Related question:
As noted, my current subs are JBL PRX815XLF. As I think about logistics (and my aging body) I find myself wondering if I could accomplish my goals by adding another pair of 15” subs. I could then bring additional subs in spaces I need more bottom. But I don’t really understand the physics with this approach. Air is air, after all, and has to be moved. Would that approach produce a result similar to a pair of single 18s? How important would it be for the subs to be matched?
Thanks for smartening me up!
Air is air, and there is no replacement for it's displacement at low frequencies.
Low frequency speakers are governed by Hoffman's Iron Law: Efficient, Low, Small, pick any two.
Matching the phase response of additional speakers is important, if not close (within 90 degrees) the different speakers can reduce output. Even if the cabinets are nearly identical, the DSP processing in different powered speakers could result in cancellation of output.
The JBL PRX815XLF uses a 15" 2275 driver, which has an Xmax (linear excursion) of +/-8mm.
There are subs available using drivers with double (or more) displacement than the 2275, such as JTR, Bassboss, DSL etc. Doubling displacement gives +6dB more low frequency output, but requires two to four times (+3 to 6dB)more power.
The JTR Captivator 215PRO drivers have 19mm Xmax and 30mm Xmech (60mm peak to peak).
Unfortunately, like most speaker specifications, Xmax can be calculated many different ways. JBL is pretty consistent, though mistook Xlim/Xmech (mechanical limit or failure) for Xmax on some past data sheets for the 2268 used in the SRX series.
Xlim/Xmech should be about double Xmax for pro woofers, though gets less with longer excursion to keep the surround a reasonable size.
Finding specifications with Xvar, which is 70% Bl motor strength is a more reliable figure.
Driving a driver past Xmax can increase harmonic distortion from around 10% to 100% or more. As anyone hearing a guitar amp knows, more distortion sounds louder..
Doubling cabinets and power results in +6dB level (if the output coherently couples), which sounds about twice as loud to our hearing/feeling in the low end.
-6dB is equivalent to doubling the distance from the source outdoors.
A sub system that is +6dB will sound as loud at 20 meters (65 feet) as the weaker one at 10 meters (32.5 feet).
An 18' of the same excursion in the same size cabinet with the same low frequency box tuning (Fb) will be capable of about 3 dB more output than a 15". That said, larger drivers generally require larger cabinets for the same low frequency corner.
Horn speakers like the DSL TH-118 "tapped horn" are as much as +6dB more sensitive than a (smaller) bass reflex, so can do more with less power, but are larger. The DSL-118 uses a driver with an Xvar of 16mm and over double the power handling, so could have more than 12dB more output than the PRX815XLF.
This calculator is useful for getting a rough idea of what a driver of a given Xmax can do assuming a sealed box or infinite baffle alignment:
http://www.baudline.com/erik/bass/xmaxer.htmlBass reflex can add 3-6dB around the tuning frequency (Fb), large horns 6-10dB across the entire sub range.
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