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
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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)