January 30, 2013, by Dave Rat
I enjoy seeking parallels and connections between various aspects of the world that surrounds us in search of clarifying analogies.
One of those connections occurred to me not long ago while doing one of my sound seminars. I was looking for a way to clearly explain the theories I implement when equalizing live sound systems.
Though mixing a live event can be a complex process with many factors that need to be simultaneously juggled, it’s possible to look at mixing audio in very simple terms as just the process of controlling the volume level and tonal balance of one or more sound sources.
Compressors and gates are just signal dependent volume controls. EQ, high- and low-pass filters, crossovers and microphone choices are all just tools that alter the tonal balance of the mix, the vocals and the individual instruments.
Yes, there are other factors like polarity, time delay, coverage, effects, panning and so on, but the fundamentals of mixing are mostly just controlling the volume levels and tonal balance presented to the audience.
To help analyze methods of optimizing our control over a sound system more clearly, let’s divide the decisions we make into two main categories: technical and preferential. Technical-based decisions encompass things like setting the time delay of a delay cluster for minimum offset, setting the polarity of two microphones such that they do not create unwanted cancellations, and adjusting the frequency response of a system to be linear and create a “what goes in, comes out the same” scenario.
On the other hand, preference-based decisions can include things like the relative volume differentials between the instruments and whether we’re trying to recreate a realistic sound or alter the sound of an instrument.
One common example of drastically altering the sound of an instrument would be the reproduced tonal balance of a kick drum at nearly every rock show. In real life a kick drum just sounds like “pop pop pop” or “thud thud thud,” lacking the significant amount of low end (and sometimes click) typically chosen to be reproduced through reinforcement systems. But hey, that’s just an evolved preference that often falls in line with listener expectations.
Now I’ll ask: what about the overall tonal balance of a mix – is it primarily a technical- or preference-based decision? I’ve been mulling over this concept for years in one way or another. Should a system be EQ’d to flat? How bright or dull should my mix be? How do I determine the technically correct tonal balance in any given situation? And how should it be measured?
I’ve made some significant progress in unraveling these quandaries by establishing headphones as a reference point. First I play music through the headphones and through the sound system simultaneously.
Then, using my ears, I compare the sound of the music in the headphones to the sound of the same music through the system. Using the house equalizers, I then do my best to EQ the system sound to match the headphone sound. Assuming the sound of the headphones is “correct,” this locks me into a “correct” tonal balance.
Next is the use of a real-time analyzer (RTA) to observe the frequency response so that I can also see what “correct” looks like. While mixing the show, I constantly refer back to the analyzer to make sure the tonal balance of my mix has not drifted from the established baseline initially established with the headphones.
Finally, during the show I compare the pre-house EQ sound of my mix in the headphones to the post-house EQ sound of the system in the room. This allows me to make sure that what’s being sent to the system sounds like what’s being reproduced in the venue.
Now let’s zoom out a bit and talk about exposure to energy differentials from the norm or average. Exposure of our humanly bodies to too much sunshine causes sunburn.
If we’re exposed to prolonged periods of below freezing temperatures, it leads to all kinds of not good things if we’re under-protected. So does exposing our eyes to the spark of a MIG welder so bright that it can be blinding and standing near a jet engine so loud that it can be deafening.
Exposure to energy in moderate amounts can be desirable, but too much exposure is often problematic.
Understand that I’m not saying how much is too much, but rather, I’m offering some observations about parallels between exposures to different types of energy. We can jump into very frigid water for short periods of time as demonstrated by the annual “polar bear plunges” around the world. We can wave our hands briefly through fire with no ill effect, glance at the sun, or comfortably lay upon a bed of nails if there are enough nails to evenly distribute the small points over a large portion of our body. Short-term exposure to larger peaks or longer term exposure to an even average is typically non-detrimental and perhaps desirable.
Sweeping back into the sound world, we expose our listeners to volume levels and tonal balances of audio energy. If the frequencies presented are in a narrower frequency range for a short period of time, like a snare drum or single bright note of a loud electric guitar, it’s all good as long as it’s not overly repeated or is offset by balancing energy shortly thereafter. Just as our bodies seek a balanced temperature for long-term exposure, our ears seek a balanced frequency response as well.
Thus while mixing, I set up the RTA to measure the room sound with long averaging times and actively switch between 10-second averages, as well as averages over several minutes and averages over several songs. The goal is to be able to measure the “tonal balance exposure” over time that the audience is immersed in. I want to assure that tonal balance over longer time frames does not drift too far from the initial comparative reference point established with the headphones.
To gain clear and predictable control over the tonal balance of a sound system, I set up three distinct stages of EQ. First are the console channel EQs, which are responsible purely for equalizing the microphone-instrument (or vocal) to sound “correct.”
“Correct” can be whatever we wish it to be, but it’s the job of the channel EQ, combined with mic choice and positioning, to get us there. When we cue that input channel up in headphones, it should sound the way we desire it to sound.
Second, it’s the job of the system processor to EQ the loudspeakers to sound correct in the enclosure they’re mounted in, and also to compensate for the EQ differentials caused by using multiple loudspeaker enclosures in an array. In a perfect world and in an ideal anechoic chamber, a signal sent to the loudspeakers sounds exactly like the source. The system processor has nothing to do with correcting for the sound of the venue.
Finally, the house EQ is tasked with compensating for the loudspeaker array-to-venue combination. During a show, knowing where to make adjustments is crucial. If the room gets duller when the audience arrives, where do we EQ? The house EQ, because it was a room change. If the drummer’s snare gets duller over the course of the show, we change console EQ on the snare channel.
Further, if someone climbs up and adds more loudspeakers to the array and re-angles everything, first we wonder why the heck they would do that, and next we would adjust the system processor to compensate. If the room heats up or cools down – house EQ. If the venue humidity changes – house EQ. If the guitar player switches to a brighter guitar – channel EQ.
What I find interesting is that most of the EQ changes over the course of a show typically occur with the biggest uncontrolled variable, the room. The tonality of the loudspeaker arrays rarely changes so there is almost no reason to alter the system processor settings, even from venue to venue when setting up similar arrays! The tonal balance from the stage may jump around a bit from song to song and over the course of the show, but these changes are either intentional or can often be managed by well thought-out usage of compressors, mic placement and mic chart decisions.
It’s the room or venue sound that is the real wild card, remaining in a state of flux with constantly changing temperatures, humidity, more people, less people, wind, refraction from changing thermal layers, and so on. If a mix is set up well and the band is halfway decent, then the art of mixing is not about hunching over the console but rather stepping back and keeping the house EQ dialed in, and presenting the optimum tonal balance exposure you desire the audience to experience.
Dave Rat (www.daverat.com) heads up Rat Sound Systems Inc., based in Southern California, and has also been a mix engineer for more than 25 years.