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:
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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.