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Discussion Starter · #1 ·
hey folks,


anybody have thoughts on boundary cancellation? this article suggests that placing a source 1/4 wavelength from a boundary will cause a 3db reduction in spl. the effects are additive, so a source placed 3.5 ft from the floor, the rear wall, and the side wall will create a 9db hole in the bass.


http://www.peavey.com/support/techno...ncellation.cfm


so, for the first question, is this theoretically correct?


second, a quick look at a table of cancellation frequencies is concerning as it suggests that most folks are going to be experience some cancellation in the bass region, since most people have speakers that are 1. about 3.5 feet off the floor, and/or 2. about 3.5 feet from a wall.


Frequency (hz).....1/4 wavelength (in feet)

60......................4.71

70......................4.04

80......................3.53

90......................3.14

100.....................2.83

110.....................2.57


now we know that each boundary *increases* spl in the bass region because the source is operating in less than full space. that is a different concept than this one which suggests that source placement can result in narrow holes in frequency response based on distance and frequency.
 

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Let's stick to the 3.5' example.

80Hz has a 14' wavelength. When the sound "leaves" the driver it is at phase 0 (degrees), at 3.5' it is at a phase of 90 degrees, at; 7' it is at 180 degrees and at 14', at 360 degrees (or back to zero).


To your issue, the sound leaving the speaker strikes a boundary 3.5' away (at a 90 phase), reflects off the boundary and returns to the speaker (after travelling another 3.5') and is now at a 180 degree phase. That 80 Hz wave is 180 degrees out of phase and causes a cancellation with the 80 Hz signal at 0 degrees. You have a 3dB notch or cancellation. With both of these waves propagating across the room together, you have a continuing 180 degree out of phase, or cancellation, issue.


The solution to this to use bass management with an 80 Hz crossover and keep the LCR speakers more than 3.5' away from the boundary (the speaker is not producing significant energy below 80Hz, therefore no notch.) Keep your subs closer than 3.5' to boundaries to eliminate notches from the subs.


Keeping the subs close to boundaries allows boundary gain without phase cancellation. Since the highest frequency (ignoring crossover slope) produced by the sub is 80Hz (or shortest wavelength), placing sub(s) closer than 3.5' provides boundary gain rather than a notch.


A simple explanation to help you visualize what's going on.
 

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Discussion Starter · #3 ·
thanks dennis! great explanation.


so in addition to room modes (axial, tangential, and oblique) and boundary cancellation, are there any other room effects in the bass with which to concern ourselves?
 

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Just an elaboration on Dennis' excellent explanation of 1/4 wave cancellation. If the reflection is not at right angles (as from a front wall), then you simply take the path length difference between direct and reflected angles to get the phase difference. Any frequency at which this phase difference is 180 degrees can produce a phase cancellation notch.


This can be an issue for side wall or ceiling reflections. Remember that subwoofers radiate sound omnidirectional. Otherwise, you wouldn't have such issues with the front wall (which is directly behind the subwoofer) to begin with!


Regards,

Terry
 

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Quote:
Originally Posted by LTD02 /forum/post/15457566


are there any other room effects in the bass with which to concern ourselves?

Just to clarify, the worst quarter wave cancellation is often from the rear wall behind you. The quarter wave distance is measured from that wall. Other boundaries also reflect and create quarter wavelength-related peaks and nulls. But as Terry explained, the angles are different so the distances vary too. More here:

Frequency-Distance Calculator


As for your main question, another important bass problem is modal ringing.


--Ethan
 
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