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Loudspeaker Wattage MECHANICAL measuring method idea?!

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9a6aag:
I have a pair of 42cm dia loudspeakers in the boxes of dimension  aprox 70cm x 50cm x 30cm....no label with spec anywhere, i was told its aprox 300W a box...but who knows....

i have a power amplifier of 2 x 100W RMS...tried up to max and seems speakers can handle a lot more power.....question is how much more?!?!

i have no higher then 100W amplifier and i would like to find out what is the actual max power of loudspeakers it can handle till distortion begins (read maximum useful sound power)

i know how to measure V x A...sending 50Hz audio signal through the power amplifier and make the readings on the speaker itself....but again, with just a 100W of power i cant reach its max and i believe with 100W RMS i am driving it it looks like speakers are on half of its possible power they can handle....

i was thinking about sending a sine wave signal from a power amplifier and on other side of speaker put some  microphone  connected to a probe of oscilloscope so i can receive the sine wave signal from the speaker through the air....and monitor the shape of sine wave....raising the input power to the speaker and monitor the sine wave on scope until sine wave form becomes distorted what means the speaker has reached its max and became unstable, so its membrane cant follow the sine wave signal anymore logically....

but again, for that method i need a higher power amplifier...which i dont have but 100W max....grrrr

so i got some other idea...alternative idea you might be all laughing now....but keep in mind, this isnt a precise measuring, but only aprox...if i get the precision of + / - 15% i would consider it awesome....

i was thinking measuring the membrane travel height moving it by hand (with some reasonable force pushing it) to some max and reading the measure with the micrometer lever dial gauge that is used in machining (i have several i am using on my drilling/lathe table) ...set it up and then by using an AC transformer of high power simply connect the transformer to the speaker starting from 1 Volt and stepping it up by 1 volt till i reach the max membrane movement (i have a lever that can make a HOLD effect at the max peak)...so when i reach the max membrane movement compared to measuring by hand moved, that would be the max power this speaker can handle....of course by measuring the Voltage and Current  what makes the max movement of membrane....

all measuring would be like less then a second....to avoid burning the coil of speaker....but more then enough to make the membrane do its movement...so that result of Wattage i can reduce for like 20% to be sure the permanent but maximum possible power that can handle


alternative method is, measure max membrane movement by hand and then measure it when 100W is applied and get aprox possible  power it can handle...but then again the gain is exponential so the calculation might be much worse then with the transformer source method i guess....

after you calm down of  :-DD all i could ask is; any other suggestion?!  :-//

langwadt:
power handling is also about heat. I guess you could measure the DC resistance of the voicecoil cold and after running and based the thermal properties of copper(or what ever the coil is made of) calculate the temperature

9a6aag:
that sounds like a level pro of measuring now....but measuring the speaker coil should be at aprox 70-80 C* ...i have thermal measuring equipment, but how to reach the coil to measure the temp is some bigger problem....

thats why i said i would reduce aprox 20% of peak measure of V x A

pqass:
One possible method to determine safe power of an unknown speaker..

According to https://www.diyaudio.com/community/threads/cone-displacement-vs-power-input.9793/  Cone movement is directly proportional to voltage applied. 

Attach your multimeter (set to ACVrms) to the speaker terminals and apply single tone (20-50Hz) from your amp at a low volume.
Video the setup using the method to determine cone movement shown in the video at several voltage level steps until you max-out your amp.   

You will have ACVrms+movement pairs, and with the speaker impedence you can calculate power.  Compare/extrapolate to the cone movement of manually bottoming-out the cone.

On second thought, as John mentions, some speakers can move greater than XMAX at which point they can cause audible distortion. So this method may not be fool-proof. If you can bridge your stereo amp then you'd get closer to knowing.  Also, the speaker needs to be in an appropriate box otherwise the value you'll determine will be lower than the speaker can actually handle.






CaptDon:
If the speaker is 'Loaded' acoustically the load should limit the cone movement to less than XMAX before you reach the safe RMS power level. A speaker in free air is not loaded at all and you will hit XMAX crashing the voice coil support into the magnet structure long before you hit maximum rated RMS safe power level. The 'safe' RMS power rating of a speaker is basically the safe temperature limit of the I2R heating of the voice coil. Trying to determine power rating from cone movement is witchcraft at best and would be influenced by the design of the cabinet. You can physically destroy a woofer speaker by using it at frequencies lower than the cabinet cut-off even at less than RMS rated power.

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