The Loneliest Number
When there is only one measurement microphone available on a project, move it around a lot. If there’s more than one, time can be saved and accuracy improved by instantly comparing one part of the room to another.
In either case, take note of the differences in each room location in respect to others. Don’t adjust EQ for one “sweet spot.” A typical approach is to measure where the console is located, as that’s the quickest and easiest thing to do. But it’s not the optimal approach. Moving the measurement mic around the room, as time permits, will lead to a far better outcome.
It’s vitally important to not use the full system when taking measurements and making EQ corrections. If the room is symmetrical (and most are) start by measuring and EQ’ing only one side of the PA then transfer those filter parameters to the other side (assuming left/right sources). Otherwise you’re measuring the acoustic addition and subtraction of the various L/R PA elements interacting with each other, rather than identifying the true room resonance.
Of course, after all elements of the system are initially adjusted, listen (and measure) the entire system as a whole. Inevitably the LF region will be more pronounced and dominant than when only measuring one side of the system. This is an excellent time to use an LF shelving filter to gently reduce the LF build-up, rather than to add more attenuation to the surgical attenuation filters.
An LF shelving filter can easily be trimmed on the fly by ear, with no dire consequences to worry about. A narrow Q surgical LF attenuation filter cannot.
Sooner or later, audience members will enter the room, thereby displacing a measure of air volume with their relatively solid human bodies, and thus raising the resonant frequency centers. This is a simple effect of physics: the room now has less air volume so it resonates at higher frequencies, just as a shorter pipe on a pipe organ produces a higher frequency than a longer pipe.
Acoustically, the presence of the patrons will be very noticeable in most rooms, both by ear and by measurement. If you have the means to re-measure the system’s response with the audience in place, then you can make some real magic! Products such as Rational Acoustics Smaart, Meyer Sound SIM, SATlive, and others, foster accurate measurements using walk-in music or even the opening strains of the performance itself.
What you’ll see is that the resonant modes, so carefully identified when the room was empty, have now shifted upward in frequency. Sometimes it’s a lot, others only a few Hz. But keep in mind that a few hertz in the lower frequencies is actually a considerable part of the LF spectrum; e.g., 20 Hz to 40 Hz is a full octave, but that octave contains only 20 actual Hz of LF information (in terms of integers).
Due to the surgical cuts instead of broadband swipes, the audible difference that you (and your audience) will hear when you touch up those precise cuts, causing them to once again fall on the new band-centers, can be astounding. Shifting a filter upwards by a fraction of an octave can have a profound effect? Hearing the results, particularly when the program material comprises natural instruments, makes it obvious that the resultant accuracy is well worth the time and effort it took to get there.
At this point it should be pretty clear that this type of precision tuning can’t be accomplished with anything other than a parametric equalizer, and one that has 1 Hz or 2 Hz of resolution in the LF section. Not all do – some jump around quite a bit in the LF region – so choose your weapon wisely, Mr. Bond.
Ken DeLoria is senior technical editor for Live Sound International and ProSoundWeb has had a diverse career in pro audio over more than 30 years, including being the founder and owner of Apogee Sound.