I was at an event the other day and the mix was good, but lacked the clarity heard in many great live mixes. The technique to make a clear and punchy mix is straightforward. Most mixing boards — even the cheaper units are – equipped with the basic equalizing (EQ) tools to sculpt out the low frequencies as necessary. When given a chance to peak at other soundguy’s mixes (I avoid backseat mixing but sometimes peak to see what they are doing, or not doing), I see the lows not cut out to the degree necessary to make a clear, yet punchy mix.
The lows frequencies in any instrument, voice are often too heavy due to the proximity effect inherent in close mic’ing techniques necessary for live sound. The proximity effect is a bass boost from any source that increases as the source gets closer to the microphone. Try it by singing on foot from a vocal mic, then one inch from a vocal mic. Of course the overall sound is louder, but the low frequencies increase ten fold. This is how radio announcers and crooners get that full bodied sound. They are practically eating the mic. At a show, I usually don’t want the singers one foot from their mics, otherwise drums, electric guitars and other sources will leak in to a degree that makes the mix hard to control and splashy sounding. It is ideal to have the singers very close to the mics and cut the lows on the mixing board minimizing drum, instrument and monitor leakage.
Low frequency information requires much more amplifier power from the PA system to seem the same level to the listener. Ten times the power per octave. If 20 watts were used at 5000Hz in a PA, then at 500Hz 200 watts are required, and at 50Hz, 2000 watts are required to have it perceived as uniform to the human ear. The rumbling and other unneeded low frequency information is wasted PA energy that ought to be stripped out. Only the few instruments that have deliberate content in the low frequency range, should be allowed to pass through low frequency information. The result is a punchier low frequency sound.
This is accomplished by applying low cut filters, sometimes called high pass filters, to all mics and instruments except , bass instruments, kick drum, low tom toms, certain keyboards, low brass and the like, depending on their performance context. Most mixing boards have a low cut filter button that cuts around 80Hz and below. Start by engaging ALL of these except on the aforementioned instruments. The low-mids 80 to 500Hz will still be adding mud to the mix which is resolved by turning the low frequency EQ down in that range. I often hear live mixes where some low cutting has been applied — but not enough to clarify the mix.
Soundwaves departing the speaker into the air are a composite of all the waves in the music. The high frequencies ride on top of the lower frequencies, like a surfer riding a big wave at the beach. Imagine if some smaller choppy (low mid frequency) waves interfering with the big (low frequency) wave. It would make the surfer’s ride a lot less stable. Removing the unnecessary (non musical) chop smooths out the ride.
Large PA systems may divide the sound frequencies into two, three or more separate frequency ranges clarifying and adding efficiency to the speaker performance by reproducing only a limited part (bandwidth) of the frequency spectrum. Subwoofers often produce 20 to 105Hz, midrange speakers 105 to 1800Hz and tweeters 1800 to 20,000Hz. Despite a bit of real world frequency overlap, the highs are not riding on the subwoofer waves. The ear and brain does a good job putting the frequencies back together, though there is a little phase shift and comb filtering effect which is an acceptable trade off for increased sound system efficiency and performance.
Why not simply brighten the mix by adding high frequency EQ to desired channels?
EQs are an imperfect solution to frequency sculpting and by design add phase shift in the processes of boosting or cutting. It is subtle, but to EQ more highs into many mixer channels will create a more strained sound as many many high frequencies are phase shifted. Try it and see. So if I’m cutting lows and and incorporating a bit of phase shift to the low frequencies removed, who cares. The phase shift is going to be less detectable by a level of magnitude. I’m not afraid to EQ highs into an instrument in an artistic manner, but the low cut technique routine prevents me from doing so to every channel. It is possible to alter the frequency response by means of thoughtful meticulous mic placement, but not always practical, time efficient or realistic. Motown legend Berry Gordy repeatedly stressed to his audio engineers never to make mixes shrill as it is a turn of to the ladies (who have better hearing than men).
I was working on a tour as house engineer and one of the bands had a great engineer that left me with one bit of advice. “Dump (EQ out) 500Hz from the main and monitor mixes.” How could this engineer speak so matter of factly regarding one specific frequency. Well, it speaks to the nature of PA speaker performance. Amplifying 100 to 500 is pretty easy for speakers. But it is not where the perception of musicality lies. The human ear discerns speech and pitch best between 500 to 2000Hz range. We want to make sure these frequencies are discernible from the muddy 100 to 500Hz range. The actual 500 frequency mentioned by the engineer would vary system to system, venue to venue, band to band. Realize the low mids are the murky beast. In fact, if I want more clarity and tonality from the bass guitar and it is already throbbing on the dance floor, I add midrange (600 to 1500Hz) to it. Try adding 2500-3000Hz to the kick drum and enjoy the added clarity within the mix. (Thanks Johnny Rod for that tip in the early ’90s)
That said, low frequencies are awesome creating sonic impact unlike anything else in the audio spectrum. Establishing pure low frequency waves without the low garbage from non-low frequency sources is key to a great mix at every event.