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
DC of any given amplitude will have a constant RMS value. AC does not, especially when using non-symmetrical waveforms like that of music.

DC of any given amplitude does not have a duty cycle. AC does not, especially when using non-symmetrical waveforms like that of music.

I didn't suggest that your test results were based upon false data. I said that the manner in which the tests were conducted may not be revealing enough to explain the criteria that i mentioned. Assuming that such data was all that one needed to formulate theories that were set in stone would be nothing less than incorrect and / or misleading to the general public. As i publicly stated, i appreciated the fact that you were willing to conduct such tests. Sean
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Sean..."the manner in which the tests were conducted may not be revealing enough". The test SPL values went down into the inaudible range. What would you like?

Again I say, go make some measurements. I can see that, like St Thomas, you won't believe until you see it for yourself.
El: I never said that i doubted your test results, so why would i bother trying to duplicate them? What i'm saying is that this is probably not the most appropriate manner to try and ascertain the type of information that we are looking for. Sean
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Sean...What are we looking for? I thought it was a variation of sensitivity (dB/volt or dB/watt) as a function of SPL, particularly evidence of a reduction at low SPL. That's what I looked for.
Thanks everyone for your comments.

I put in an order for a black DD-18 subwoofer. Originally I thought I wanted the maple, but I really liked the gloss finish on the black.

In total, I listened to the DD-12, DD-18, BagEnd Infra18, Rel Stadium III, B&W800 and 850. My impression was that an 18" subwoofer could be played well at low volume. I had never heard a home subwoofer before, and was amazed at what it added to movies and HT, hence I decided to get the biggest one I could. Although the Velodyne retail salesperson and various other people on this forum and elsewhere have assured me that the smaller 12" and 15" subwoofers should work about as well in smaller rooms (they said that room size should be my only consideration), I didn't have the chance to test this. In the small to mid-sized showrooms (250-400 sq feet?), and sitting at a close distance to the subwoofers, I found it hard to believe any number of 12" subwoofers could ever match an 18" subwoofer. The 12" ones didn't seem to have authoritative impact, and in fact I didn't really like the 'punchy' bass they put out- I see why someone might call them a toy, no offense intended to anybody. Is it a non-contested fact that multiple 12" subwoofers could match or best a single 18" subwoofer in terms of quality of low bass? It seemed so hard to believe in the store.

Also, a reviewer for a hifi magazine was in the store that afternoon, and I got to listen in for his review of the 1812 subwoofer for about 45 minutes of home theater, and I decided I wanted to get as close to what i heard there as possible

The DD-18 will arrive in about 2-3 weeks at my apartment, I'll try and post a follow-up around then.