I'm wondering: Is there any rule of thumb for estimating continuous power dissipation in a speaker you're building from scratch?
Say you got a magnet and a basket with absolutely no information whatsoever about the power rating of the previous speaker.
Once you get a voice coil for it, the main surfaces involved in heat transfer will be already defined: The lateral surface of the wire coil itself, the surface of the voice coil former (important if its aluminium) and the surface of the top and bottom plates of the magnet.
Obviously, air flow in the speaker as it moves is very important for cooling, and things aren't easy to estimate if it's a vented pole piece.
But I'm just looking for a rule of thumb, and for continuous power dissipation, not transient (in which thermal masses matter a lot).
I could try to find a pattern across several speaker datasheets, if they have all the dimensions (including voice coil height) plus voice coil former material.
But I thought that perhaps someone here might know about some easier way to do it.
Say you got a magnet and a basket with absolutely no information whatsoever about the power rating of the previous speaker.
Once you get a voice coil for it, the main surfaces involved in heat transfer will be already defined: The lateral surface of the wire coil itself, the surface of the voice coil former (important if its aluminium) and the surface of the top and bottom plates of the magnet.
Obviously, air flow in the speaker as it moves is very important for cooling, and things aren't easy to estimate if it's a vented pole piece.
But I'm just looking for a rule of thumb, and for continuous power dissipation, not transient (in which thermal masses matter a lot).
I could try to find a pattern across several speaker datasheets, if they have all the dimensions (including voice coil height) plus voice coil former material.
But I thought that perhaps someone here might know about some easier way to do it.
There isn’t one to my knowledge because it will vary depending on the enclosure and frequency. For example, the convection cooling will be much lower at 80hz than it will at 40hz due to excursion but conduction cooling will remain the same.
One method would be to destroy a driver to find its thermal limits and then turn it down 1db with the replacement driver ;-)
One method would be to destroy a driver to find its thermal limits and then turn it down 1db with the replacement driver ;-)
There's a rough relationship between voice-coil diameter and power handling.
I had a book with a table of the expected power handling of speakers that had different diameter voice coils.
It was an old book, though, and cooling methods have improved.
Compare the coil diameter in datasheets to see if a pattern forms.
I should add that if increased power handling comes at the cost of reduced efficiency, you aren't gaining anything if sound output is what you want.
I had a book with a table of the expected power handling of speakers that had different diameter voice coils.
It was an old book, though, and cooling methods have improved.
Compare the coil diameter in datasheets to see if a pattern forms.
I should add that if increased power handling comes at the cost of reduced efficiency, you aren't gaining anything if sound output is what you want.
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As the power rating of a driver doesn't simply depend on the thermal load of the coil, you may miss something by just looking there. A naked chassis has no reasonable continious power.
First you have to decide in what enclosure you will use it. In free air, even at a very low power level, it will hit the end of the magnet gap. So what is this for continious power? Then place it in a closed enclosure, too small, feed it too low a frequency and the coil will not move and burn. Use a vented box and you see another "max power" rating.
A better way is to test it inside the planed cabinet, with a frequency that is suited to the chassis size and look for distortion. If the Speaker is reaching it's limit, distortion will rise. So for a woofer a 10% distiortion level should set a limit, much less for a midrange.
So you build your box and measure what kind of power it can take, then print a label and glue it to the back. It may make someone happy.
I use 50W rms speaker with 400W rms amps, so what. I could use a 4W valve amp, too.
This is why most experienced builders don't care too much for any rms numbers on a chassis. TSP are important and a good construction.
First you have to decide in what enclosure you will use it. In free air, even at a very low power level, it will hit the end of the magnet gap. So what is this for continious power? Then place it in a closed enclosure, too small, feed it too low a frequency and the coil will not move and burn. Use a vented box and you see another "max power" rating.
A better way is to test it inside the planed cabinet, with a frequency that is suited to the chassis size and look for distortion. If the Speaker is reaching it's limit, distortion will rise. So for a woofer a 10% distiortion level should set a limit, much less for a midrange.
So you build your box and measure what kind of power it can take, then print a label and glue it to the back. It may make someone happy.
I use 50W rms speaker with 400W rms amps, so what. I could use a 4W valve amp, too.
This is why most experienced builders don't care too much for any rms numbers on a chassis. TSP are important and a good construction.
Thanks for the replies, but my problem is exactly that I'm in a context where the power handling has to be estimated, not measured.
I know well the procedures to measure the power handling, I've done it many times, but the problem is that I'm developing an instruction video for building mechanical wave drivers out of old speaker parts, similar to this one by PASCO:
The idea is that people will be able build it themselves, and they don't need to find a magnet with specific dimensions, so they might be building it with different magnets and voice coil diameters, each will have its own power handling.
And all I need is a way to conservatively estimate the value for the protection fuse. Proposing a thermal test is completely overkill, the builders are not going to be experienced in this area.
I know well the procedures to measure the power handling, I've done it many times, but the problem is that I'm developing an instruction video for building mechanical wave drivers out of old speaker parts, similar to this one by PASCO:
The idea is that people will be able build it themselves, and they don't need to find a magnet with specific dimensions, so they might be building it with different magnets and voice coil diameters, each will have its own power handling.
And all I need is a way to conservatively estimate the value for the protection fuse. Proposing a thermal test is completely overkill, the builders are not going to be experienced in this area.
I wonder if there's a correlation between driver diameter, resonance frequency and power?
I would think that a driver with an Fc=37Hz is meant for woofer duty and therefor needs to be able much more power than the same diameter driver that resonates at 200Hz and obviously is a mid driver. Roughly.
So measuring the resonance frequency might give a pointer.
Won't work with the naked no-cone driver of course.
Jan
I would think that a driver with an Fc=37Hz is meant for woofer duty and therefor needs to be able much more power than the same diameter driver that resonates at 200Hz and obviously is a mid driver. Roughly.
So measuring the resonance frequency might give a pointer.
Won't work with the naked no-cone driver of course.
Jan
"I'm developing an instruction video for building mechanical wave drivers out of old speaker parts"
There are so many variables here; I have three 8" drive units to hand, with different coil diameters, different magnet diameters and different x-max capability.
Build one and see how much power you need to do what you want it to do. You might only need 10W for its maximum effect, especially at low frequencies.
Once you have built one, you can give guideline instructions.
There are so many variables here; I have three 8" drive units to hand, with different coil diameters, different magnet diameters and different x-max capability.
Build one and see how much power you need to do what you want it to do. You might only need 10W for its maximum effect, especially at low frequencies.
Once you have built one, you can give guideline instructions.
I think I found something that might be useful.
Checking some datasheets, in particular those of Visaton, which provide both voice coil diameter and winding height, I found an approximate linear correlation between continuous rated power and winding lateral area, as follows:
It looks off because I forced it to intersect at 0,0.
Obviously far from being something exact, but for a conservative estimate I think this is useful, using this and then halving the estimated power for safety, and choosing a fuse according to P=I²R
Checking some datasheets, in particular those of Visaton, which provide both voice coil diameter and winding height, I found an approximate linear correlation between continuous rated power and winding lateral area, as follows:
It looks off because I forced it to intersect at 0,0.
Obviously far from being something exact, but for a conservative estimate I think this is useful, using this and then halving the estimated power for safety, and choosing a fuse according to P=I²R
But as I said, the magnet and voice coil dimensions are not going to be defined. And they shall not be.
It's a matter of reality, I'm talking about a low-cost replication for physics education that should be accessible in my country (Brazil), in most of our cities you won't have options to choose from, you'll need to use the closest you find, and the gap diameter might vary considerably.
So there needs to be a way to estimate, roughly and conservatively, the protection fuse just to prevent destroying it.
And, as you asked, the same driver is used in highly different experiments, some low frequency and large displacement, like in the vibrating string, and some high frequency and low displacement, like the Chladni Plate. It's the same driver for all of them.
Obviously, the fuse must be sized according to worst case, which is the high frequency one, where displacement is low and there's less convective cooling.
I used an old speaker to vibrate a bath of etching solution when making PCBs.
You might be surprised at how little power you actually need.
I wouldn't worry about fuse rating.
Like I said, you need to build one example, from something readily available in Brazil, then you will have a better idea of the limits. At least, that is what I would do.
You might be surprised at how little power you actually need.
I wouldn't worry about fuse rating.
Like I said, you need to build one example, from something readily available in Brazil, then you will have a better idea of the limits. At least, that is what I would do.
"Something readily available in Brazil" isn't a single thing, that's the point, there's no single magnet universally available everywhere here.
In the small cities you'll find 2-3 guys that work with speakers and luckily you'll find one magnet of a reasonable size for this project, I can't restrict more
parameters on top of that. Whatever gap size it has, that will be it.
The one I built has a 31.75 mm VC, and I need to dissipate over 16 W thermal (2 A RMS on a 4 ohm VC) for the Chladni Plate to work in the higher frequency modes (which have nearly no excursion, < 0.1 mm).
It doesn't burn, but it heats quite a lot and I can't let it at that power indefinitely either.
And perhaps a 25 mm VC could burn quite faster and would need to be restricted.
This is why the fuse is needed and that's why there needs to be some parameter to properly size this fuse according to the VC they're employing.
For the lower frequency and larger displacement experiments like vibrating string and ripple tank this is less of an issue, they need a lot less power.
Vented pole pieces may have been new in 1948 when Jim Lansing (pre-JBL) used them on his D-130 15" speakers, but 75 years later it's not.
The way the venting is done can make a big difference in heat dissipation, improvements have been made since then..
Not so obvious is a fuse conservative enough to protect the voice coil of a speaker will often burn up on transients that can easily be handled by it.
For instance, the tiny 5watt voice coil in the EV T35 in the photo above can take 50 watt peaks for 10milliseconds.
The PASCO mechanical wave driver uses a 1.0 A, 250 V fuse, about 250 watts.
A mechanical wave driver similar to the one by PASCO driven with sine waves would require a fuse with less than half the RMS rating of a typical voice coil to have a chance of protecting it.