Can someone check this for accuracy?

Message

Tim A · #1 Post by **Tim A** » Mon Mar 24, 2008 8:08 am

I'm making/copying useful charts to carry along to gigs. Someone here recently posted an excellent Near Field/Far Field transition chart which I've copied and kept. I'm also using the boundary cancellation chart on Padrick's website. Charts are cool! It let's me look at the situation and make some quick decisions.

This is one I found and modified to include double wave lengths for cancellation between subs. It looks accurate based on numbers I've heard here, but would like some confirmation.

Also a question about cancellation. If a guy were to place his subs 25' apart, which per this chart is a cancellation distance for 90Hz, would that mean everything from 90Hz and lower would be affected to some degree, and everything above 90Hz would be fine?

Tim A · #2 Post by **Tim A** » Mon Mar 24, 2008 8:14 am

Crap. I've tried to get it to align and it won't. It might be kind of hard to decipher. In case it is:

each line = frequency, length in feet, 2X length, 1/2 wave, 1/4 wave.

#3 Post by **Bill Fitzmaurice** » Mon Mar 24, 2008 8:25 am

Tim Ard wrote:Also a question about cancellation. If a guy were to place his subs 25' apart, which per this chart is a cancellation distance for 90Hz, would that mean everything from 90Hz and lower would be affected to some degree, and everything above 90Hz would be fine?

No. What is affected is the one frequency where the difference in the distance from each source to the listener is 1/2 wavelength, and that frequency changes as the listener moves within the coverage zone. The problem is not the distance from sub one to sub two, it's the distance from the listener to sub one minus the distance to sub two. By placing the subs at least 50 feet apart for the most part the null frequency will be raised to outside of the sub passband, and/or the listener will be hearing the output of either one or the other but not both.

Tim A · #4 Post by **Tim A** » Mon Mar 24, 2008 8:33 am

Bill Fitzmaurice wrote:
Tim Ard wrote:Also a question about cancellation. If a guy were to place his subs 25' apart, which per this chart is a cancellation distance for 90Hz, would that mean everything from 90Hz and lower would be affected to some degree, and everything above 90Hz would be fine?
No. What is affected is the one frequency where the difference in the distance from each source to the listener is 1/2 wavelength, and that frequency changes as the listener moves within the coverage zone. The problem is not the distance from sub one to sub two, it's the distance from the listener to sub one minus the distance to sub two. By placing the subs at least 50 feet apart for the most part the null frequency will be raised to outside of the sub passband, and/or the listener will be hearing the output of either one or the other but not both.

Got it, and that explains a lot. I had only part of the info, the rest of it resolves much.

I got the table fixed too, if anyone wants it, although based on Bill's explanation it's rendered almost useless.