Quote:
JohnM wrote:
 Dips are generally caused by phase cancellations, which means the sound reaching the measurement point is a combination of sounds which have a phase difference of half a wavelength at the frequency the dip occurs. Adding two signals at the same frequency which are half a wavelength out of phase results in cancellation at that frequency and a sharp null or dip, the closer the signals are in level the more severe the cancellation.
With a sub and main speaker this can be caused by the sub phase adjustment being out compared to the main speaker. It most often occurs at or close to the crossover frequency.
Another cause of dips is sound reaching the listening position by 2 different paths, a direct path from the loudspeaker and a delayed path reflecting from one of the surfaces of the room. How sharp and deep the dip is depends on how close the level of the reflection is to the level of the direct signal, the lower the level of the reflection the shallower the dip. The frequency at the lowest point of the dip, 130Hz in the case of your plot, gives a clue to the other path. At 130Hz the wavelength of sound is about 2.6m or 8.7 feet (wavelength = 343m/frequency or 1125 feet/frequency). To have a half wavelength phase difference the reflected path would be travelling half that distance further than the direct path, so 1.3m or 4.35 feet. That rules out reflection from a very nearby surface (e.g. a speaker very close to a sidewell) but is fairly typical of the differences that occur from a floor reflection or a sidewall a few feet from the speaker. It could also occur if your measuring position is a couple of feet from the rear wall. To work out which surface the reflection is coming from, move the measuring position and see what happens to the dip. If the dip gets lower in frequency you have moved closer to the surface the reflection is coming from, if it gets higher you have moved further away. When you know what surface is the cause of the problem you can try some acoustic treatment on it (e.g. a thick rug on the floor if the problem is a floor bounce) or move your listening position or speakers to somewhere the effect is lessened. |
I found this post when I did a search for dip. It looks like the dip (around 130 Hz from the center spkr) was probably caused by a reflection off the rear wall. When I moved the listening position away from the wall the dip got much shallower. Now for the dip for all three speakers around 195 Hz. If I do the same math that JohnM did, I get 1125 (not sure why he didn't use 1130.....but oh well close enough) divided by 195 then divided by 2 equals 2.9 FT. Below is my room (before I moved the couch). The couch has been moved to about 2.2 FT from the back wall. I have 1" linacoustic from the floor to 48 IN above the ground. I wonder if it could be the back wall/ceiling corner that is causing the dip. Or maybe it's the ceiling. Any more thoughts?