in that Legacy speaker the drivers are both facing forward in contrast to the clamshell. Do we know for a fact that the Legacy drivers’ cones move in the same direction as each other ie acoustically in phase just like the clamshell ?
YES. If you do a quick search for "Legacy Audio Whisper compound-dipole" in Google you will find some links that talk about it, and even the MFG specifications state: Low Frequency Alignment: 6th order differential (compound dipole)
The direction that the drivers face does not change the radiation pattern, which is a dipole just like with a single driver. The separation between the two top woofers is about the minimum that should be used. I was not able to figure out how the lower two drivers are configured since I did not find any pics of this part of the speaker, but since it is two 15" drivers and the speaker is not large I assume those are also set up as a compound dipole.
Edit: I found a pic here that shows the the rear at bottom is open and you can see the rear of the driver. It looks like there might be a smaller subwoofer at the rear, midway up. See pic about 1/3 of the way down this page:
https://audioexchange.co.za/product/legacy-audio-whisper-speakers-rare-rosewood-set/
The direction that the drivers face does not change the radiation pattern, which is a dipole just like with a single driver. The separation between the two top woofers is about the minimum that should be used. I was not able to figure out how the lower two drivers are configured since I did not find any pics of this part of the speaker, but since it is two 15" drivers and the speaker is not large I assume those are also set up as a compound dipole.
Edit: I found a pic here that shows the the rear at bottom is open and you can see the rear of the driver. It looks like there might be a smaller subwoofer at the rear, midway up. See pic about 1/3 of the way down this page:
https://audioexchange.co.za/product/legacy-audio-whisper-speakers-rare-rosewood-set/
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We seem to have two opposing views being put forward on this thread:
A) subject to the drivers being sufficiently far apart, the output from the clamshell = the SPL from 2 drivers side by side.
B) The output from the clamshell is something less than the SPL from 2 drivers side by side.
A) subject to the drivers being sufficiently far apart, the output from the clamshell = the SPL from 2 drivers side by side.
B) The output from the clamshell is something less than the SPL from 2 drivers side by side.
When:
When you deviate from these conditions the SPL may less, for example when
- the drivers in a compound dipole are sufficiently far apart from each other and
- you observe them in the far field where the distance to each is roughly the same and
- when each driver is getting the same power as a single driver,
When you deviate from these conditions the SPL may less, for example when
- the drivers of the compound dipole are wired in series and connected to the same amplifier
- the separation of the drivers of the compound dipole is very small or zero
- the measurement/observation location is very close to the compound dipole
I do not have data on that. Perhaps you are willing to make both arrangements and measure.
I did dipole vs bipole long time ago, but with baffle, not naked. Its in my blog.
Hope this isn't too OT. I'm currently experimenting with a similar concept with my OBs. Not clamshell, the opposite. Added a second Eminence Beta 15 to the first front mounted one. Tried it both Legacy and Emerald Physics style. Settled on the latter for now, but that could change. Wired out of phase (so there is a null in between the bass drivers). Sorry, no measurements just subjective. There is an improvement in performance over using just the single driver, in my set-up and smallish room. Advantage here is the second driver is removed from the front baffle, allowing it to be a lower height, with less added vibration. Probably the same as a single driver in a H frame, but an easier addition to what I already had. Plan on adding some Sonic Barrier material to fill the inside cavity of the back baffle and actually attaching it to the base (as is, just free standing).
https://imagizer.imageshack.com/img924/4096/dhLUKe.jpg
https://imagizer.imageshack.com/img923/6218/0xxqrV.jpg
https://imagizer.imageshack.com/img924/4096/dhLUKe.jpg
https://imagizer.imageshack.com/img923/6218/0xxqrV.jpg
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For $13K, NOTHING should lacking. Those bad boys better be flat to 20hz ground plane.
"This unequalized response drops rather smoothly and is down 9 dB at 50 Hz and 13 dB at 25 Hz."
"Their weaknesses are a lack of bass capability below 25 Hz and a lack of bass punch below 100 Hz."