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Boundary loading for non-perpendicular planes
Posted: Thu Jul 17, 2008 6:42 pm
by Gauss
Just a quick question:
I've got my front-radiator style bass cabinet on a kicked-back stand. The speaker ends up being about 2-3 feet from the wall when I roll the cabinet against the wall, and it's about 2 feet off the ground angled up at about 40 degrees. I've not been having the best of luck with sound consistency lately. Is boundary loading to blame? Am I unknowingly affecting the signal coming out of my single 12" speaker? In other words, how does boundary loading affect the sound when the speaker plane is not perpendicular to the wall/floor?
Thanks for your patience,
Gauss
Re: Boundary loading for non-perpendicular planes
Posted: Thu Jul 17, 2008 7:00 pm
by Sydney
If you plug those distances 2 - 3' into
http://www.padrick.net/LiveSound/CancellationMode.htm
You see you can have a cancellation in the range ( 140Hz and 95 hz )
Cancellations starts when you pass 90deg out phase, and peaks @ 180deg,
It affects a range of between 1/4 and 3/4 of a wavelength.
Have you tried other distances ( closer or further ? )
From Bob McCarthy's outstanding book "Sound System Design and Optimization"
Is this graph of harmonic wave summation:
Wave Summation and Cancellation follow this phase relationship:
- Relative Summation vs Phase
Relating Phase to distance:
A 100Hz wavelength has a 180 degree space of 5.65' ( 1.715M )
a 120 degree space of 3.7' (1.14M).
For a lower frequency and longer wavelength: 80Hz has a 180 degree space of 7' (2.14M) and a 120 degree space of 4.7' (1.43M).
Re: Boundary loading for non-perpendicular planes
Posted: Thu Jul 17, 2008 8:35 pm
by Gauss
I measured it and it's not as bad as I had thought, 19 inches both ways from the center of the speaker to the wall and floor. My concern is that the soundwaves traveling from the speaker will reflect at every distance up the wall and out along the floor and then cancel wherever they meet.
I now realize this isn't a question-posing section of the forums, so if the mods want to move this, please do.
Re: Boundary loading for non-perpendicular planes
Posted: Thu Jul 17, 2008 9:18 pm
by Bill Fitzmaurice
Gauss wrote: My concern is that the soundwaves traveling from the speaker will reflect at every distance up the wall and out along the floor and then cancel wherever they meet.
.
Not with bass frequency wavelengths at least eleven feet long. The only cancellation you might get close to the cab is off the ceiling or adjacent walls.