??? on RMS of amps and speakers

Shawshanks

need the facts and opinions on the true meaning of rms for amps and speakers.

1) got some CDT components, if they say rms is 150watts, should i get an amp that produces an rms close to what the speaker people say? are all component speakers 4ohms or can they be bridged into 2ohms?

2) i'm looking at amps and see that the fosgate power T8004 series has a "rated power" colum of 50x4 @4ohm or 100x4 @2ohm, then an "avg. actual power" colum 96x4 @4ohm or 137x4 @2ohm respectively... what should i pay attention to more? w/ the cdt speakers, can i use the 2ohm power to run the speakers? should i look into the model up which has more watts?

3) what are your takes on the Fosgate T8004 amps vs. the MTX Thunder 7804 amps?

thanks all for the info and remember now.... RED STRIPE, HOORAY BEER!!!!

MacLeod

Its best to get an amp thatll match the speakers rated power. Doesnt have to be exact but it gives you a ball park figure. The only 2 ohm speakers I know of are made by ID, otherwise theyre all 4 ohms.

The RF amp is like MTX, Crossfire and a bunch of others in that they are extremely underrated especially the Power series as its a competition level amp and they like to cheat. The "actual" power is just that, the actual power the amp puts out.

You cant get a 2 ohm load with only 1 speaker per output so you only need to worry about the 4 ohm rating.

The RF Power amps are flat out awesome. Definitely cannot go wrong with them, but Ive always been a big MTX fan, especially of their amps. Plenty of clean power and great build quality. Flip a coin, theyre both excellent choices.

Shawshanks

Thanks Mac for the info and i kinda knew what u would recommend b/w the two amps and i guess i'll have to go w/ the MTX 7804 b/c their rms is closer to what my component speakers are rated at. if i went w/ the rf, i would have to step up to the T15004 and the actual avg power is 326x4 @4ohm and that would be way too much.

so if "power" amps are comps and underrated, could the actual birth sheet say more than what they list as the "avg actual power"? would having a x-over on the 4chan amp that can be either 12 or 24db be beneficial than one that has only a 12db? can u explain that?

thanks

MacLeod

It depends. My Crossfire birthsheets pretty much claim about what theyre rated at but yet in ever test Ive ever seen, they usually put out a ton more than their ratings. I would worry too much about it really. For 150 watt speakers I would get an amp rated at 100-125 watts and be done with it. If its from a quality manufacturer, youll have the power.

The difference between a 12db cutoff and a 24 db cutoff is simply a steeper slope. Take a 12 db high pass filter at 1000 Hz, it works by attenuating the sound by 12 db every octave. So a 24 db x-over would attenuate by 24 db an octave. A steeper slope wouldnt let in as much below the crossover point as a 12 db would.

As far as which is better, it depends on your tuning preference. Midrange speakers for instance sometimes work better when they let in some high range frequencies above their crossover point in order to blend a little better with the tweeters, so a 12 db would be better. But tweeters usually work best when cut off pretty steep as they dont do well with lower frequencies at all so a 24 would suit it betters. So it depends on your application and tuning goals as to which is better.

PoweredByDodge

technically, average power and rms power are two completely different animals.

V[peak] is 1/2 of V[peak to peak]

V[average] true average of one half period of V[peak]

V[rms] is "radical 2" * V[peak]

For a true sine wave, V[average] and V[rms] are very close ... not that close, but they're close. give or take 10 percent.

For a square wave, it's dead on perfect, but that's only becauce you've dc'ed it all out already anyways by using a damn square wave.

example...

assume a 4 ohm single load for all calculations.

A) standard sine wave with a V[peak to peak] of 6.00 volts.

..... V[peak] = 3.00 volts
..... V[average] = 3.00 * 0.637 = 1.91 volts
..... V[rms] = 3.00 * 0.707 = 2.121 volts

..... Power[average] = 0.912 watts
..... Power[rms] = 1.125 watts

..... discrepancy = 23%

standard square wave with a V[peak to peak] of 6.00 volts.

..... V[peak] = 3.00 volts
..... V[average] = 3.00 * 1.00 = 3 volts
..... V[rms] = 3.00 * 1.00 = 3 volts

..... Power[average] = 2.25 watts
..... Power[rms] = 2.25 watts

..... discrepancy = 0%

C) standard triangle/sawtooth wave with a V[peak to peak] of 6.00 volts.

... i'm tired, and too much of a PIMA cuz I forgot what it's sposed to be... but it doesn't match up, I can tell you that much, it's further off than a sine wave is.


Moral of this story... ?

Amplifier companies are on acid.

Pick a standard and a terminology and stick with it.

If you're going to rate in rms, rate in rms, not "oh ya but the average is such and such" --- average power is NEVER higher than rms power for a sine wave. I'd like to say it's never higher for ANY waveform, but I won't stick my foot in my mouth just yet.

And what the hell is "actual" power ?

Actual power, is, i suppose, another way of saying "Peak Power".

Peak power is based on V[peak] not V[rms] nor V[average] since rms and average are manipulations of what is ACTUAL-ly the voltage, V[peak].

"Average Actual Power" is like saying "Boared and Stroked Stock Engine Displacement"

i'm getting pretty disgusted with the total disregard for customer common sense by some amplifier manufacturers. they assume that a person can't understand what "rms" means, so they say "average actual" or some other crap, then they decide to put a "1500 watts max power" label on the box, when it's actually an amp that doesn't do 1/3rd that power.

even the term max is misused. max should be power based on V[peak] with a square wave dumped in there. that is truly the MAXIMUM amount of juice that amp is going to produce.

i can respect someone saying "maxinum rms voltage" when its with regard to something like a head unit or some other device that has a volume knob of some sort that you actually dial... because you are varying your output manually.

personally - ignore any "actual power" / "average power" / "music power" / "maximum music power" or other horse-**** statements on the box.

rms power is rms power... and it is just as valid to use to compare amps as true peak power. TRUE peak power... not fudged numbers peak power, not "15000000 watts TOTAL OUTPUT!" or some other crap.

the only reason we ended up in this rms deal is because its a convention. home electronics run off 110 - 120 Vac[rms] ... this was so as to show a more realistic comparison between DC power and AC power consumption due to the sine wave that is indemic to AC power - it's not always at its peak... it floats up, then floats down... then back up... rms gives you a better idea of how much "juice" is actually being sucked up. So everything AC pretty much defaulted to rms ratings... which is fine and well... it's a better system for measuring... and that's all fine and dandy, but you know what, it needs to be consistent.

if everyone does an honest rms rating - great... if everyone does an honest average rating, that's fine too - or everyone does SOMETHING the SAME WAY, then you can compare amplifiers and not have to worry about jack ****.

5 years ago you couldn't even get amplifier companies to agree on what dc power input voltage to rate their rms power at. ... now they can't decide on how many friggin ways they're going to say that their amp is somehow a hundred thousand watts over what its "actual rms power" is...

note - "actual rms power" and "actual average power" are perfectly fine... "rms actual power" and "average actual power" while seeming to be the same damn thing, are 100% not, and i'll bet you my **** that the **** who wrote that to begin with on the amplifier's box knows damn well that a lot of people may misconstrue it - either that or they had no idea what they were doing in the first place.

Shawshanks

PBD... dang thats alot of info and i appreciate the lesson on rms and how its calculated. i went ahead an got the Rockford Power T3002 (my sub amp) and the Rockford Power T8004 (running the seperates). i'm hoping that i have enough power to run everything and i guess i'll find out when i hook everything up.

does anyone have any opinions on the products i've got:

RF T8004 (power cdt & inifinity coaxs)
RF T3002 (power Polk Momo)
CDT EF-61 CF 6.5 components @150w rms
Infinity Ref. 6.5 coax @ 100w rms
Polk Momo 10" sub @ 300w rms

as for a headunit, don't know as of yet, still doing the research. any thoughts? do i have enough power to run the speakers well? should i change the amps to a step higher w/ more watts?