guitar and a low piano mic. DI’s and other direct feeds don’t count because they aren’t picking up ambient sound. A sharper knee and a steeper slope will allow you to set the filter to a higher frequency without degrading the natural tone of the source up to a point. Too steep of a slope can cause a filter to “ring”. Filters have resonances too. Then, during the show, solo each mic with cans that provide good low frequency isolation and response (I like Beyer DT-770’s) and you may find you can cheat your high pass filters up in frequency a little higher. Oh and by the way, have your monitor engineer try this too, only he/she can be quite a bit more aggressive with it. The performers on stage don’t have high pass filters on their ears and even most ear molds don’t do a great job of isolating lower frequencies.
Using this approach you will find yourself cutting less in the 125-200 range on the system EQ because you are solving the problem at the source. You will also be surprised at the increased clarity in your overall mix. Your system will have more headroom as well since the frequency ranges we are dealing with are real energy hogs. Remember, garbage in garbage out. Why deal with it in your mix when you can cut it off at the pass? The high pass, that is.