15" vs 18" subwoofer - which to buy?

Eldartford: I probably didn't state things very clearly. You are correct, it's not really inertia in the classical sense, rather it's the amount of current needed in the voice coil to get the driver moving in the first place. You have to overcome that intial friction. I've been told by speaker manufacturers that this is an issue for larger drivers. To move them properly with good control you have big magnets and big voice coils and this can become an issue playing at low volumes. (A speaker manufacturer is probably better to comment on this than I am) You are also correct about the perceived loss of bass at low volumes, this is the work done by Feltcher and Munsen, that shows how our ear sensitivity drops off in the lower octaves and that it is a non-linear phenomenon and becomes a greater discrepency at lower volumes. Lastly, with regard to your comments about the servo. You are probably right too, though some servos are better than others (I've measured some that weren't too hot), my guess is that Velodyne has been doing this for so long that there's is probably pretty close to spot on--but having never measured it I don't know for sure. Keith Yates did a few articles on subwoofers and some very extensive measurements. I don't think they published the low level data, but he may have it. Now I'm curious and will likely ask him.

To follow-up, I called Bag End and the sales guy told me that there was no difference in quality between the Infra 12" and 18" at low volumes (I said I heard a large difference at any volume between 12" and 15" with B&Ws, and he said, "well, not with ours"). This was also quite different from what the Velodyne guy told me, which was that anything 12" and below is basically a 'toy' and that the move to a 15" is a dramatic step up at any volume. The Martin Logan guy gave me the opposite advice- that the smaller Depth may have an edge in clarity and response over the Descent at lower volumes, because of its agility.
Three different manufacturers, three different basic responses...bigger is better, no difference, and smaller is better.....

Bag End's are designed COMPLETELY different from ANY other subwoofer made. As such, they will respond differently. The one thing that will be consistent with Bag End's regardless of size as compared to other subs is that they will require more power to achieve the same level of volume. This is directly related to the design choices that they made when building these products.

Other than that, all drivers have a "knee in the output curve", just like all semiconductors do. The subs using a servo system can take this into account whereas those without some type of active equalization / monitoring system can't. I'm quite certain that the Velodyne's have this factored in. The Bag End's also vary bandwidth as spl is varied i.e. the lower the spl's, the deeper the sub goes. As spl is raised, the active EQ system reduces the boost applied at the extreme low frequencies. While this occurs naturally with ALL woofers to some extent, the Bag End's do this not only acoustically, but also electrically. Like i said, they are kind of a novel design. Sean
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I have had alot of experience with both, DD-15/DD-18 and ended up with the DD-18s each time.

I ALWAYS run subs in pairs. Aerial SW-12s, Velodyne F-1800R's, HGS-18's, HGS-18II's, DD-12's, DD-15's, and DD-18's. I had a pair of DD-18s, sold them to go back to the Aerial SW-12s.

Now, these are the only subs I like personally. The SW-12s did better at all volumes/material than did the HGS series of subs. I preferred the 15s over the 18s from the HGS series, as the 18s didn't have enough power to keep up with the driver, so it was slow and needed volume/room to 'catch up' to my mains, but was quickly overpowering even set at about 2. Then the DD series emerged. Again, I got into the 18's, and was shocked at the presence at low to mid level volumes and the spectrum they can cover. I know Velo told me they increased the output by 2DB, which normally isn't a big deal, but in subs it is a HUGE deal, and mid level volume is where I noticed it most. No, wait, maybe that was because I was finally able to dial in the room with the built in spectrum analyzer?
No wait, maybe it was the increased servo speed of the DD versus the HGS? No wait maybe the spectrum? No wait, maybe..........anyway, you get my view.

The change between the HGS and DD was a huge step. Now, 15 versus 18s? I doubt VERY much at normal / moderate listening levels anyone is to notice differences between the 15's and 18's. Anyone unless trained professionals with perfect rooms and ears. As you probably know, the room will change the sound more than anything else, so probably no reason to get hung up on the sound differences between these two at normal levels. My bet is they will act / sound the same.

The big differences kick in as the volume levels increase and you start to walk away from the mid-low bass of the 15s and start getting into the quick, deep push of an 18. There is nothing, imo, like a correctly powered 18" sub driver, actually 2, in a system for 2 channel. Just incredible. I have someone that has a full scale ORGAN (like 100ft tall, 200ft wide) that bought 2 DD-18s for reproduction of sound in the hall! And home theater, well, suffice it to say, if it is good for 2 channel, it is great for theater. I don't think there is a better sub for 2 channel or theater, low to high levels of output as the DD-18s. I definitely feel bass is directional, despite what many others say, and in stereo, dialed / spectrumed (is this a word?) in there is no better.

Alot of folks in the 18" game look into Bag End. They never had enough power for my liking, which led to weak/anemic tonal control of the driver. I am not a huge fan of ported subs, as the Aerial SW-12s are. The had to be to achieve volume but the mass air push on my legs during huge passages was quite annoying despite their nice presence at normal levels. Servo's of the DD series are so much more advanced than those of the HGS series (alot of hum, buzz, etc.) and seem to get bass right.

Now, again, this is all my experiences, my opinions, and you know what they all say about opinions? Like asshol_s, everybody has one. But, that being said, I am not a dealer, I don't 'represent' any lines, have no affiliates, and have had the ability to try / demo many subs (powered and not - passive like Snell 18's, JBL 18's, and some older Kinergetics) and have landed 100% into the DD line. If space (physically) isn't an issue, go 18's, you will not be disappointed!

For what is it worth, I am running 2 pair of JBL K2S9800SEs, matching center, Levinson 33's, Meridian 861/800 for theater, and the Conrad Johnson Premier 8A's and the CJ ART in 2 channel. Oh yeah, a trio of DD-18s, stereo pair in front, single in back (for theater - as if really needed).

Dan

Sean...Other than compression at some high level of SPL, I can't think of a reason for a low SPL "knee" in driver sensitivity. Can you provide any further info/references?

Equalization that is SPL-dependent is another one of the tricks that the ubiquitous Behringer DEQ2496 can do. I haven't tried it yet, but I have hopes that it might provide the "Loudness" function which my preamp lacks.