I've been in car audio for about five years now and have done countless installs ranging from mind-dullingly simplistic to tedious and time consuming and very elaborate, as well as heard and owned many and many different products, but just tonight I learned how to calculate swept volume. This is a way to get a general idea of how much air a particular sub setup is capable of displacing. Obviously the more air a setup can displace, the more output it is capable of. I still need to know how many swept litres would result in a say 1, 3, or 6 dB increase in output, but maybe someone can chime in with a general rule on that all things else considered equal. I realize some people may know this already, but I for one didn't, so I'll help other people that dont know as well. All you need are two figures, Sd which is cone area and Xmax which is of course one way linear throw. Here it goes... First, convert Xmax to cm if it's listed as mm (Example: 10mm = 1cm, so for any given Xmax in mm, divide by 10. 15mm = 1.5 cm, 20mm = 2cm, 30mm=3cm...) Next, take your converted Xmax and multiply by the Sd or cone area, which will be listed in cm^2 in most cases (Example: 450cm^2 x 2cm = 900 and because you are multipling equal units at different powers, the powers add up, leaving you with 900 cm^3). Now, all you need to understand is that 1000 cm^3 = 1L, which is what swept volume is expressed in usually. So if you have 900 cm^3, and it takes 1000 cm^3 to equal 1L, you're left with 0.90L, but this isn't the swept volume yet. One more simple step. Since we are using Xmax and this of course is one way throw, and subs move two ways, you're going to need to multiply by 2 to get the final swept volume. In this example, you're left with a sub that displaces 1.8L Just multiply that by the number of subs you're using if you're using more than one and it's an easy way to see how much output you're capable of. This wont give you an SPL number per say, but it is good on comparing different setups like if you were thinking of running two 10" subs or a single 15" sub, you could calculate the swept volume of both setups and see which has more output capability. Very helpful info, glad I finally took the time to learn this. Quick formula for ya.... (1) Xmax / 10 x Sd / 1000 x 2 = Swept Volume in Litres Just thought I'd share.
Thanks Josh. I'm sure that will come in handy sometime. Looks like you are becoming more and more of a bass head huh? Of course you must also remember that different companies use different techniques in measuring Sd. Also, this doesn't take into account BL or power compression. So it is possible that one driver with a higher swept volume won't be as loud as another driver with a lower swept volume at some output levels. So this won't be completely accurate, but it will give you a very good reference. So it's definitely helpful information.
Was this really deserving of five paragraphs? I always thought swept volume was kinda self-explanitory. I'm pretty sure double swept volume is a gain of 6dB or is it 3? I'd also like to see some sort of formlula that takes into account displacement and frequency and could give you anechoic spl.
Just wanted to explain it in detail. Like I said, been in this hobby for five years and just never took the time because its not something thats useful to me as an SQ guy, but I figured I'd see which of these new setups I'm look at would give me the most oomph.
I thought it was very well written. It reads much more smoothly that way rather than throwing up formula's without explanations.
You gain 6dB each time you double your displacement. Doesn't matter how many liters that is... as long as it's double what you had before, you'll create a 6dB increase in your output. dB scale is logarithmic. B) You gain 3dB when you double your amplifier power... because doubling your amplifier power doesn't double your excursion... ...it takes a 4x increase in amplifier power to do that.
OK, so lets make this more personal then. Here are the two real setups I'm looking at, which would YOU personally go with by numbers alone... Dual 10" Setup: Swept Displacement = 1.79L 2.0 net cube ported box tuned to 30 Hz or slightly lower, 120w per 10" (150w RMS) Single 12" Setup: Swept Displacement = 1.59L 2.5 net cube ported box tuned to 30 Hz or slightly lower, 240w input (400w RMS) Both are relatively efficient subs IMO. The single 12" setup only has 89% of the displacement capabilites of the dual 10" setup, and the single 12" setup would have a single 4" port while the dual 10" setup would have roughly double the port area (10.5" x 2") and just by looking at the numbers, the dual 10" setup would make better use of the power I have available (more closely matched to recommended RMS). Now, just by swept volume alone, the dual 10" setup wins by a little bit, but probably would be less than a single dB louder than the single 12" setup, BUT taking into consideration that I'd have more port area I feel may make the dual 10" setup be noticably louder than the single 12" setup? Personally I'm leaning towards the single 12" setup just because of my personal manufacturer preferences but the dual 10" setup beats it out in every way possible save price (dual 10" setup would be roughly $25-$40 more expensive). Thoughts? Opinions? Not a basshead, diehard SQ, but some days you just want to crank up some rock or throw in some rap and just blast it, and I want to have that ability. I dont need my ears to bleed, I just want enough output to fill the car, which I feel either setup will do just fine judging from my experience with trunk cars and this power range, just wanting to get some other input.
You gain 6dB each time you double your displacement. Doesn't matter how many liters that is... as long as it's double what you had before, you'll create a 6dB increase in your output. dB scale is logarithmic. B) You gain 3dB when you double your amplifier power... because doubling your amplifier power doesn't double your excursion... ...it takes a 4x increase in amplifier power to do that. [/b][/quote] That's what I thought, I edited in the 3dB later just in case . Just some random math... double power = (sq. rt 2) more displacement? Maybe even x times more power = (sqrrt x) displacement?
