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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
 
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.
sean
6,231 posts
 
A driver has both a mechanical suspension and thermal losses. Until you can overcome the majority of losses in both of those areas via increased drive levels, the driver itself won't be running in the region where it has the greatest linearity. At the same time, driving the speaker harder will eventually go beyond the sweet spot of operation, causing dynamic compression due to saturation of the mechanical suspension and / or thermal losses.

In effect, a driver is no different than any other type of device. They all have a region where they operate most linearly. Above and below that, they still work, just not as linearly. This is why various speakers sound better at certain spl levels i.e. don't come alive until cranked up or sound worse as they are driven harder. Bass alignment also comes into play here as a higher Q design will sound "fuller" at lower volumes, but becomes bloated and tubby as drive levels are increased. This is why speakers are the most specialized component in the system i.e. they vary the most in terms of room interaction / placement, listening style ( spl ranges, seated listening position, etc.. ) and personal preferences in terms of tonal balance, transient response, etc.. Sean
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eldartford
4,262 posts
 
Sean...Mechanical losses are actually thermal in nature, like the electrical loss in the voice coil. In neither case is there any mechanism for a "threshold" to be overcome. But rather than debate, I will make some measurements.
eldartford
4,262 posts
 
SOME DATA

Signal......SPL
..-67.......50
..-52.......55
..-45.......60
..-40.......65
..-35.......70
..-30.......75
..-25.......80
..-20.......85
..-15.......90

Both the electrical signal and the SPL were measured, in units of dB, using my two Behringer DEQ2496. The SPL at 60 and 80 were crosschecked with my Radio Shack meter, and agreed within 1dB. The signal was white noise from a DVD player (used for channel balance). The measurements were RMS. Because this bounces around a bit, some estimation was necessary to get an average value. I set the SPL using the preamp volume control, and then read the associated electrical signal. The speaker measured was one MG1.6 backed up with a subwoofer, and the mic was about 4 feet from the MG1.6. I could not go higher than 90 dB because my preamp volume control maxed out using the DVD player signal. 90 dB RMS is pretty loud, and there is no indication of compression.

A plot of the data shows that at low volume below 60dB the SPL does not increase as steeply as it does over the rest of the range. However, I do not think that this reflects the kind of low SPL inefficiency which Sean suggests, because the background noise of the room ranges between 45 and 55 dB, as a function of traffic on the road outside. This background noise is pulling up the SPL data for the lowest two points. Late tonight, when background noise is low, I will make some more measurements.
sean
6,231 posts
 
El: Thanks for taking the time to not only perform the testing that you did, but for sharing the results with us. I look forward to your confirming both your earlier test results and my prior comments in your next post : ) Sean
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PS... Try a bandwidth limited signal for greater accuracy. The larger / heavier / less efficient the driver, the more apparent the variances in drive levels will be until the threshold or "knee" in the curve is reached. After that, the input / output ratio should remain relatively consistent until driver compression comes into play. As can be seen by your initial testing, this is exactly what took place and what i said would happen.
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