Pros / cons of a WWMTMWW design?

[fjhuerta]

Well, besides being physically huge, that is...

Consider a WMTMW with RS270 drivers as W's, vs. a WWMTMWW with RS225's as woofers.

Interior volume should be roughly the same - 200L.

Output levels would be very close, but impedance (with the 8 Ohm models) would be about 8 Ohm for the RS225's, vs. 4 Ohm for the 270s.

The thing I'm not so sure about is - vertical dispersion.

If I had a WWMTMWW would I need to roll off the outer woofers if I'm planning to use a 250 Hz crossover to the M's?

The reason I'm thinking about 4 RS225's per side is price. Buying 4 270's costs about $440 with s/H to Mexico. I already have 2 225's, so 4 extra 225's are $280 USD.

What do you think?

[charliemouse]

I'd imagine as long as you're making sure the tweeter is at ear height, any comb effects shouldn't spoil things too much. I'd hazard a guess you'll get more of an effect from the interaction of the speakers location with the room (WWwmtmw vs WwmtmwW) than with their vertical dispersion?
Once you're onto meters between the furthest appart driver, you're in the same region as the distance between speakers. I always wondered about this. If, for example, a design would have horrible comb patterns if the woofers are 1.2m away from each other... just how much more of an effect would 3 m between the speakers have on the interplay between the drivers? Not much relevance to your questions, but you got me thinking about this again. When working on my speakers I ended up giving up with theory, and played with lots of designs and found dipole bass was much more room-tolerant than sealed boxes whether WWwmtmw or WwmtmwW.

[Dennis H]

I don't think either would be much of a problem as long as you cross the woofers low enough. Yavuz's line array tool is pretty slick for modelling that stuff. Set up an array of drivers and set the center one(s) to have a power of 0 to simulate the space where the MTM section is.

http://www.pvconsultants.com/audio/radiation/vpr.htm

[fjhuerta]

I'll be crossing the woofers at 250 Hz at most. The wavelength is 1.36 meters. If I understand correctly, The distance between drivers can be separated by half the wavelength. 68 cms.

I could get close to that distance with a WMTMW. A WWMTMWW would have a much bigger distance between the outer woofers.

Hmm.

[---k---]

Output levels would be very close, but impedance (with the 8 Ohm models) would be about 8 Ohm for the RS225's, vs. 4 Ohm for the 270s.

My models show that 4-rs225 give about 4db more max output - maybe I got something wrong.

Does the woofer impedance matter? I know you were talking about using the RS28a for a tweeter. In both the Khan's and CJD's RS-3ways, the impedance dips way down due to the tweeter.


I know you can raise this, but that just makes the crossover more difficult, and maybe needlessly so. Is the impedance the only reason you're looking at going with more drivers? Or is it SPL or Extension?

With 2-RS225 the tweeter is already the limiting factor on max SPL. After that, probably the 2-RS150s. Going with 2 RS270s over 2 RS225s gives you more bottom end, so that is worthwhile. But, I'm not sure what going to 4 RS225s will gain you.

Maybe the crossover guru can give you more insight. I need to start doing crossovers, so i'm not talking out of my butt so much...

[fjhuerta]

Ryan, I'd use a series-parallel combination, so final impedance would be close to 8 Ohm.

Although impedance (theoretically) shouldn't matter - I'm using a pair of Class-D amps, and supposedly they are stable down to 1 Ohm

[looneybomber]

Why do you want to go with a wwmtmww design? As already stated, two rs225's are plenty. There's really no need to up their size and double their quantity unless you're wanting some massive bass. In which case, I'd suggest building some very capable subs.

If SPL is your goal, ditch that tweeter.
If SQ is your goal, save money, cut your woofers in half, and build the Khan's.