How to calculate floor bounce reflection(cancellation)
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 23rd September 2011, 04:28 PM #1 jtsaudio   Account disabled at member's request   Join Date: Dec 2007 How to calculate floor bounce reflection(cancellation) I have a 6" two way tower speaker that I may add a larger woofer to for more extended bass. I read somewhere that there is a floor-bounce cancellation related to woofer height (Allison effect?). The 6" driver is approximately 30 inches from the floor. How do I determine the frequency at which this occurs, and how will/should this affect my choice of lower-end crossover frequencies? Thank you. Joe
 23rd September 2011, 04:48 PM #2 dantheman   diyAudio Member     Join Date: May 2008 Location: Mountain View, CA __________________ My Blog My Music Recordings
 23rd September 2011, 05:10 PM #3 CharlieLaub   diyAudio Member   Join Date: Mar 2007 Location: Michigan This Excel spreadsheet based modeler written by Jeff Bagby does the same, and more: Loudspeaker Design Software -Charlie
 23rd September 2011, 05:17 PM #4 DBMandrake   diyAudio Member     Join Date: Nov 2010 Location: Glasgow, UK For a really quick, back of the napkin calculation, there is also this useful page: Floor/Ceiling Reflection Calculator __________________ - Simon
Michael Chua
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Join Date: Jul 2006
Location: Calais, ME
Quote:
 Originally Posted by jtsaudio I have a 6" two way tower speaker that I may add a larger woofer to for more extended bass. I read somewhere that there is a floor-bounce cancellation related to woofer height (Allison effect?). The 6" driver is approximately 30 inches from the floor.
The floor bounce is real. I picked this up while I was designing a 2-way with Zaph's ZA14W08. Speaker was on a 30" stand.

The cancellation doesn't show up in near field measurements. However, when the mic was placed 1 meter away, an in-room 20-20K ungated sweep registered a deep notch at approx 150Hz.

That's one reason why mid-bass is missing sometimes.

More measurements can be found at SWIFT
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CharlieLaub
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Join Date: Mar 2007
Location: Michigan
Quote:
 Originally Posted by Michael Chua The floor bounce is real. I picked this up while I was designing a 2-way with Zaph's ZA14W08. Speaker was on a 30" stand. The cancellation doesn't show up in near field measurements. However, when the mic was placed 1 meter away, an in-room 20-20K ungated sweep registered a deep notch at approx 150Hz. That's one reason why mid-bass is missing sometimes. More measurements can be found at SWIFT
A 3-way with the woofer located at or very near the floor, and the woofer-midrange crossover point at or above 200Hz, can completely eliminate this problem.

-Charlie

Wayne Parham
diyAudio Member

Join Date: Feb 2004
Quote:
 Originally Posted by Michael Chua The floor bounce is real. I picked this up while I was designing a 2-way with Zaph's ZA14W08. Speaker was on a 30" stand. The cancellation doesn't show up in near field measurements. However, when the mic was placed 1 meter away, an in-room 20-20K ungated sweep registered a deep notch at approx 150Hz. That's one reason why mid-bass is missing sometimes. More measurements can be found at SWIFT
Floor bounce definitely is real. So is the notch from the wall behind the speakers. Vertical axis room modes also tend to fall in this frequency range. Really messes up the lower midrange.

I tend to suggest flanking subs to mitigate this problem. Run a "helper woofer" just a smidge on the high side, like just north of 100Hz. It will smooth the lower midrange, sort of filling in the holes from the self-interference notches.
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Wayne Parham
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Join Date: Feb 2004
Quote:
 Originally Posted by CharlieLaub A 3-way with the woofer located at or very near the floor, and the woofer-midrange crossover point at or above 200Hz, can completely eliminate this problem. -Charlie
Definitely. I either run a three-way with mid and woofer blended between 100Hz and 250Hz or, if I run a stand-mounted two-way, I run flanking subs, which effectively do the same thing. I don't usually run flanking subs quite that high, but I do let them blend in the octave above 100Hz.
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bbggg
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Join Date: May 2007
Quote:
 Originally Posted by jtsaudio How do I determine the frequency at which this occurs, and how will/should this affect my choice of lower-end crossover frequencies?
It's dead simple both to visualize and to calculate. You have to imagine a mirror image of the driver deep inside the floor. When sound emission from the driver itself and from this subterranean image are in antiphase, there is cancellation. This happens when path length difference is half a wavelength. You calculate the path length difference by calculating two hypotenuses and subtracting one from the other, and then divide the speed of sound by the result: this number is twice the frequency in question.

 14th April 2013, 03:41 PM #10 fatmarley   diyAudio Member   Join Date: Aug 2003 Location: Cheltenham Could I use a 10" wide baffle and still cross over around 200Hz with a floor mounted woofer? Or do you have to use a wider baffle for this to work?

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