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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
sean
6,231 posts
 
El: How is it possible for the motor structure of a loudspeaker, which is fed a non-steady state AC based musical signal, which varies quite drastically in both amplitude and duration, to operate anything like a DC motor?

After thinking about the above question and coming to a logical conclusion, look at your own quote here: "Perhaps you are thinking of AC motors, which do not develop much torque at low rpm."

Perhaps i am. The fact that we are feeding AC into a magnetic motor structure could have something to do with my thinking that. The fact that you acknowledge that such a system doesn't develop a linear torque curve, and is "weaker" at lower drive levels, is exactly what i stated above.

Like i said, the testing that you performed may not have been specific enough to reveal these problems. Making comparisons to different motor designs and modes of operation, which an AC motor or voice coil is to a DC motor, just because it fits a specific set of test results, doesn't make it so. It just means that the tests performed weren't capable of revealing what is really going on. The fact that you already knew the answer pertaining to AC motor operation as demonstrated by the above quote, but adjusted your train of thought to support your later test results and twist the comparison around, is neither consistent or scientific. Sean
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eldartford
4,262 posts
 
Sean...A loudspeaker driver is EXACTLY a linear version of a rotary PM DC motor, where force (torque) is proportional to applied voltage. The applied voltage is variable DC. What do you think would happen if you had a DC-coupled power amp (like my Kenwood LO7M) and applied a staircase voltage, 0.2, 0.4, 0.6, 0.8...1.0....etc? (The cone would move out in steps determined by the suspension elasticity).

The AC motor, induction or synchronous, develops low torque at low RPM basically because of inefficient
commutation. (A synchronous motor has zero torque when stopped, and requires either a mechanical spin, or auxillary windings to get it going. Sometimes, as with the motors used to spin gyro wheels of inertial instruments, a periodic overvoltage is applied to the motor so as to get it to "lock in" to synchronous opearation).

Your suggestion that my test hid the effect that you theorize is a cop out. Truth is my test would be INCAPABLE of hiding it. Do you suggest that I reported false data? Why don't you try it for yourself?
sean
6,231 posts
 
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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eldartford
4,262 posts
 
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.
sean
6,231 posts
 
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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