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[scott foster]

When trying to avoid the worst cancelations in a room related to various placements of subs - where on the sub does one measure from to determine your distance from the boundary?  Specifically with ported, dual 18's.  Is it just the mid point between the 2 speakers?  Or the mid point between the speakers and ports?  Or the outside edge of the nearest speaker to the boundary?  I can't seem to find this info anywhere.

[Dave Rickard]

I measure from the driver.  Some people point them into a wall, if the xover point is low enough and delays can be matched.

I also try to place subs so that inches are irrelevant.

[Mac Kerr]

Since cancellations starts when you pass 90deg out phase, and doesn't peak till 180deg, cancellation will happen over a range of between 1/4 and 3/4 of a wavelength. The wavelength of a 100Hz acoustic sound wave is about 11 feet, and 80Hz is about 14 feet, for half waves of 5.5 feet and 7 feet, lower frequencies are longer. The dimension of the subs is pretty small compared to the lengths you are dealing with.

Mac

[scott foster]

Thank you for replies.  Anyone know of a simple freeware program to model room modes/cancellations vs sub placements?

[Dave Rickard]

This will get you started, but it's not software.  Meyer has MAPPonline, but I don't remember if it models boundaries.

[Dave Rickard]

Mac Kerr wrote on Tue, 13 March 2007 19:39

Since cancellations starts when you pass 90deg out phase, and doesn't peak till 180deg, cancellation will happen over a range of between 1/4 and 3/4 of a wavelength.

I never thought about it not being so black and white, but it makes sense.  Thanks for todays lesson, Mac!

[Tim Padrick]

This chart may save your calculator finger some grief: http://www.padrick.net/LiveSound/CancellationMode.htm

[Mike Pyle]

Tim,

Would the sub-to-sub cancellation distance at a given frequency equal twice the boundary cancellation distance?

[Tim Padrick]

On that I have no idea.  I don't think the Power Alley article gets that in depth.  Plus indoors you have so many reflections that computation would be exceedingly complex I think.