How to calculate floor bounce reflection(cancellation)

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jtsaudio

jtsaudio

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#1
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
 
#5
jtsaudio said:
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.
Click to expand...

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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#6
Michael Chua said:
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
Click to expand...

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
 
#7
Michael Chua said:
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
Click to expand...

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.
 
#8
CharlieLaub said:
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
Click to expand...

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.
 
#9
jtsaudio said:
How do I determine the frequency at which this occurs, and how will/should this affect my choice of lower-end crossover frequencies?
Click to expand...
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.
 
#12
Wayne Parham said:
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.
Click to expand...

I don't know how I stumbled onto this old thread but I also prefer stereo subs "flanking" usually behind the mains as you do. There is an interesting analogy but you have to consider it in reverse. Modes in a room are much like the signal strength contours seen with multipath FM. The solution for FM is diversity antennas (spaced out) and for speakers are spaced drivers covering the same range. PSB mentioned spaced drivers (the woofer and midrange) crossed over so that the floor bounce was helped by having 2 sources in the literature for the PSB Stratus Gold. Speakers are somewhat like antennas in reverse.
 
#14
Multisub approach is a good tool for taming room modes at low. 150 to 800 Hz is very prone to frontwall and floor bounces/reflections/cancellations. If one has a measuring system like REW it is necessary to make several measurements also by moving the microphone! Also the wall behind the listener is problematic. Wayne Parham has a very good writing of this (linked earlier)!

One issue not often discussed is that reflections have a different phase from direct sound. Cancellation at 180¤ is most easily measured but milder deviations mess up sound too. I have recently fought with response and phase matching of my 4-way AINO speaker. Outdoor and indoor measurements are other worlds! By the way AINO's downfiring woofer plays is crossed at almost 300Hz and thus range of of 100 to 400Hz is played from two points some 60cm apart. I have measured so far only one speaker playing, but the response looks promising
351886d1370027741-aino-gradient-collaborative-speaker-project-ainog-ice-v1-room-spl-phase-200ms.png


Acoustic design and treatment is important, but difficult to do in living rooms.

I have found that sometimes it is not wise to smooth out all of the measured response electrically - the ear-brain system has more clever way to even out direct and reflected sound!
 
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