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if you want to test the real RMS power, you need a resistive dummy load for teh amplifier (per channel), a power supply capable of delivering the required current for the amplifeir at 12VDC to 14VDC, and you'll need an oscilloscope and DMM.
all totalled you're looking at a serious investment in bench equipment.

you can buy a used hand held oscilliscope on ebay for around 100-150 bucks. as far as measuring db, there is a formula that will give you an estimated db range your system will be capable of. it is very simple math. the physics don't change just because you are in a car, you simply have to take the environment in to account. I have used this formula in every system I have built and it is usually dead on. I used it to calculate the db in my db drag car and came up with 150 db, my max in competition was 154.7. my average was 150.4

here is the text form a worksheet I created when I was in retail very simple on calcualting wattage:
AMPLIFIER REAL WORLD WATTAGE CALCULATOR

FIRST, WE NEED TO HOOK UP OUR SYSTEM WITH ALL THE CORRECT COMPONENTS. THEN WE NEED TO TEST OUT SYSTEM WITH A SINE WAVE. USING A 1Khz TEST TONE, WITH OUR METER SET AT 200 AC-VOLTS, WE PLAY THE DECK TO MAX UNCLIPPED SIGNAL. THE READING WE GET IS THEN PLUGGED INTO OUR FORMULA

AC VOLTS X AC VOLTS
_____________________
IMPEDANCE
THIS GIVES US THE ACTUAL OUTPUT @ 1K. THIS IS JUST A CLOSE ESTIMATE. IN ORDER TO BE TRULY ACCURATE, YOU WOULD NEED TO KNOW THE IMPEDANCE AT 1 KHZ. WHICH WE CAN VERIFY USING AN IMPEDANCE TESTER.
If you don't have an impedance meter, you can use nominal impedance as stated on the woofer, but it won't be as accurate. if you have access to an audio toolbox, it will have all you need. you can buy one for around 300 on ebay. I use mine all the time AWESOME tool.

here is the spl calculator, this really works for real world systems. db Drag cars may not perform like the worksheet since the boxes don't really fit the profile of the worsksheet. but if you have a typical box, even custom built this will work:

Decibel work sheet

Using the sensitivity rating, we can calculate within +-3 dB the spl for any given system at any given power.


Sensitivity =


add 3 dB per doubling of cone area
ie 2 subs, 3 dB, 4 subs, 6 dB

take sub sensitivity, with dB per sub added
and calculate 3 dB added per doubling of power starting at 1 watt\meter

1=
2=
4=
8=
16=
32=
64=
128=
256=
512=
1024=
2048=
4096=
8192=

Next we add in Transfer Function
add 12 dB for a standard car\truck
add 16 dB for a hatchback car

Now we figure in box type
for a sealed box, 0 dB, sealed offers no dB gain.
for a ported box, 3-9 dB depending on tuning frequency and air ripple. the higher the ripple, the more gain but at a cost of sound quality.
for a bandpass box, 6-12 dB again depending on the tune freq. and ripple.

using the ranges for the boxes, you will get a db range that your car will fit into. This is simple physics and works.

154.7? Not bad, what were you running?

2 15" Crossfire XT15fs and 4 RF bd1500s. I had a kenwood excelon in my podium and a kenwood z828 in the dash. I have an Ohio Generator 200 amp alt with an optima yellow top batt. heres the kicker, it was in my 1971 Buick GS convertible. It has been touted as the world's loudest convertible. which it still is. for those who doubt, I have the pics and the dbDrag member page to prove it. I am the kind of person who gets pissed off if I am told I can't do something, so I go out and do it. It was a 1-2 no wall car, I took the back seat out and built a 10 cf box with a 175 sq in port tuned at 50hz, and I played 55 hz as my tone. it was friggin brutal! made you dizzy, sick to your stomach and was quite hard to breathe if you sat in it. I am currently in the middle of a complete rebuild with a different approach,I have achieved what i set out to do in db drags and building a show car again out of it. It has been featured in 4 magazine articles I wrote for Mobile Electronics Magazine on the build up. the latest comes out in january. a few days actually.

Sounds good, now see if you can take the harder step - win SQ competitions :)

I took fifth regionally in Iasca in 2000 in oklahoma. I don't like competing in SQ, its too boring, subjective and the judges can be biased. I prefer a more objective approach to competiton. I really don't even compete in car shows that much, there just isn't any adrenaline. but i love the butterflies I get when I roll into the staging lanes and burp that funky stuff. its cool to see a ragtop in the lanes. but SQ is more difficult in a ragtop because yoy can't silence the exterior! no matter how much sound deadening you put in (and I have about $1500 dollars worth at cost!) you still only have an 1/8" of vinyl and canvas keeping the outside noise out. no way around that. but the road noise is gone and the doors shut like its a lexus. no buzzes or rattles, even at 154.7. quite amazing actually. I blasted the entire interior, spryed it with v-block, sprayed that with a bedliner and the covered every un-posed inch with dynamat. the carpet has a 1/2 inch decoupling mat (1/4 foam, 1/4 deadener) between the floor and the carpet. but I still have wind noise!

That real world SPL calculator is pretty cool. I calculated 135.7 dB for my current setup while I'm hitting 140.4 dB (141.1 dB if I lay on the roof). I had to stop at 512 because I'm at 600, but I'm not sure how much 100 or so watts affects it.

Does this apply to 12.5 volts only? I'm within the margin of error if it is at 12.5 volts (I hit 139.x dB at 12.5 volts -loss of about a decible compared to 13.9 volts).

Oh, and my transfer function is 15.1 if I remember right. Did some testing :) .

*edit: my memory was almost right, it's 15.7 at 50 hz and at 41 hz

yeah that's with in the margin of error. what type of box? the added spl could be because you box is more efficient in the location you have it placed. there are lots of variable that will get you little boosts and gain. flipping down the visors or just one visor will actually gain you 2 db in some car! its weird. its all about focus. like I said, it will get you close to what your doing, if you've really worked on tuning, you can get those little gains and really top out your system each car has a different transfer function. My convertible reacts more like a hatchback with a high transfer function, where as other hatches have a lower TF. but it is pretty cool, just the physics of car audio. home audio is the same way, it all depends on where you place your speakers as to where you get the best sound and bass. the same applies to a car but you have an irregular environment compared to a house with lots of different surface types and irregular angles you don't find in a living room. pretty cool huh?

4.5 cubic foot, slot ported to 30 hz. 4.5 doesn't include port displacement or driver displacement though. I believe it is closer to 4.2 cubic feet.

I placed my box and all into winISD pro. The SPL calculator is quite accurate. For my box I get 123.052 dB + 15.7 dB (for transfer function) = 138.752 dB or 138.8 dB which is much closer to what I hit at 12.5 volts 139.x. I found this with data for a 4.3 cubic foot box.