...
Of course, one cannot really deal with room modes with an equalizer anyway (as Andrew has emphasized), since they are highly dependent upon location...
That very much depends. I was able to clean up a huge 15db peak at 56Hz that appeared almost everywhere in my room. The suck out at 72 Hz was a lost cause.
I would agree on localized peaks and nulls though.
Hi Fred -- This is an interesting / excellent observation. Let me elaborate.
All room modes (standing waves) will be intrinsically position dependent (they represent sine/cosine type spatial transitions -- or more complicated functions in oddly shaped rooms -- between pressure maxima at the room boundaries). Another way to say this is that energy conservation implies that a peak at one location must be compensated by a null elsewhere -- it can't be peaks everywhere in a zero-sum game. However, your counter example emphasizes a really critical feature: for very long wavelengths, the position dependence may be so broad (spanning across all of the listening positions in some cases) that it can really be interpreted as a global/constant effect for practical purposes, at least over the relevant confined listening area. The wavelength at 56 Hz ( note wavelength = speed of sound / frequency, where speed is ~ 343 m/s) is around 6 meters, close to twenty feet, so this caveat certainly applies.
The lowest room mode, taking parallel walls, will occur when the wall separation is half a wavelength. For a typical 20 foot dimension the lowest room mode would have a wavelength of 40 feet, or about 12 meters, which corresponds to a frequency in the ballpark of 30 Hz. In this case, there would be a null at the midpoint, with accentuation (an amplitude peak) at the boundaries. The second mode would be a full wavelength wall to wall with peaks at the boundaries (as always), but also at the dead center, in the interior listening area. The wavelength of 6 meters would correspond to a frequency right around the value you noted (say 56 Hz) and it would have positional minima (nulls) at the quarter wavelength locations, i.e. 5 feet from the front/back wall. As long as you were within, say, the inner 8 feet or so of the room, it would be pretty fair to call this a constant / global effect, as long as no one was seated close to the locations of the nulls. The third harmonic would correspond to the room dimension being 1.5 wavelengths, yielding a wavelength of 20 feet * 2/3, which is about 4 meters, corresponding to a frequency of about 85 Hz. Being an odd harmonic, there would be a null at the midpoint, which is prime listening real estate. It might just correspond to your experience (although there are effects from all three room dimensions that are more complex)
There is a nicely done writeup (which spared me from some trouble)
here.
Cheers - DSQ
