Critique my "unique" dipole speaker

[Mazeroth]

I had originally planned to build a dipole much like the Arvo but with dual RS315s, duals RS180 mids and a single RS28a tweeter. The more I thought about it the less I wanted a 14" wide speaker and something that's already been built. I did some calculating and am almost sold on this design:

I'd like to run four RS225s at the bottom in a 10" wide x 8" deep dipole config, with a slight angle on the back, much like the Arvo. The angle would only be 2" in at most, and the 8" depth is not set it stone. After calculating the dipole output using Linkwitz's spl_max spreadsheet (http://www.linkwitzlab.com/spl_max1.xls) with 200 Sd, 28mm peak excursion (four RS225s with 7mm xmax) and an effective path difference of 660mm I get the following results of max output at 1 meter:

25hz=82dB, 35hz=91dB, 50hz=100dB, 71hz=109dB, 100hz=118dB

This is for one speaker, so theoretically two speakers should yield 6dB higher. My listening position is 3m away, so that should yield 86dB at 35hz and 95dB at 50hz, plenty loud for music listening. I have an infinite baffle sub in this room that can handle the lows, so I'm thinking of crossing over to that around 40hz to alleviate the dipoles.

For the top section I'm stuck with what to do. I like the four RS225s because I can create a slimmer and less expensive speaker, but at the same time I should be able to cross over to the mids higher, due to using a less deep "enclosure" and the better high-frequency response of the RS225s. With that, I'd like to use something that's better suited for mids and that can cross over higher to a tweeter, so I can alleviate the tweeter from having to produce too much low-end. I was thinking of going with a single mid like an RS150, dual mids in an MTM like dual RS125s, or anything else you guys can recommend. For the tweeter I was looking to use the Vifa XT25, due to it's stellar frequency response and distortion figures above 2khz.

Alas, I will be attending the Parts Express Tent Sale on July 15th and they will more than likely have a 20% off coupon. I'd like to stick with drivers I can get there but am 100% open to any other suggestions. Spending a few hundred extra won't kill me, and I was even considering going with a Seas Excel magnesium mid and one of the Seas tweeters. Again, I'm open to suggestions.

Lastly, and sorry for sounding stupid, but what are the drawbacks of going with an MTM over an MT? I would think there would be some serious cancellations due to the increased center-to-center spacing but seeing so many MTMs I guess it's not that bad? I do know they have worse dispersion? than an MT does so if you guys think I can get away with going with a single mid for a killer dipole speaker (in this config with four RS225s) then please, let me know!

Sorry for making this so long but I wanted to throw it all out there. I'm open to ANY suggestions/criticisms you may have and would really appreciate them.

Thanks! :T

[Paul H]

Sounds interesting Maz.

Personally for the upper portion I would go with a single smaller mid like the rs150 rather than doing an mtm - as you noted the 225's should be able to cross plenty high enough to do that without straining the mid at all.

Mtm's will provide more output, but do have limited vertical dispersion because of cancellation between the 2 drivers - that's sometimes considered advantageous, especially for movie watching where less acoustic reflections are sometimes considered desirable, but may not be so desirable for music - opinions vary.

A single mid will allow smaller centre-centre spacing between drivers at the higher crossover frequency, allowing a higher tweeter crossover.

8" deep with only a 10" width sounds like you're pushing at or past the limits where you'll get resonance from the cavity formed by the 8" wings. It's hard to say for sure as the math is so impossibly complicated that virtually everyone just builds a test baffle to hear it rather than try to crunch the numbers I have seen a number of comments from dipole builders that state that a clearer more open sound is obtained with the baffle closer to flat rather than having severe wings. You might consider trying a 12" front with 6" wings to compare to the 10" and 8" if/when you're making cheap test baffles.

The XT25 has shown some poor distortion test results below 3000Hz from the on line testing crew. I'd look at others - the RS28?

[Dennis H]

I think your D estimate is optimistic. I get closer to 400 mm assuming 7" average depth and that's assuming the acoustic center is close to the baffle. It's probably actually a couple inches behind that which would knock D down to around 300 mm. My understanding of D is hold one end of a string at the driver's acoustic center, run it around the back of the U and forward even with the acoustic center on the side. That shortest path will be how much 'time' the back wave is delayed relative to the front wave.

All that said, it's no big deal. With a dipole, crossing to a sub 1/3 octave higher buys you 6dB so you could do as Paul suggested and go with a shallower U and just cross a bit higher. That would help with the cavity resonance problems. Note that Jon has abandoned the U-frame for his latest version of the Arvo.

About the mid(s) you're going to need something that can cross pretty low or you'll get lobing. Your line of woofers plus a small mid will be about 3.5' long so that limits you to an XO below 350 Hz or so. I don't know if a single 5-6" will be enough to cross that low in a dipole.

[CraigJ]

Paul wrote: "I have seen a number of comments from dipole builders that state that a clearer more open sound is obtained with the baffle closer to flat rather than having severe wings."

Interesting you should mention that because, to my ears, Jon's Arvo Type 3s sounds more "open" than the Type 2s.

Craig