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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
eldartford
4,262 posts
 
Rives...I don't understand your comment about "inertia". The effect of cone mass does not have any threshold to be "broken". If the parameter were "friction" what you say would make sense, but a cone driver has no friction, just mass.

If sensitivity diminishes at low power level something other than inertia is at work. I, like every other audiophile, have the impression that this occurs, but I am of the opinion that it is the human ear sensitivity that is diminishing rather than the speaker output.

Because it is servo controlled, I would expect a Velodyne to perform well at low volume. If there were any nonlinearity of the driver at low SPL the servo would counteract it. As you suggest, Velodyne should have something to say about this.
no_slouch OP
48 posts
 
Rives...good idea. I called Velodyne and the guy told me that for a 4000 cubic foot room, he would recommend two DD-15s or one DD-18 (he said these are equivalent, as are four DD-15s and two DD-18s). When I mentioned cost wasn't an issue, he recommended DD-18s, and said that there was absolutely no downside to getting a larger subwoofer, as far as low-level playing and neighborly relations. The DD-18 can always be adjusted to a lower volume and match any low volume DD-15, and in fact at low volumes one would be hard-pressed to tell the difference between the two boxes.
rives
953 posts
 
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.
no_slouch OP
48 posts
 
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.....
sean
6,231 posts
 
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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