Someone very clever, a long time ago, figured out that room resonant characteristics tend to fall somewhere around one-third of an octave in width. Not always, but fairly often. This makes it seem like those 31 bands on your graphic equalizer can take us anywhere we want to go in terms of room correction.
But room volumes and reflective characteristics vary considerably from one architectural marvel to another (and from one trashy club to another).
And this is the important part: room resonant modes have not yet formed a committee to agree that they will always resonate at the ISO center frequencies that are the backbone of the graphic equalizer.
While the foregoing is intended as a joke, it’s absolutely true that ISO centers are arbitrary and the room you’re working in is completely ignorant of them.
So what happens when you identify a room resonant mode with a band center of 140 Hz, very narrow in width but high in amplitude, and the only way you can cut it is to adjust the 125 Hz or 160 Hz filter on your graphic equalizer? Well, it might perfunctorily help the room problem…but it probably will not. Instead, it is more likely to exacerbate the issue (Figure 1).
Figure 1: The top graph (A) depicts room resonance centered at 140 Hz. In the middle graph (B), we see the response of a graphic equalizer attenuating 125 Hz and 160 Hz. In the bottom graph (C), it’s clear that the two filters only served to exacerbate the room resonance problem instead of solving it. (click to enlarge)
Think about how a 1/3-octave RTA and a companion 1/3-octave graphic EQ are like audio venetian blinds. Look through a partially closed blind and you see an image, but you don’t see it all. The brain fills in the gaps. It’s easy to tell that the girl outside the tour bus is a girl. But can you precisely see her form, her clothing, all of her features? You’re viewing a limited amount of visual information and your brain does it’s best to fill in what’s missing.
A similar effect occurs when measuring a sound system with a 1/3-octave RTA and then tune the system with a 1/3-octave graphic EQ. Unfortunately, the end result is not going to be the same as looking at the girl through the venetian blind. I can’t emphasize enough the importance of viewing a high resolution response trace (at least one-twelfth octave), and then using an equalizer that has at least the same (or better) resolution, if the results are to be precise.
Anechoic chambers are built to have almost zero resonant modes because they absorb the sonic energy rather than reflect it, but virtually all other rooms (and for that matter any vessel that contains a volume of air without equivalent absorption) will resonate in response to the air mass that’s being excited by sonic energy. It’s why flutes work, it’s why saxophones work, it’s why pipe organs work…and it’s also why large theatres (and sports arenas) tend to have a lot of acoustic “mud” in the low frequencies.
Resonance is much more pronounced in the low-frequency region than the high-frequency region, except in special cases such as tent structures in which the LF energy is absorbed by the flexibility of the tent walls. In very large rooms (Radio City Music Hall, for example), it’s not uncommon to see LF peaks that are so high in magnitude that it takes ganging two analog filters on top of each to provide the required level of attenuation.
These days, that’s largely become a non-issue because most DSP-based equalizers have a very wide range of cut capability (often -40 dB), but it was a genuine problem in the “good ol’ days” when most analog filters maxed out at -15 dB.
While a well-designed graphic equalizer can be a great tool for shaping the sonic qualities of an individual instrument (I once spent two hours with an 11-band graphic to get a problem snare drum to sound “just right” on an album project), is it also the right tool for tuning a sound system in a resonant room? Numerous professionals make it their first choice. Some believe it’s not even possible to tune a system with a parametric equalizer. Let’s look at this more closely, because some basic engineering concepts can help you make the best choices.