Loudspeaker Wattage MECHANICAL measuring method idea?!

[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  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.

[daveismissing]

I'm with CaptDon on this one. The real risk is warping the coil former and have it rub forever. I think mech excursion limits would be audible and a cease and desist would not harm. Cutting  the cap you might be able to ramp up the power and measure the voice coil temps IF it was an aluminum former that had thermal conductivity to an area that an IR thermometer could see. If it has a kapton former suspect the heat stays locked up in an area you cannot sense. Seems like a lotta work...

[Conrad Hoffman]

Most speakers have very little continuous sine wave power capability. IMO, the copper resistance method is the best. It's done with transformers, so it should be easy to find info on the best method. I'd think you'd put a DPDT switch in the line close to the driver and quickly switch over to an ohm meter.

[9a6aag]

So if i did get it right, movement of the membrane is linear compared to the power input...what means i can make some test of movement in mm p/p at input 10W, 20, 40 and 80W and if movement measuring showing linearity i can calculate what is the max power till it reach maximum possible movement of the membrane....well...of course, i dont expect accurate calculation, but just aprox...

so for example....if moved by hand a membrane can make 20mm highest peak and with power input of 10W it makes 1mm, at 20W it does 2mm, at 40W 4mm and at 80W it does 8mm...that means with a 200W would be aprox 20mm, but to be sure to not override it, i can assume 150WRMS would be pretty fine to drive that speaker to its max, but still not in damage zone...

[Conrad Hoffman]

What frequency are you planning on using?

[coppercone2]

can you try a high speed smartphone camera?

[pqass]

So if i did get it right, movement of the membrane is linear compared to the power input...what means i can make some test of movement in mm p/p at input 10W, 20, 40 and 80W and if movement measuring showing linearity i can calculate what is the max power till it reach maximum possible movement of the membrane....well...of course, i dont expect accurate calculation, but just aprox...

so for example....if moved by hand a membrane can make 20mm highest peak and with power input of 10W it makes 1mm, at 20W it does 2mm, at 40W 4mm and at 80W it does 8mm...that means with a 200W would be aprox 20mm, but to be sure to not override it, i can assume 150WRMS would be pretty fine to drive that speaker to its max, but still not in damage zone...

Yes, that's what I was suggesting BUT there are issues with that apparently.

a. when measuring movement, it should be done with speaker in-cabinet; not in free-air because the vacuum/compression will "load" the speaker causing less movement for same power also it's the truer use-case so do that,

b. BUT, as per CaptDon, the true power limitation is determined by the heat dissipation of the voice coil.   And how can we measure that?  By measuring the coil resistance as Conrad pointed out. 

eg. using the temp. coefficient of +0.00393*R/°C, a rise of 50°C (from 25°C to 75°C) would result in a 0.8ohm increase in resistance of a 4ohm speaker.  You'd need a trustworthy multimeter/bench meter do determine hundredths of an ohm with accuracy.


So, the method may be to:
1/ note the ambient temperature and initial coil resistance,
2/ apply 20W continuous single sine wave (20-50Hz) for several (?) minutes; periodically measuring the coil resistance until it levels off,
3/ calculate the temperature delta by taking the difference in current coil resistance minus initial,
4/ apply another 20W, measuring the coil resistance until it levels off, goto 3/
5/ stop when the calculated temp. diff. + ambient is 75°C (?) or if you run out of amp. power, extrapolate to temp. ceiling.

Anybody see anything wrong here?

The labelled power rating of a speaker is just so subjective because music isn't a continous sine wave so you can double/triple/etc. for short periods anyway. I guess you'd need another variable where:     avg. continous power * fudge-factor = music I listen to power


FYI: I picked 75°C for no good reason; hoping it won't melt anything.

Relevant links:
https://geoffthegreygeek.com/understanding-amplifier-power/
https://www.cirris.com/learning-center/general-testing/special-topics/177-temperature-coefficient-of-copper

[Berni]

The max power where a speaker might hurt itself is dependent on too many things.

It will only hit mechanical travel limits on low frequency, but at the same time the low frequency large movements will pump more air past the voice coil keeping it cooler and so able to handle more power.  All this is again affected by the enclosure the speaker is placed in, a small restrictive enclosure will let the speaker barely move, a large open one is almost like free air, a tuned resonant enclosure might actually make the speaker move even more than normal at a certain frequency etc..

You will definitely hear the voice coil bottoming out to prompt you to reduce volume. The more hidden danger is overheating. You can't hear it or see it and when you smell it then its probably already too late. Normal music actually has a really low averaged out RMS power(especially older music that actually used dynamic range), so you can crank it up well past the rated power of the speaker and it will be fine. But if you play a bass test(constant full amplitude tones) for long periods of time, you might be able to blow a speaker even at under the rated power.

[CaptDon]

Remember to use the 'actual' cold resistance reading in your calculations. Most 4 ohm speakers measure anywhere from 3 to 3.5 ohms and 8 ohm speakers anywhere from 5 to 7 ohms. Also, a '4-Wire' meter would almost be mandated in this situation and even the meter connections to the speaker could vary several 10's of milliohms up or down each time you make the connection.

[langwadt]

Remember to use the 'actual' cold resistance reading in your calculations. Most 4 ohm speakers measure anywhere from 3 to 3.5 ohms and 8 ohm speakers anywhere from 5 to 7 ohms. Also, a '4-Wire' meter would almost be mandated in this situation and even the meter connections to the speaker could vary several 10's of milliohms up or down each time you make the connection.

I'm not sure it needs to be that accurate

https://www.stereophile.com/reference/1106hot/index.html