Finding out the power an unknown speaker can handle

[NIC1138]

Hello. I found a couple of good-looking old Philips speakers in my parent's home, and I was thinking about using them in an amplifier I want to build. But I would like first to be sure that the speakers would be able to handle the power.

Is there any way to discover the maximum power a speaker can handle by making simple measurements in it? Any theory or rule-of-thumb?

They are 8 ohms and have something like 15cm diameter...

[HK26147]

I would compare it to what is commercially available in a 15cm or 6" size. Try to get an approximation of the VC size.
Personally, unless this was something other than a typical 6" ( not a high end 6" ). I would limit it to 60 watts or less.

I consider Max power ratings to be functionally irrelevant. It is what is done with the wattage from -10 to -3db ( 1/10 - 1/2 ) power that is most significant.
Thermal/Power Compression:
Within 30sec of play a voice coil heats rapidly. This changes the electrical characteristics of the driver. So as more power is supplied more get wasted as heat, so it becomes a matter of diminishing returns - Power in gets no louder - just hotter. Depending on the materials, the heat will start to damage/melt/burn the voice coil.
I know someone who has devised a means of calculating VC temp, but that won't tell you how much abuse it will take, short of destructive testing.
With a volt meter and an SPL meter you can plot the SPL out vs Vin and you will see it start to plateau, that gives you an indication of thermal effects.

[Steerpike]

The old Philips full range drivers - even up to 8-inch - were only rated at 5W

Can you identify the drivers from the model number on the magnet?

[Ron E]

There are two components to power handling, thermal and mechanical. At bass frequencies the limitation is usually mechanical - the coil or spider will hit some sort of hard limits - usually this is the coil hitting the back plate of the magnet or the VC leads fatiguing or tearing off, or the spider hitting the front plate and tearing off the coil, etc...

The other element is thermal power handling. As a very rough rule of thumb, it would vary with diameter, say a modern driver with a 1" voice coil can handle ~50W, A 2" voice coil similarly constructed could handle 100W and a 4" voice coil 200W - a half inch would handle 25W. Thi s is just an example and the real power handling would vary a lot based on how the coil is constructed and what the coil insulation is made out of, etc... Things you can't necessarily know....

If the driver has a paper coil former or was built before 1980, it probably won't handle much more than 10W. The advent of quality car audio and dB drag races, etc have led to an increase in power handling and reliability over the last ~20 years. The major manufacturers have ironed out many of the more typical failure modes.

Usually you can avoid damage to a speaker, no matter how much power you are feeding it, by listening to what it is telling you - drivers usually complain before breaking, with distortion or nonmusical noises. But not always...

I remember trying to hear 20kHz with some cheap bookshelf speakers when I was about 25, turning up the volume slowly. I heard nothing but amplifier hiss from about 10 feet away until the right tweeter made a nice spark and cut out. It handled almost 60W RMS at the point of failure, which is fairly impressive even today.

I agree that the most "average power" you would want to feed a driver is typically 1/10 or 1/20 it's peak rating. Nothing wrong with having an amp too large, as long as you know to listen for teh signs...

[HK26147]

Another consideration:
How would you use this 6"?.
If you feed it sub 100Hz signal, it will handle a LOT less power, If you filter out the lower octaves it's more likely to be thermal limited than mechanically. For every octave drop - excursion demand goes up 4x.

Ron E observation about age vs power handling mirrors my experience: A Rectilinear speaker I used around 1972 used a Phillips 5" full range speaker as a midrange. It blew during a period of "youthful exuberance".

[NIC1138]

Thanks so much for the great reply, guys!...

I thought some pictures of the speakers in question could be interesting, here is the front.

[NIC1138]

Here is the back of the speaker

[HK26147]

Pop the back of the cab to see the magnet assembly.
1st impressions: Don't put very much power into these.

[NIC1138]

...and finally the inside of the speaker.

The application I have in mind is a stereo amp capable of at most 14+14W to use with a computer. For now I am not much concerned with getting low frequencies for example, but just finding out the most I can get from these speakers without much headaches.

Let me try to summarize the limits. We must take care of the maximum distance the cone can move safely, what implies a maximum current, and also the maximum temperature the coil can get to before starting to distort the signal, or melting. Right?

Should I just throw in a 1kHz sine wave and increase the current until I start to hear distortions then? Let's just be empiricists, isn't it?

Thanks again for all the great info. If I do any more interesting tests with these speakers I'll let you know. And it would be great to hear if anyone has ever used this exact model before.

[NIC1138]

A better view of the magnet, and mysterious white inscription on the cone.