Beth,
If someone else already answered this issue for you, I appologize. I thought I would give it a shot before we run out for Christmas Eve at Mom's.
The physical size of perceivable sound waves range from over 56' long down to a fraction of an inch. Therefor (especially in smaller rooms) low frequencies interact with a room in dramatically different ways than mid or high frequencies. Below about 80hz (approx 14' wavelength) you will not develop a complete waveform in a typical small room and therefor the direct sound from the speakers interacts with a partial (reflected) waveform at a particular point in its phase (from 0 to 360 degrees). This interaction will cause deep comb filtering, that changes from point to point in the room dependant on the phase of the reflected wave. At 180 degrees, the two waves are fully out of phase and will cancel completely, causing troughs of 30 to 40 db. This phase relationship changes for every point in the room! If you eq your speakers for a point where you sit or place a microphone, a few inches up/down/right/left of that will be very different. Just the act of moving over to adjust your eq (in the rack) and then moving back is enough to invalidate your adjustment.
Low frequency problems are omnidirectional and can only be perceived as gain changes. Because music dictates that frequencies change constantly, it is difficult to accurately perceive the exact frequency of interference. Trying to eq a room at low frequencies in MHO in futile and doomed to failure. You are trying to eat your soup with a fork. Better to arrance your monitors and listening position for minimum low frequency interaction and maximum distance from directly radiating surfaces. This usually means placing your monitors as close to the front wall as possible and moving your listening position to the front half of the room.
This direct/reflected relationship remains in lesser and lesser degree up to about 400 hz, at which point there are usually enough reflected waves from enough surfaces that the phase relationship becomes complex and less influencial. At this point the room is not contributing so much to the raw gain of the signal through phase interaction, but to the perceved gain through ambient extension (reverb). Some frequencies sound louder because they last longer, while others die away more quickly and therefor sound softer. Eq'ing monitors in the mid range affects how LOUD the signal is, not how LONG it lasts, which is the real problem. While it is certainly possible to subjectively affect the perceived loudness of a mid range signal with eq, it is preferable to either redirect the early reflections away from the listening position (RFZ Reflection Free Zone) or absorb them with mid frequency absorption. I suggest that you refer to Ethan Winers posts for solid insight here.
At extremely high frequencies the room does not really contribut ambience without obvious and directional effects like flutter echo and "pinging". These noises are easily observed and so when somone wants to "treat" a room, they put carpeting or fiberglass in to "deaden" it. Unfortunatelly, this has no affect on the lower frequency problems and serves to just confuse the performance all the more. This range is also is where eq is most effective. Most "treated" rooms have very little high frequency reflections and the perceived signal is almost all direct from the monitors. Turn it up or down at your leisure.
All of the above of course, relies on a ruler flat monitor system. We know that until very recently, there were very few of these around. If you eq a room to compensate for non linearities in the SOURCE, (those JBL's in the wall) then you are correctly using a spanner on a nut, otherwise your eating your soup with a fork.
Best Regards, Happy Holidays!
Bill
