# Wattage

Discussion in 'Electrical Engineering' started by Dave McMahon, Oct 29, 2003.

1. ### Dave McMahonGuest

I know that James Watt gave his name to this measurement, Do you guys know
how to measure it? I am trying to measure the max wattage of a sub woofer I
have a mutimeter and I know the sub has a rating of 4 ohms, I don't want to
find out by testing it (bang)

2. ### John GilmerGuest

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.

3. ### daestromGuest

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.

http://level2.phys.strath.ac.uk/ScienceOnStreets/jameswatt.html

(last paragraph)

daestrom

4. ### Cameron DorroughGuest

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

Enjoy!
Cameron

5. ### Jerry G.Guest

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:
http://www.djzone.net/pg/0008/te00084.shtml

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.

--

Greetings,

Jerry Greenberg GLG Technologies GLG
=========================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
=========================================

I know that James Watt gave his name to this measurement, Do you guys know
how to measure it? I am trying to measure the max wattage of a sub woofer I
have a mutimeter and I know the sub has a rating of 4 ohms, I don't want to
find out by testing it (bang)

6. ### Cameron DorroughGuest

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

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

7. ### Don KellyGuest

---------
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.
----------

8. ### Cameron DorroughGuest

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

9. ### Cameron DorroughGuest

No you bloody don't.

I'm merely trying to be helpful and learn something on the way.

Cameron

10. ### Cameron DorroughGuest

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.
...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

11. ### Don KellyGuest

-------------
No problem here -provided that you know the maximum current rating.
That was not addressed or stated in either your post above or the
I will stand by the statement that impedance <alone> 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.

12. ### Don KellyGuest

----------
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.
.. -----

13. ### Guest

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.

14. ### Cameron DorroughGuest

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.

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

15. ### Guest

Actually, the impedance of the speaker has nothing to do with
how much power it can handle. The op said:
"I am trying to measure the max wattage of a sub woofer"

Did you have that in mind when you posted the above? Or
are you thinking of doing the calculation based on wire
size in the coil of the speaker? If the latter - you would
have to measure the wire size in addition to the impedance.
I guess I'm totally missing your point.

16. ### Guest

Yup! And I didn't even think of it. Where's the dunce cap
when I need it?

17. ### Don KellyGuest

---------
Wire size is not necessarily a good measure of current rating. Note that
nominal ratings apply to wire in air- not wire wound in a coil and enclosed.
There may be charts for this situation which fit particular devices (they
cannot be general) - in fact, I am sure that there are. However this does
beg the question- if one knows the manufacturer and the speaker model, then
the manufacturer will have the information needed, without the need to run
tests.
If not, then the impedance test won't tell you anything about the power
capability of the speaker and looking at the wires won't help. .

----------
----------
No problem, It is like using a fuse tester which runs the current up until
----------
I thought that I said or at least implied or assumed as "a priori" knowledge
that the load determines the current for a given voltage. It is true that
one doesn't control the power directly- in fact I don't know of a device
where this is true- one controls something else to control the power and
often the power change is a side result of trying to control some other
variable. In this case: Double the voltage- double the current and
quadruple the power- This is true at any frequency independent of the
impedance at that frequency. I stated fixed frequency to avoid comparing
thepower/voltage relationship at one frequency to that at another frequency.
---------

18. ### Cameron DorroughGuest

Cite? Now you're making yourself out to be the Audio Fool.

The output impedance of most audio circuits is *never* nil - it is always
some intrinsic value. Some examples: Speakers = 2/4/8ohms, Balanced Line
= 600ohms, AES/EBU Digital audio = 110ohm..

Have fun.

Cameron

19. ### Guest

You've missed the point. It's an ellipsis, based on the content
of the post he was replying to, where the impedance changes
with changes in the frequency.

Ellipsis: "the omission of one or more words that are obviously
understood but that must be supplied to make a construction
grammatically complete." (From Webster's)

Had Keith not used that ellipsis, he would have written something
like:
"Sure, in most audio circuits the output impedance DELTA is
nil, so you're varying the voltage."

20. ### Cameron DorroughGuest

Yes, I guess I *did* miss the point. I let his groundless insults get to
me - and I shouldn't have done that (this is Usenet after all).

In my (brief) time doing concert audio, all we ever needed to do to measure
speaker characteristics was to hook them up to a Tektronix impedance
analyser, run the tests, print the graphs and hand them over to the cabinet
designers.

I have learnt quite a bit though this discussion - thanks to you and Don.

Keep up the good work

Cameron