Wattage

in article bnpa84$bit$ snipped-for-privacy@hercules.btinternet.com, Dave McMahon at snipped-for-privacy@btinternet.com wrote on 10/29/03 1:08 PM:
How do you know that? I doubt that Watt had anything to do with making a watt a unit of power. He had a name that was available for appropriation.
Bill

You are better off trying to get that from the manufacturer. It is a nebulous figure anyway and lies are rampant. I assume you really want to know if your amp will smoke it. Bear in mind the numbers for amps are also more marketing than science.

Watt improved the steam engine invented by Newcoming (sp?). One or the other invented the term, "horsepower." It was a natural to name the basic unit of power after Watt.

Well, more correctly it was named after him to honor his work in early thermodynamics. You're right he didn't have anything to do with it directly.
(last paragraph)
daestrom

The easiest way to do this is to read the label on the back of the speaker.
If you *really* *seriously* want to know what your sub is rated at, you would have to: 1. Hire/borrow/etc/etc. an impedance analyser from some place. 2. Hook it up in an anechoic chamber (or wear suitable hearing protection, or both ;-) 3. Chart the speaker coil impedance over the range 0Hz to 20kHz (for a sub, the impedance after about 5kHz won't tell you anything useful, but if you've got the gear you might as well measure it). 4. Take the lowest impedance value recorded and plug it into the standard formula to calculate the Max Power (RMS) at this point. 5. You now know your maximum Wattage. Congratulations!.. Depending on the quality of the speaker, the measured value should be near enough to, but slightly higher than, the value written on the label on the back of the speaker.
The only use for a multimeter in an audio environment is to check the continuity of your speaker leads. ;-)
Enjoy! Cameron:-)

Measuring the wattage rating of a speaker driver is fairly complex. You cannot do this with a simple multimeter. For true sinewave power handling it would require driving the speaker with a low distortion sinewave, sampling at a group of frequencies to represent the full design spectrum of the speaker, via the proper amplifier. The current draw of the speaker, voice coil temperature, and acoustic distortion factor would have to be also measured under driven conditions. The speaker unit may over saturate at different levels, depending on the frequency. Speakers usually have problems to handle the lower frequencies than the higher ones, because of the larger mechanical movement demand.
Some manufactures publish the base power rating of their speakers at 400 Hz, while some at 1,000 Hz. Some will publish the true sinewave power, and others will publish so-called "music power". The best power rating spec should be published for the full spectrum of the speaker design. Music power is not an accurate or real way to rate power. I would guess that they put some type of music in to the speaker and then judge the point where the speaker would be destroyed, or may start to over-extend itself.
There are low end speakers that I see at flea markets, and shops that have published power ratings that are way out by real standards. I have no idea of how they get their numbers. I would think that they feel to put any good sounding number on them. I have seen small cheap 6 inch shelf speakers have numbers such as 200 or 500 Watts on them! I am sure if I would connect up a typical Crown or Altec amplifier on these, the voice coils would be shot across the room as soon as the first drum roll or click comes along. The best one are these little computer speakers that are almost pocket sized, and they say 300 Watts on them. I think they should divide this by about 100! I have no idea where these numbers come from...
Usually when testing speaker types for their maximum power handling they may be damaged during the process. It takes the proper conditions and sophisticated test equipment to do it properly. A speaker is also a reactive device, thus this is how the term impedance was derived. The actual load or impedance of the voice coil is also dependent on the applied frequency. The 4 ohm rating of a speaker means that the impedance of the speaker should be 4 ohms at the rated reference frequency. At the same time, the voice coil may have a DC resistance of 4 ohms, but using DC to measure the voice coil is not a proper test. In many cases, the voice coil may measure lower than the rated impedance. Impedance is a reactive quantity, not a DC quantity. Most manufactures use 1000 Hz as the reference frequency for the impedance test. This frequency is also used as the start reference base for the testing process. A very good simplified explanation of speaker reaction and characteristics can be looked at: Many manufactures do not publish true specs, and only the very high end ones can be trusted. Many also estimate the theoretical power handling capability. The power handling capability of a speaker is also arbitrary. If the amplifier is putting out some distortion, or some clipping from being slightly over driven, the speaker voice coil will overheat quicker, thus causing a break down at lower power than what the speaker was rated at, even if it was properly rated.
To find out the rating of any speaker from what the manufacture says it is, it is best to contact the manufacture for such details.

Kind of.. I must admit that the ratings they put on amps and speakers these days can be a bit erroneous depending upon where you are and where they were made!
Most (all good) amp/speaker manufacturers will give a rating in "True RMS" - probably somewhere in fine print on the back page of the manual and as far from any advertising material as they can get - and you can reliably use these ratings to configure your system. Rule-of-thumb is that the Peak RMS power of the speaker should always be higher than the per-channel Peak RMS power of the amp.
Totally ignore ratings like "PMPO" (Peak Music Power Output) - usually the absolute peak (not RMS) power of all channels of the amplifier into 2(!) ohms x the maximum possible number of speakers (note: 1 mid/high/sub = 3 speakers) you could physically connect with destroying the entire unit - but sometimes they include a large fudge factor as well to improve sales.
To put all this in perpective (FWIW), a comfortable listening level for music at 1 meter from a speaker is 1 Watt RMS. And since volume is logarithmically related to power, 10Watts is twice the volume of 1Watt...
Cameron:-)

protection, --------- You are assuming that at the lowest impedance value the speaker is resistive. However, the power with this impedance depends on the voltage- that is a measurement of impedance alone will tell you nothing about the power unless you have this other information. However, given knowledge of the impedance and the voltage or current- you then only have a figure for the power input under the conditions of the test. This is not a measure of the maximum allowable power for that speaker. Crank up the volume and the power into the speaker goes up but the impedance at a given frequency will not change or if it does- it will be at or near the point of no return. That is a sure way to test so that you know what not to do with the replacement speaker. ---------- > The only use for a multimeter in an audio environment is to check the > continuity of your speaker leads. ;-)

No...ish. Given the impedance at it's lowest value and knowing the max current rating of the coil (Amps) should be enough to calculate the max power, assuming coil burnout is the most likely cause of failure (not over-extension of the core, etc. etc.). P=I^2*R last time I checked (but I haven't got a textbook nearby ;-).
But you don't have any control of the voltage - that's completely up to the amplifier manufacturer. At some point in the impedance curve, the *amplifier* will current limit (at Max Output Power of the amp), but the speaker may be stuffed by then...
Not the power per se - the *voltage*.
Certainly one way to calculate max power would be to feed the speaker under test from a humungous amplifier and wind up the volume on white noise until the coil burns out. Instantaneous volts and amps at the time would give you a good indication... but I assumed the OP wanted to use the speaker afterward.
Regardless of voltage, if the current that the amp delivers exceeds the coil rating you get burnout - so the Max Power of the speaker must be related to impedance and current only (excluding coil dynamics and acoustic effects). No?
Cameron:-)

"Peak RMS power"? "True RMS power"?? Oh, good grief! These audiophools will buy anything! Also ignore *anyone* who talks about "RMS Power", peak, average, sustained, or any other qualifier.
Good grief gert! I think we have a prime audiophool here.

Check your book again. You have a non-resistive load. Power I*E. The voice coil burn-out mechanism isn't power, rather current. Whether the speaker cone rips first is another issue.
Nope. Most amplifiers today have a very low output impedance. You have control over the output voltage. That control is called the "volume" knob.
Sure, the amplifier will clip at some point, but this is above the power rating of the amplifier. You'll likely be deaf long before. ;-) Huh?
...indeed! LOL!
"Instantaneous"? Please. Power is the failure mechanism here, and it's *not* instantaneous by any approximation of the term. Heat is the problem, not some magical "instantaneous power".
No. Current is the killer (at least up unit cone failure). Speakers aren't resistive, nor even linear loads.

No you bloody don't.
I'm merely trying to be helpful and learn something on the way.
Cameron:-)

As stated below. Please read the *entire* post (and the thread too)
Please read the entire post. My point was that amplifier manufacturers can use whatever output voltage they like. If they want to drive a speaker using 2 volts @ 100 amps or 100 volts @ 2 amps there is nothing to stop them - the output power is unchanged.
Crap. That *is* the power rating of the amplifier - regardless of what they might say in the manual.
Like I said - please read the entire thread.
..And exactly where did I talk about "instantaneous power"??
Oh, go away. I was having a good discussion with Don until you turned up.
Cameron:-)

message ------------- No problem here -provided that you know the maximum current rating. That was not addressed or stated in either your post above or the original thread. I will stand by the statement that impedance won't give the power rating. That was the point I wanted to make. True -I should have said current rather than voltage (recognising that losses and cone excursions are current related) but, for the minimum impedance situation under discussion , the speaker will appear purely resistive and the voltage and current are intimately related and power can be expressed in terms of voltage. (P=VI =I^2R=V^2/R) At other times where the impedance is not resistive,, it definitely makes sense to consider current rather than voltage as you indicate. ----------- -------- True enough -However, at any point in the impedance curve, knowing the voltage is sufficient to determine the current. You can't have one without the other. If you know the impedance in both phase and magnitude (hence the reference to the minimum being the resistive case)then knowledge of the voltage can be used to determine power. Use whichever approach is convenient and fits your comfort zone best - although, admittedly measuring current is preferrable..
The speaker may or may not crap out at the amplifier current limit so the current and the power at this point is not a measure of what the speaker can handle. What the amplifier can do is not a measure of what the speaker can handle (without going to destructive testing).
It boils down, as you indicate to having to know the maximum allowable current. The manufacturer may be the only one with this (or the "power" rating) information. The power rating would also have to assume that the nominal impedance is resistive and applicable -otherwise it is a can of worms. -------------------- ------------ As the voltage goes up with a fixed impedance (as stated above) so will the current and the power. (I^2R or V^2/R are not basic but are derived relationships) ----------------- ---------- In the sense that the thermal limit of the speaker and the maximum force produced (affecting coil excursions are current related -Yes. In the sense that voltage and current are directly related at any impedance and, in the minimum impedance case, as discussed, ignoring the L/C and mass/spring factors , the speaker appears as a resistance -then using I^R or V^R are equivalent. At other impedances, it is best to use curren tas the criterion. That is - don't exceed a given current is a better rule than don't exceed a given voltage as the latter requires more information than is generally available. Is it more sensible to use current as a limit rather than voltage ? Definitely- because the impedance may not be resistive and the terminal voltage higher but the current limit and the approximate real power will be the same. In fact it makes more sense to have a current rating on a speaker than to have a "power" rating.
However, we have been discussing the resistive case -mid frequency situation. ignoring L,C,Mass ,spring etc and being stuck with coil resistance assuming coil losses far exceed the mechanical power output- in that case -use of V or I are a matter of choice in determing power.
-- Don Kelly snipped-for-privacy@peeshaw.ca remove the urine to answer

---------- 2V @100A implies a speaker load of 0.02 ohms. 100V @2A implies a speaker load of 50 ohms . Since the speaker impedance is independent of the amplifier, one must consider what that amplifier can do with a given speaker load. 2V across a 4 ohm speaker results in 0.5A and a power input of 1W (assuming 4 ohms resistive) 100A in a 4 ohm speaker requires 400V and a power of 40KW Quite different kettles of fish. The voltage/current/power characteristic of a speaker is determined by the speaker (i.e. what is downstream of the speaker terminals), not the source. ----------------------- --------snip------------- ----------- You mentioned instantaneous voltage and current as a measure - these determine instantaneous power, not average power. There is no relationship between instantaneous current and voltage and average power. If you are referring to the rms voltage and current and average power -that is different but still it is not a good measure unless failure occurs after a sustained period of time at this level. . ----- -- Don Kelly snipped-for-privacy@peeshaw.ca remove the urine to answer

Well, since you've snipped all your nonsense, let me add it back in so we can discuss:
There is *NO* such thing as "RMS Power". It's an absolutely meaningless munge of terms, used to make the speaker sound knowledgeable. Anyone who professes such should be ignored immediately and permanently.
Power is power. Though I did misspeak. Average is the only thing important from an audio amplifier.
"1 Watt" is meaningless as it relates to loudness. You have not specified what that electrical power translates into sound pressure.

...done that phool.
Huh? The power to the constant (same speakers) load is certainly a function of the voltage driven to the load. This *should* be (but phools have strange units of measurement) the rating of the amplifier. However, the issue was the speakers. This has *nothing* to do with the speakers. They'de limited bu what power they can take before melting. You just said that amplifier manufactures can use whatever output voltage they choose! PLease get your thoughts together and come back. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ "Instantaneous volts and amps" translates to instantaneous power, no? I give up. Please tell us what you really meant! I take it that you're not an engineer. You haven't a clue. I trust you like it that way though.

C'mon Keith, there *must* be. After all, if the manufacturer rates the equipment with that term, people will pay more for it. Of course, if you want to maximize the "true RMS power", you must connect the amp to the speakers using gold plated, argon filled, 4000psi pressurized, pre-stretched monster cables that you wash weekly with "Signal Kleen". Someone will be willing to sell you a bottle of that for 10 dollars an ounce. :-)

Given the resultant confusion, I should probably have originally stated that the only thing you need to *measure* to do the calculation is impedance.
The maximum current rating is directly proportional to the wire size (in mm2) used to make the coil (basically, from manufacturer's charts). I'm sure there is a formula to calculate this, but I have only ever seen the charts - and then only in distant memory.
Note here: I didn't really expect the OP to try my method when it is easier to read the label, so I didn't bother spelling it out in fine detail - perhaps I should have.
< snipped for brevity>
I don't disagree. I was trying to clarify your point that by changing the volume of an amplifier you are directly controlling the voltage only - not the "power" (voltage x current) as such. The load determines the current draw. It's a minor nit - assuming the impedance is fixed, which being frequency dependent to some extent, it is not.
This comment was relating to empirically calculating the maximum current rating of the coil. The amplifier voltage & current at the time of failure will give you an upper limit on the max power handling capability of the speaker.
Cameron:-)