A good point. A multi-sub arrangement for example typically overlaps with the mains. The thing is that I usually go sealed for the mains, the subs do extend the response and I still don't count them as an extra per se.
I wouldn't really call a half way a crossover. In camplo's case the woofers could be fixed passively after being actively crossed as a pair.
You seem to have a goal of ruining what could be smooth frequency response and consistent polar response 😉
The best (sounding) 15" midwoofers are designed for vented cabinets, but also work in sealed boxes of course.
Because the cab is 'undersized' and SPL requirements are 'substantial'?
It seems Americans are bass heads par excellence.
Germans as well.
This is also related to the size of the average (living) room.
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From the outset the low-end extension and SPL targets dictated that subwoofage would be needed. High-SPL midbass extension below 80 Hz was therefore unnecessary for the two-way main speakers. This allowed the consideration of midwoofers which would have been impractical for use in a full-range two-way. The midwoofers are each in their own isolated subenclosures, which are not "undersized" for this application.
Here's what they look like paired with their 21" subwoofers (mostly hidden by the console), photo courtesy of Mix Magazine:
What do you not like about the idea of slotting the two 15"s? The polar will be wider than the horn at crossing.
Considering your large horn, I'd be inclined to sandwich it between 2 of these (placed horizontally):
You could also vent such cabs in combination with an 'early' LP.
I am trying to not grow on vertical baffle unless it were in the fashion of MTM. Rather, is this a bad thing or what? I am going to cross low, lets face it. All I am trying to do is widen the pattern so that it is wider than the horn at XO.
Someone did some ground work for me. Look how much better behaved the null is. These are 15" woofers, 18" would not be major difference? ballpark null at about 500hz. I also have no idea how he took his measurements and with this being open baffle.... big question mark on this as a sealed enclosure, Seems like the slot null would be the same.
Thanks!
We went with the HF1440 (not installed when that photo was taken), crossing around 800 Hz.
Terrible peaky response @200Hz, though.
Not entirely clear to me what this solves vs. two normal front-facing woofers... especially if the intent is to cross them over to a large horn in a 2 way.
Two solutions for the peak, first is EQ. In simulation, an EQ'd response shows a well behaved Spectrogram in HR. Not to say I have Sim'd every possible situation. Some of the situations slotted still leaves a slight hint of excess decay. In the situation above, Damping the rear 50% of the slot should improve things to, very good, according simulation.
I can only speak personally as to why I am playing with this opposing woofer in the slot business. Headroom and 1 meter listening. As I've said, sitting so close, makes the loudspeaker vulnerable. I find myself sensitive to baffle position of the drivers. I don't have huge issue to forward facing woofers but 2 18"s would make the cabs about 4" wider. Plus this opposing deal is supposed to cancel out mechanical vibration, and summing between the two woofers is obviously excellent, separated by only 5.5" inches. The reducing terminus area vs radiation area is beneficial to imaging, in my opinion. A lot of people like to say bass is not apart of imaging, I disagree but this is most true at close listening positions such as 1meter.
So above is what I supposedly would have if I fill half my Slot with Rockwool. I have already been very happy with what I have been able to do with low cross overs and the horn+PPSL as a two way. I moved on to trying to improve the slot, which is how I ended up with the above simulations. The "mks rayls/m" are low balled btw, cause thats how I like to design, of course, actual measurements would seal the deal. At this point any space between the Horn and the 18"s woofers in PPSL is essentially dead space. I could, add the single front facing 15".... but why add one when you can add 2... 2 seems appropriate anyways. The narrow parts of the horizontal polar (as a result of side by side woofers) are not desirable for crossing to the horn. So I proposed a slot as shown below to widen the pattern. I can fit this above the 18" ppsl without raising the height of the enclosure. I don't want to raise the height of the enclosure either, its pretty close to perfect at 26" high and any bigger it can't be moved from room to room without disassembly of some sort. I also come to the conclusion that distance from each source is important to me, so no reason to differ from the good job I've done so far in regards to XO point and center to center. The ratio of CTC spacing vs XO point is that of a small 4-5" woofer+1" tweeter two way, but its a Horn+PPSL that can hit 115db within Xmax down to 30hz. To me, thats winning for a floor standing 2way that can potentially have the Tweeter near or below ear height. I don't have any intentions in particular in raising the XO point, but its always good to try to incorporate some versatility into a design and more woofers means less excursion. So once again, theres open baffle space and more headroom to be gained, in my eyes.
I am also one urge away from stacking 2 ppsl on top of each other as TMM or in MTM. All the MTM's that @Ro808 showed are symmetrical. If I were to do MTM I would be trying 18"s at the bottom and 15"s at the top. Excursion would be pretty low, potentially <2mm and that should take care of some of the differentiating signature between the two un-identical sources but thats a roll of the dice if I would like it. I am also crossing at just above what some would consider Subwoofer territory so signature difference maybe not be such an issue, but its still guess work at this point.
Keeping all the woofers together allows me to use them redundantly as much as possible, increasing efficiency. If I evert convinced myself to just build a larger enclosure for the 15"s and use the 18" PPSL as rear subs, I still.... I would build another PPSL out of the 15"s to mate to the horn.
I digress, in the meanwhile, whats the deal with these baffle slotted woofers lol
I get that I want to keep any resonances created, out of the passband. A thing that I wonder about is the idea that Resonances, are excited by all kinds of acoustical stimulus and not just signal playing at the resonances. What I am most interested in is seeing how the comb filtering is affected by slot. Unfortunately there's no simulations of side by side woofers behind a slot.
The crappy frequency response and resonant time delay that will result are what turn me off to the idea.
Other than that...
The largest reflection, I think, exists from the proposed slot edge to the edge of the driver... That dimension would be ~7.5" so the resonant frequency ~451hz. I can cross under that.
I know you said a slot would not effect comb filtering, but in my ripple tank sim, it did. Below are demonstrations of the same frequency, 2 drivers spaced on a quasi infinite baffle, one sim has the addition of a "slot" of sorts.... Maybe I did it wrong, I dunno.
Fullrange Fane 12-250tc and NIWO 18" Eckhorn (2.1 system). Fane fullrange without crossover at 90 liters closed enclosure and the NIWO bass horn active filtered with Thommessen Subway x. But the Fane can combinied with other 18" bass drivers as a F.A.S.T. A crossover is only neaded for the bass driver.
Troy recently compared 2 vertically positioned woofers to 2 horizontally positioned ones with interesting results. As a consequence he has used 2 x 10” SEAS woofers mounted horizontally in a recent design, system 2800 if I recall correctly.