Say if I'm designing a 3 way floorstanding speaker, and to eliminate cabinet reduction, I will have 2 woofers dual opposed. I know that bass is omnidirectional, so dual opposed subwoofers are awesome, but the woofer will be crossed to the midrange at 300Hz. At that frequency, would the fact that there is a woofer firing backwards negatively affect the <300Hz sound quality?
There will be comb filtering due to the inherent delay of the sound coming from the rear woofer.
When the woofers are 90degrees out of phase, the overall magnitude drops 3dB:
F3 = 340/(4*d)
And a null when they reach 180degrees:
Fn = 340/(2*d)
So for a cabinet 0.3m deep you get:
F3 = 340/(4*0.3) = 283Hz
Fn = 340/(2*0.3) = 567Hz
This may not be all bad news if you plan to cross at 300Hz - you can use the combing to steepen the acoustic slope of the woofers so you might only need say a 2nd order electrical filter instead of 4th order. Only problem with doing this is that as you move off axis, the nulls move as the effective distance from the listener to each of the woofers becomes less than 0.3m different. So you may only hit the correct acoustic slope for the woofers in one listening axis.
Also if you place the cabinets too close to a wall, reflections from walls will also alter the effective distance of the rear woofer and may reflect some of the higher frequency output of the rear woofer which would otherwise not diffract around to the front of the cabinet.
When the woofers are 90degrees out of phase, the overall magnitude drops 3dB:
F3 = 340/(4*d)
And a null when they reach 180degrees:
Fn = 340/(2*d)
So for a cabinet 0.3m deep you get:
F3 = 340/(4*0.3) = 283Hz
Fn = 340/(2*0.3) = 567Hz
This may not be all bad news if you plan to cross at 300Hz - you can use the combing to steepen the acoustic slope of the woofers so you might only need say a 2nd order electrical filter instead of 4th order. Only problem with doing this is that as you move off axis, the nulls move as the effective distance from the listener to each of the woofers becomes less than 0.3m different. So you may only hit the correct acoustic slope for the woofers in one listening axis.
Also if you place the cabinets too close to a wall, reflections from walls will also alter the effective distance of the rear woofer and may reflect some of the higher frequency output of the rear woofer which would otherwise not diffract around to the front of the cabinet.
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I actually tired that once, a few years ago. It does not work very well. You can do a push-pull, slot load, but in my opinion 300Hz is too high for that approach.
Another option would be to do a push-push, listening to the backs of the drivers. I don't have an exact picture to show you what I am talking about, but you would want to mount the woofers on slant boards and couple the magnets with a wedge board. Please see the included picture for the general idea, but not the exact idea. The picture was for a subwoofer to where the configuration didn't matter for up to 100Hz. You would want angled boards in both orientations such that here are no parallel walls to set up standing waves. This apparatus would then be inserted into an enclosure which would be calculated for the remaining volume to be correct for the woofers. Clear as mud ?
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