Don't give me all the credit as this has been copied and pasted.
You'll need a good digital multimeter.
you'll need a good digital clamp meter.
a pen and paper
and a buddy.
you hook up your amp, sub, etc... get it all connected.
then you clamp the clamp meter on the positive speaker wire. ONLY on the positive wire.. and clamp it as close to the amp as possible. (i clamp about 2" from the amp terminal)
then you connect your dmm to the positive and negative speaker wires. pos to pos and neg to neg.
set your clamp meter to 100 or 200 ac amp range. whichever is the lowest your clamp offers.
set your dmm to 200 ac volts.
now, get ready to play some test tones...
as you play your tones.. you want to play them long enough for the meters to stabilize thier readings.
you and your buddy should each be in charge of reading one meter. because it can be difficult to try and remember the peak you saw on each of them. (if you have a higher dollar clamp with PEAK hold (not data hold) then you can set it to hold the peak amperage it sees, then you only need to remember the ac volts)
after each tone, you should write down the peak ac amps and the peak ac volts.
now is the fun part.. the math...
acvolts X acamps = real watts used (this is the actual amount of watts your amp produced at that frequency and volume control)
acvolts / acamps = reactive ohm load (this is your REAL ohm load when playing that frequency.)
so.. lets look at some real world examples....
joe dirt has a 4000 watt amp.
it's rated for :
1000 @ 4-Ohms
2000 @ 2-Ohms
4000 @ 1-Ohm
he has read that it only does around 3000 watts @ 1ohm, on forums (mostly from people who have no clue and have never used or tested that amp) so everyone says that amp is waaaay overrated, by at least 1000 watts.
so, joe goes and gets one. he hooks it up in his car on his super duper fancy beefy competition sub with giant motor and super cool dustcap, this is a competition sub so it has dual 1 ohm coils, so he wires the coils together and hooks it up to the amp at 1/2 ohm because at 1/2 ohm it should put out superunbelievableginormous amounts of power.
he hooks up his clamp meter and his dmm.
he gets out his pad and paper, and starts going through his bass cd's test tones, playing each tone looking to find out what his peak frequency is, and then playing that peak frequency at every volume level until he thinks its distorting so he stops there.
at the end of it all, he looks to see which volume level and frequency gave the msot power..
low and behold, he's in for a few surprises.
first of all, he thought his peak frequency was 35 hz, because thats what his enclosure is built to.. but hediscovers that with in car gain.. his peak is actually 53 hz .
then he discovers that even though his sub is connected to his amp at 1/2 ohm.. he's actually waaaay above that.
he figures it out with the math.
we'll say that his peak frequency gave him the following readings...
71.4 ac volts
31.8 ac amps
71.4v X 31.8a = 2270 real watts
so dangit.. they were right on those forums.. his 3000 watt amp hooked up at 1/2 ohm only does 2270 watts.
man, now he hates that manufacturer and slams them on every forum because his amp is overrated by almost twice as much...
had he done the REST of the math....
71.4v / 31.8a = 2.24 ohms
he would have realized that his sub, while hooked up at 1/2 ohm.. was actually playing at 2.24 ohms because he has such a huge box rise.
so now... is his amp still overrated?
it claims to do 2000 at 2 ohms... and he is doing 2270 at 2.24 ohms.
looks to me like his amp is actually UNDERated.. and worthy of some praise instead of being slammed all the time.
so, joe takes this newfound knowledge and builds himself a new enclosure, he has no idea if it will be better or worse, but hes prepared to find out...
his new box, with the same amp and the same sub, gives him the following result....
70.1 ac volts
59.5 ac amps
70.1v X 59.5a = 4170.95 watts
70.1v / 59.5a = 1.17 ohms
holy moley joe is making 4170.95 watts @ 1.17 ohms from an amp thats only rated for 4000w at 1 ohm !!! looks to me like his amp is UNDERrated even more than he thought it was.
and look at that, he went from 2.24 ohms down to 1.17 ohms just from building a more effecient box.
AND... the best part.. he gained 1900.95 REAL watts... and didnt buy any new equipment to do it... all he did was build a new box.
Now joe will go out onto forums with REAL WORLD numbers, and actual tests that he hs done, and instead of jumping on a hate bashing bandwagon, with a bunch of people that dont have real world experience wit hthat particular amplifier, and he'l lgive them real results and a real opinion, based on real experience, not based on something he read on a forum that was posted from peole without said real experience.
and this is the reason that people that dont know what they are doing tend to claim that certain amps are overrated or underrated, when in fact they have never clamped the amp to see what thier box rise, ohm load, or power was.
and of course.. lets not forget that joe saw these results on a burp, with his peak frequency, ie: the most perfect note for his car, on the most perfect volume for his car.
but can you guess what happens when he pops in a music cd.... you got it.. the music is dynamic. the volumes and the notes are ever changing. so the reactive load is going to change CONSTANTLY, as is the overall power output.