15" vs 18" subwoofer - which to buy?


If price and room space/aesthetics were not a big issue, is there any reason to get a 15" subwoofer over an 18" one? My main issue is not disturbing the neighbors too much. I'm confused about the difference between a 15" and 18" subwoofer. I want to get the best sound quality possible for low-to-medium volume use.

I've heard that an 18" subwoofer can be played at low decibels and provide good bass resolution and fullness of sound, while to achieve the same volume with a 15" subwoofer, the power has to be much higher. So for any given sound volume, the main difference is in distortion- and higher distortion sound may be more obtrusive. Is this true? it seems counterintuitive that an 18" subwoofer could be better for neighbors than a 15" subwoofer.

However, I compared a B&W800 (12") vs B&W850 (15"). I listened from the other side of the store wall (not as thick as my apt), and at minimum levels which I found satisfying for HT, I found the 12" more obvious and 'boomy' sounding while the 18" produced a more subtle (though actually more powerful), lower frequency vibration, like a very low-level rumbling background earthquake. The 15" subwoofer did seem more of a disturbance because of it's 'obvious' sound. However, I worry that an 18" won't even start producing quality sound until a certain volume that was much higher than a 15", and consequently potentially more disruptive for neighbors. Is this true? What is the relationship between subwoofer size, low sound volume, and subjective listening experience?

Unfortunately, it will be difficult to get an opportunity to hear both the DD-15 and DD-18 which I am considering, so I hope someone can help out here.
no_slouch
Just so you Know I run a Velodyne F1800X behind my couch in a HT setup and now I don't need bass shakers in the couch because it moves the couch plus it doesn't have to be quite as loud, you must crossover at 80hz or below becuase it is in the back of the room. So under my screen is now uncluttered! For movies go for the single 18", if it was music dual small subs but the Electronics will take care of the room problems.
Dialing in a pair of subs is quite tough, and I have not been able to figure it out quite yet. In some respects, using a single, dialed in correctly can be much more accurate than two, not so dialed in!

I run a pair of DD-18s and they seem to be pretty close to dialed. Not being a professional, I am sure there is alot of room (sorry for the pun) for correction. I will be trying to locate a JBL Synthesis professional to EQ my room with the SDEC, which should help a bit.

The DD-12 are incredible. I know several folks that run electrostats that use the DD-12. Slouch? Well, I suppose that depends on who/what you compare it to. The DD-15 and DD-18, for output levels and impact, sure. To a midfi levels of Paradigm, Sunfire (sorry guys, it doesn't compare, imo) and just about every B&W sub (I do like the older ASW-800) The DD-12 rips for 2 channel and HT.

Now, onto Aerial. I have owned / lived with Aerial SW-12 subs at least three times. Each time I used a pair as I have not felt a single can give me what the Velodyne subs can, so I ran them in pairs. The SW-12 is one of the best 3-4 subs I have had experience with. (Velo, Aerial, Rel, ATC, Earthquake (nice sleeeper here), Talon, and the others like Sunfire, Paradigm, Energy, HSU, M&K, etc)

My issue with the SW-12s are that in large passages (be it an organ or explosion) the excursion of the 11.7" driver is so much that the ports BLOW ME. On hot summer nights, it is a nice feeling, but the port shaffing noise and air does bother me. Now, at any other time, I love, love, love the Aerial. If they made an SW-15 or SW-18 I would be in as THE SUB!

One gets alot with the DD series subs. Ability to dial them in, they are incredibly quick, output levels unmatched, and their shape is much more appealing (though, in my case, 18's in any cabinet are imposing) than the older HGS series.

I am having Sound Anchors make the first Aerial SW-12 like stand for a DD-18 so they will have the platform in about 2-3 weeks. I will set the DD-18s on the stand and place a very heavy piece of custom black granite on the tops. This is the single largest improvement I have heard with subs. Placing something very heavy on top. Not sure why, but it has made a nice addition, appearance improved as well and when my cats use the top as a catapult, it doesn't ruin the finish (sm!).

Dan
Thanks guys. If you click on my system and view the pic of my current horn sub, you'll have an idea of where I'm coming from. It works wonderfully, but at 42"x42"x22", it's a monster.

I had a 9' sofa sitting on top of it acting as a 22" riser for the second row of my HT. Out of sight it provided not only great bass, but acted as a huge butt-shaker. Now that it's out in the open, my wife says it has to go. Thus my looking at Velodyne and Aerial subs, and my quandary of whether to go with one 18" or two 12".
Kenl,
With that monster sub you own now, I'd want either two 15 inch subs or two 18 inch. I'm going with two Revel Sub 30s having heard one B-15 that sounded incredible. Everything I've read reinforces my belief that two subs are better than one, and if you go to Revels site and look at their white papers, 4 subs are the best, one in each corner of the room. This gives the smoothest response no set up headaches according to Harman Specialty Group testing.
Sgr,

I wish my room could accomodate 4 subs, but there's really only one workable corner. I could put 2 smaller subs under my front screen, with a 3rd in the back. Ideally I'd like to get by with one great sub if possible, especially since the reasons for replacing the horn sub are space considerations and room aesthetics.

What do you think about three Velodyne DD-12's -stereo for 2-ch listening and the trio for HT? I'm assuming they can be connected for audio and HT simultaneously like the REL? Would three 12" subs equate to two 15"?

I've read good reviews of the Revel B-15, although it came in middle of the pack in the recent Sterophile Guilde to HT review. I've not read much on the Sub 30 though. How would you compare the two?

I really like your listening room. I wish my wife would allow me to treat the ceiling the way you have. Lately I've been thinking about going with a TACT TCS that would provide digital equalization rather than treating the room. But I think that's more dreaming than reality.