Actually, the "huge" problem is due to the enclosure, not the dipole arrangement itself.
Correct, but in order to separate the front wave from the rear, there must be some separation baffle, either wide or deep. Through my own testing, I have found significant resonances in these open chambers of air resonate. Both the Orion and the Phoenix Dipole woofer use a variety of notch filters to deal with these resonances.
And with proper filtering and crossing about an octave away from these frequencies it is not a huge problem at all. In my design, I took the route of a sealed box woofer over a dipole woofer system for several reasons:
1. Allows for a higher frequency crossover from midrange to woofer. This places less strain on the midrange.
2. Much greater output capability (dynamic range) in frequencies below 100Hz. than a similar dipole arangement with two woofers per side.
Simpler overall system design, with better overall sound quality, and greater dynamic range really leaves no other option.
The Linkwitz Orion uses a 4th order LR electrical crossover at 120Hz and the excursion of the driver isn't all that large.
Correct, but the midrange dipole losses are also compensated for, so remove the 4th order High-Pass, and the midrange will play flat down to around 30Hz, where the natural roll-off of the driver occurs and the Low-Shelving filters run out. So, the 4th order electrical filter in that case is a 4th order acoustical filter since driver impedence dosnt matter with active filtering.
In fact, I'm told that the woofers run out of steam well before the midrange shows any sign of stress.
This all depends on the music you are listneing to really.
The midrange is most limited in the range right above the crossover frequency. At 140Hz, the midrange would be limited to 108dB at 5mm of excursion. The woofer system is at or below that point from 82dB on down. So, again, it really depends on spectrum. Technicaly, the woofers are more limited, but in my experience, placing the Seas W22 on an open baffle of about 11-12 inches wide, crossing 4th order at 120Hz, there is significant movement required by the midrange. At these frequencies, I would prefer slightly more dynamic range.
I'm curious if you are using an XO point of 250Hz in this speaker. What order is the crossover?
Acousticaly, 3rd order. Even power response... goes with the whole theme.
As to my quote, That was 2 years and 7 months ago, and I had just started playing with dipole speakers about 6 months before. I had recomended that as a 'rule of thumb'. At this point, and back then, I feel/felt that if properly done, any crossover point can be done to sound good. Actually I prefer the sound of a tweeter crossed pretty low, cutting that 300-3000 range rigth down the middle
. If you have heard the Orions, or any of SPTech's speaekrs, I am sure you would agree. There are very few woofers that dont exhibit very bad properties above 1500Hz or so, such as stored energy and harmonicaly exciting higher frequency resonances. Tweeters do a much better job in this range.
For advanced speaker designers that have designed 20 different (good) speakers using passive, active and DSP crossovers, I dont think that it holds water. For a novice that had posted on the DIY Audio boards, I still think it is a good general guideline, the project that he was about to embark on was nearly impossible, I have no idea how it turned out.
