Originally Posted by doublewing11
What I also find interesting is I moved the microphone across the front row seating.............graph were very similar, but the null at 65 hz lessened as I moved closer to wall on either side. Hmm.............
First, if you are going to work with the graph you posted earlier, you need to be certain that the only speaker playing was the sub (the S2 is the whole system, right?). If that's the case, then my comment about crossovers is irrelevant, as there can't be a phase interaction between two loudspeakers with only one playing, right? (
Also, you called this 65Hz, and I'd say that's pretty close - but maybe it's a little lower? My hunch is that it is a little lower, and that is to be expected (though I'm not sure how much lower should be expected) because "real" walls are not infinitely stiff. The flex they undergo acts to delay (in time) the reflection of the bass wave, making its period longer and thereby decreasing the resonant frequency somewhat. Working from the dimensions you posted in the first post of this thread, I put them in an online calculator (my favorite one)
to see what the modal response should
be like. This screen capture shows that there should be a second order (like a second harmonic) standing wave established axially, between the side walls of your theater. There are other standing waves in this frequency range, which may complicate the interpretation, but the 2nd order width mode is strongly coupled to your sub when it's placed along center line of your room (it's still under the center channel speaker, right?).
If the mode I've highlighted here is the culprit, the null should be deepest 1/4 of the way from each wall and there should be no null at the walls or in the dead center. In the image, dark regions are high pressure (loud sound) while white space is nulls.
If you were to play a 65Hz (or maybe 64Hz?) pure tone and walk across the room while running REW's RTA function (or any other RTA) you should see the SPL follow that pattern - high at the wall - then through a deep null - high again in the center of the room - another null centered at the 1/4 point - and high at the other wall. Don't forget your ear protection and don't play it too lound for too long. Pure tones can be rough on the driver's voice coils - it doesn't need to be very loud to get the pattern and confirm the resonance. Alternately you could use pink noise, but it's more difficult to watch the RTA that way.