why subwoofers cancel each other? (1 Viewer)

[Chris Tsutsui]

I believe that you don't need a dedicated HT or acoustical treatment in a room for a good start in bass EQ. All you need is a little room tuning knowledge, and then you can have fun with the DIY absorbant panels or helmholtz resonator/bass traps.
When you have a single subwoofer in a corner, you are activating all the bass modes of the room. This means you'll have areas of loud bass, and areas of low bass in specific areas of the room. If you move the subwoofer forward into one of the pressure minimum areas, you can effectively cancel out that mode. This means that with a given room, the position of the subwoofer and the position of the listener determines how the bass response will sound.
The reason behind separating the subwoofer in home theaters is so that the sub can be tuned like this. Low frequencies are greatly affected by the room, so what better idea was it to allow ppl to move the subwoofer in the room where it will sound best? Another plus is you can position and tune the mains/center independent of tuning the bass. (Gotta love that LF channel)
But.. One subwoofer can only be tuned so far.. Which reminds me that some of the best dedicated home theaters I've been in have 2 subwoofers. With 2 subs, it's possible to effectivly cancel out more bass modes for the listening position if placed right. With a cancelation of bass modes, the response of the subwoofers can become smooth without any room treatments or equalizers.
As for mains producing upper bass... This is why I believe the mains play a smaller role in mode cancellation. But since they start playing sound that's directional at 80hz+, they should be stereo.
IMO, full range towers are more for 2 channel music. What happens is the speaker positioning for 2 channel listening doesn't always agree with the positioning that home theater calls for. Another drawback of full range towers is the price you have to pay for smooth levels down to the 20-30hz region.
See the http://www.harman.com/ white papers for more info on the subject.

 

[Yee-Ming]

I'll just chime in that the first time I came across this phenomenon was in high school: I'd studied it in physics class (constructive and destructive interference), but when working to set up a school function outdoors, the huge speakers on the stage were playing, and as I walked across I could actually hear the volume go noticeably up, then down, then up again, as I crossed the points where interference was constructive, then destructive, and then constructive again.
one of the few times in school when I realised what we were learning wasn't necessarily all complete rubbish and irrelevant...

 

[Peter Johnson]

Im pretty sure nobody has answered the actual question
Towers will cancel out at high (all) frequencies in different positions from each speaker. However, at higher frequencies, due to the small wavelengths, they are in phase again at high intervals , ie very close together(depending on frequency), so you dont tend to notice.
Bass waves have LONG wavelengths, and once they go out of phase, in an average size room, they dont tend to come back into phase again, hence they cancel out.

 

[DanielSmi]

If you have to subs shouldn't you stack one on top of the other? Won't this cut down on cancellation issues?

Daniel Smith

 

[John F. Palacio]

"If you have to subs shouldn't you stack one on top of the other? Won't this cut down on cancellation issues?

Daniel Smith"

This will indeed eliminate cancellation issues between subs. Placing them next to each other would accomplish the same. The original question in this thread was for subs placed some distance apart.

No matter what there will always be irregularity issues do the the room interaction and the source.

 

[Chris Tsutsui]

"No matter what there will always be irregularity issues do to the room interaction and the source."
Have you no hope for equalization?