amp and speaker compatibility

I've looked online into this issue and I've seen two opinions regarding amp and speaker "continuous average power(CAP)" (commonly, but incorrectly called RMS):
~It's better to overpower than to underpower
~You must match the CAP of the amp and the speakers (closer is better)

I'm not sure of what to believe, so I come to 12Volt with this question:
Will this be a PROPER amp and speaker buy?

Cadence F100 5-channel amp with:
~100x4 @4ohms + 300
~125x4 @2 ohms + 500

[will be powering]

(4x) MTX TN653s
~rated at 45 CAP.
(1x) Stock 8" Sub
~rated at ? CAP

Will there be damage to the speakers on normal function? Is this too much CAP for the speakers?

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'97 Lexus ES300
(Expertise Level: Recently gotten into working with cars)

You can use any amp with any speaker as long as you know what you're doing.  In general it is better to buy more power than you will need.  The max continuous rating of the speaker should NOT be your target, but it gives you an idea of when the speaker will fail.  Just set your amp up properly with properly set gain and don't over-drive the speaker and you'll be fine.

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To clarify, it is advised to buy more power than needed, set the gains properly, and enjoy? If so, excellent. Thanks for your help.

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'97 Lexus ES300
(Expertise Level: Recently gotten into working with cars)

Funny that many say that speakers should have a power handling GREATER than that of the amp. I once did too based on NOT blowing the speaker by overpowering.

However, IMO the reality is as DY stated - set gains etc.

And IMO even if there are no gains as in HUs etc, then I'd rather the amp be the source of distortion rather than the speakers.


I don't recall hearing of CAP for audio outputs. However now I have seen & googled it, and I agree CAP is NOT the same as RMS.


Of googling interest was this...
"In 1974 the Federal Trade Commission (FTC) required that audio power and distortion ratings for home entertainment equipment be measured with power stated in RMS. [Amplifier Rule CFR 16 Part 432 (39 FR 15387)]."

oldspark wrote:

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Funny that many say that speakers should have a power handling GREATER than that of the amp. I once did too based on NOT blowing the speaker by overpowering.

However, IMO the reality is as DY stated - set gains etc.

And IMO even if there are no gains as in HUs etc, then I'd rather the amp be the source of distortion rather than the speakers.


I don't recall hearing of CAP for audio outputs. However now I have seen & googled it, and I agree CAP is NOT the same as RMS.

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So you agree that more CAP in the amp is better/allowable as long as the gains are set properly?

I made up "CAP" after seeing someone mentioning how RMS is not the correct term. I just thought I would use the correct terminology. :)

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'97 Lexus ES300
(Expertise Level: Recently gotten into working with cars)

Well, more RMS...

I like the "CAP" abbreviation because I can now say consistently that I don't like caps - whether capacitors, or CAPs, or upper-case letters in audio replies, or unexpected bangs behind me whilst working (LOL).


To me RMS is the correct parameter since overloading is generally related to heat which is an RMS parameter.
Things like overload punch-through and similar failures usually relate to peaks which neither RMS nor CAP relate to (in typical waveforms).



As I understand it - and may DYohn & Haemo & the other gurus correct me - an oversized amp could blow undersized speakers, but with properly set gains, the max output is limited hence NOT overpowering the speakers.

oldspark wrote:

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To me RMS is the correct parameter since overloading is generally related to heat which is an RMS parameter. Things like overload punch-through and similar failures usually relate to peaks which neither RMS nor CAP relate to (in typical waveforms).

As I understand it - and may DYohn & Haemo & the other gurus correct me - an oversized amp could blow undersized speakers, but with properly set gains, the max output is limited hence NOT overpowering the speakers.

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RMS does not apply to power; there is no such thing as "RMS Wattage" or "RMS Horsepower."   RMS is a means for measuring voltage.  In amplifiers, the correct term is "Continuous" power, and it is determined from the RMS voltage output of the amplifier.

I agree with your second statement.

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RMS does apply to power (Watts).

Interesting though that average Power is defined as Vrms x Irms, whereas for RMS Power, it is the the RMS of V x I where the only "real power" is for matched frequencies. (eg, V @ f1 x I @ f2 = 0 Watts when F1 is not equal to f2)

To me, the reality is RMS power - eg, the heat in a resistor etc.

oldspark wrote:

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RMS does apply to power (Watts).

Interesting though that average Power is defined as Vrms x Irms, whereas for RMS Power, it is the the RMS of V x I where the only "real power" is for matched frequencies. (eg, V @ f1 x I @ f2 = 0 Watts when F1 is not equal to f2)

To me, the reality is RMS power - eg, the heat in a resistor etc.

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Well, maybe it is a recognized engineering term Down Under, but here in America "RMS Power" as applied to audio amplifiers is a misuse of the term.

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No - you have the same definition of RMS power.

RMS is not CAP - that CAP Wattage will be larger for speakers than their RMS.
Whether you use CAP instead of RMS (despite the apparent FCC "ruling") is whatever it is.


Now, is the actual audio "power" from speakers from their resistance, or inductance, or both?

You know the answer to that question.

Sorry, I maintain that in the audio world, there is no such thing as "RMS power."  I quote from the Rane technical glossary: rms power No such thing. A misnomer, or application of a wrong name. There is no such thing as "rms power."  Average or apparent power is calculated using rms values but that does not equal "rms power;" it equals continuous sine wave power output into a resistive load.

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No - the R & L issue was something I considered after my last reply April28. My initial thoughts were both since both DC & AC cause e-mag force & displacement.
A speaker is a device where IMO the imaginary (inductive) power does result in real (acoustic) power output unlike classical AC loads where the pf (power factor) has no impact on the real power consumed (ie, hence no extra energy required nor extra consumer electric-consumption charges).


Hence speaker power should be a VAR (average power) relationship.
Speaker heat will only be from the resistive component (ie, one of its inefficiency factors).


As I wrote, I can't recall seeing the term continuous average power, though my main issue for amps etc has been NOT use vague or defined "peak" power etc as they have little relevance.


From what you wrote, the apparent power is simply real RMS power divided by the cosine of the phase angle.
The calculations otherwise will be the same - ie, involving the product of each frequency's voltage and current (ie, =0 if not the same frequency) across the frequency range.


My concern would be that people would merely use the Vrms & Irms values for such calculations and use that as a CAP value.

Maybe RMS specs have been used because "true RMS" meters are not uncommon (including power). I'm not sute about "true RMS Apparent Power" meters.


But speakers will have an RMS power, it should simply be the VAR or "average power" times phase angle and (electrical) distortion factor (ie, pf = W/VA).
My "RMS power" source is my ancient learnings and the heaps of relevant texts. The end result mainly falls under mathematics etc - eg, Laplace & Fourier transforms and their series expansions.


As to what should be applied to speakers and amplifiers I can leave to authoritative references. I haven't investigated nor seen what has the better correlation (ie, RMP real or apparent power). And then which is used is a case of practicality. (If simple specs such as impedance or power at 1kHz etc is used, I can understand why RMS (real) power is used... if people can't handle full bandwidth specs...)


At least we are in agreement about what RMS power is, whether real and apparent.

I checked Rane. It's not quite the RAND that I expected.

I'd disregard their "definition" of RMS power since they fail to qualify what they mean. (And it certainly is not a definition.)

And since they clearly refer to RMS power elsewhere (eg, in other definitions), it must exist. Someone should suggest they fix their RMS power definition else remove references to it in other definitions.


They do not provide a definition for continuous power which I find strange.


Maybe google's narrative accompanying Rane's ProAudio Reference is apt...
"RaneNotes A series of technical notes written by Rane's technical staff. .... who claims to have posted the largest glossary of recording terms on the web. "
I can interpret as a collection of terms or definitions - often linking to/from other sources (eg, Rane uses Wiki) - and hence not necessarily integrated nor consistent. F.ex, though Rane mention mains AC as a source of ground loops, this is not mentioned under their Ground Loop definition.

Rane did however pass my first test. I usually check definitions of "power factor" (pf) to see if it is defined as the phase angle between current and voltage. It isn't - except for non-distorted waveforms (ie, sinusoid). (Otherwise SMPS etc that have very close to in-phase voltage & current would NOT have PFs of 0.6 and lower.)
The def'n of PF is W/VA which they got right. (Or rather, they did not muck it up by adding "pf = cos-phi".)


Other sources define continuous power as the product of RMS V & I (ie, VAR).

But as some sources paraphrase, the reactive power is "power borrowed and returned" by the load to the power source. ERGO the load never used that power. ERGO the speaker can only use its REAL power.

I love these contradictions that few else none seem able to explain.   

I guess a speaker is a device that doesn't follow our understanding of the world.


PS - some seem to infer that Continuous (Audio) Power is merely the long-term (or steady-state) RMS sustainable. AFAIAConcerned for equipment ratings, that is its "RMS" rating unless otherwise qualified (eg, 1kW for 1 hour).


PPS - I incorrectly used VAR as the product of V & I (both real and reactive aka imaginary). That should be VA. It's P (Watts) + VAR that equals VA (ie, in a "power triangle").
Normally I'd back edit, but...

The fact remains: in audio engineering the term "RMS Power" is a misnomer and generally only used because customers want to hear it.  Continuous power is how legitimately rated amplifiers are rated, and is the proper term.

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An FYI interim reply...

From what I have seen, Continuous Power is merely "long term" RMS power. (Which - as I wrote - is what I expect RMS ratings to be. However I am only too aware of their undefined conditions when otherwise!)

It is NOT related to reactive etc power which some seemed to suggest.


There does seem to be quite some confusion on the subject. That IMO is probably not helped by ambiguous "definitions" like "Average or apparent power is calculated using rms values but that does not equal "rms power;" it equals continuous sine wave power output into a resistive load." (IOW, real power, the same as RMS power if RMS power is taken as the product of RMS voltage and current in a resistive load).
In some cases, "continuous" seems to confer testing with other than a sinewave. Big deal - that is still RMS power.
Even though Wiki have an incorrect definition of pf, they seem to agree with my other "RMS" stuff (ie, wiki: AC_power).


I am still unsure what point is trying to be made re continuous vs RMS unless it's a case of how "RMS power" is tested or measured, but IMO that's another case of the under-knowledged complicating an otherwise clear topic or definition.
But as I said, this is a mere interim reply. My curiosity has been rudely interrupted by other issues.

If "RMS Power" is a misnomer when applied to amps or speakers, I would like to know why. (Assuming it isn't merely a "not short term" as opposed to a long-term RMS rating. That is easy to understand.)   

Maybe you could shed some light, eg, recognised definitions or papers etc?

You don't know how to let go do you?  :)  Whatever, believe what you like.  It's Ok in this world to use the wrong treminology I suppose.  I just try to promote using the proper terms for engineering concepts n this forum.

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Ha ha - nice try,
But I totally agree. If I didn't, I certainly would not be writing here!


So you can't offer any definition (Rane does not have one), nor any explanation, nor references?   
I might have to resort to hardcopies!

To me it seems like people think that by measuring continuous power they are somehow measuring the "actual" power. Not so.
Hence I'm trying to find out WHY they want Vrms x Irms instead of the real power used. I can understand why amp designers use it, but not system users.

Can you at least say how it is measured? Or is it simply the mathematical product of the V & I RMS values (which as they say, is NOT RMS power - the real power of any source or load).


And please forgive me for expecting explanations of the type I try to provide on this forum.
I paraphrase etc rather than mere vague quotes from questionable sources else add I think or I don't know, and maybe the occasional "too difficult to explain".


I'll report back what I find, though it will probably become another of those some day curiosities.

Ah - d'oh - I think I get it. To quote from your linked Rane definition.
"... the rms value of the voltage by the rms value of the current... It's the amount of power the casual observer thinks is available (hence, apparent), but because of power factor may not be -- the real power is usually less. "

Sorry, I thought you were inferring that what I said was wrong.
I was taking you to be far more than a "casual" observer.
Meanwhile I was perplexed by what was wrong.
And sorry about the "nice try", but all too often I find litols and pushers that resort to the same tactic. As the gurus say, that's a good sign...


I still have 3 definitions for continuous average power of which Rane's Vrms x Irms is one.
And whilst no one has said CAP equals "real" (aka RMS) power, I have yet to see where and how "they" apply CAP measurements. Not that 3 different definitions helps!

I see it as people's confusion with "specs".
I often hear how "specs are crap" because they don't provide the full picture.
Of course not - people don't want to see equivalence circuits or transfer functions. They merely want a few standardised metrics so that they can easily compare different equipment.
When those metrics aren't suitable (eg, 1kHz for subs), new measurement parameters are defined.

I have seen reasonably complete specs whether transfer functions from inputs (at various mV) to output (at various Watts), or graphical or tabulated data. And even though I was capable at the time (of the simpler transfer function method), it was not easy to transfer those to a meaningful value or expression, especially for different loads - eg, speaker impedances and conditions.
AFAIK, the only "simple" method is to use simple circuit equivalences in the s-domain. The it's the simple application of Ohm's Law etc and converting back to "real" time domain expressions or frequency & dB graphs etc.
But that's probably not for people that prefer to uses "resistance" instead of "impedance" for speakers etc.


So if you have some nice link to the practical application of CAPs for audio design, I'd be most interested.
Until then, I'm happy to stick with real power as heat as the simplest "standard" one-line method on which to base performance limits.
From there it gets more complex, but that involves program content (which is one of the CAP definitions I've seen) and from there almost borderless complexity.

To me Rane's misnomer statement is like stating how the "solar system model" of atoms is totally inadequate, as is the electron or ion model of electricity. (We "know" electricity exists, yet it still cannot be assigned a single unit as can other base entities... LOL!)
Not that that effects the level of work we do nor it's validity - provided we stay within the model (eg, the guy that argued "but a half (AC or audio) cycle is like DC..." No it isn't - DC content does not hold for sub-cycles).   

In summary:
I am asking WHY is CAP the appropriate measure for speakers etc as far as YOU are concerned?   What does it specify that real power does not?

If YOU can answer that, I can move on.

I measure amplifier output power with a tone generator, an oscilloscope and an 8-ohm resister bank.  Or by using an Audio Precision 2722 in the lab.  How do you do it?

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IOW, the CAP = apparent power = "real power" where I have been using "RMS power" as "real power" (since it it calculated similar to RMS as opposed to direct multiplication of RMS values of voltage and current).

And since you are using a resistive bank and NOT speakers, there is no reactive power hence "apparent power" = "real power".
Or is it a wire-wound or coiled resistive bank that has inductance etc? In that case, what is its inductance?

DYohm - come-on - the difference between RMS power and real power into a resistive load?

And why test into a resistance, not a speaker equivalent?


Please also explain how your CRO "measures" or relates to CAP.

Audio amplifiers are normally rated intpo a resistive load.  The correct term for audo amplifier power is "Continuous Power" not "RMS Power."  That's it.

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So it's merely semantics, nothing "practical" per se.

My conclusion (based on other sources) is that your quote is merely that "power" as calculated from RMS voltage and current is in fact an "average" value, hence "average continuous" etc.

IMO the nett effect is negligible, and certainly pales into insignificance testing with resistors.
If a typical sub or speaker can double its impedance over its rated bandwidth, IMO it makes a joke simple "paper" ratings, let alone vague definitions.


Yep - I'd stick to pocket CROs or distortion meters; preferably dynamic.   


Conclusion: The correct terms for audio amplifiers and speakers is "Continuous Power" and "Impedance".


Thanks for your assistance.