Can't blow a speaker by under powering it.......

[David Jayne]

Steel has better heat transfer than air.

Yes, but I'm saying it doesn't matter. In use, the magnet assembly will rapidly reach a temperature greater than ambient, thus the ambient air temperature will always be lower than the air temperature in the gap. And that means that the coil would be cooled better if it were just hanging out in the open, ambient air. The magnet/ gap assembly can't 'draw' heat from the coil simply by being near it. Thermodynamics doesn't work that way.

 

[B-string]

Air is a poor conductor of heat. That is why you need to increase the surface area for heat transfer. The smallest amount of air space between the heat source and the radiating device is needed. Yes the magnet will get warmer than the surrounding air, but in turn the air surrounding the motor will also be warmer than the ambient outside the cab.

 

[Mehve]

I'll attach a pencil to a speaker cone and see if it doesn't work like a chart recorder.

If you do it, make sure you track the voltage input and the pencil output over time. You'll find the pencil output lags the voltage input level.

Alternatively, you can explain how voltage applied to a coil is supposed to translate into a linear distance, instead of a force like every physics textbook seems to think.

 

[fdeck]

I'll attach a pencil to a speaker cone and see if it doesn't work like a chart recorder.

Why not just use the equations at my web page? Or consider my argument, given above, about the filtering behavior of a speaker. This is not rocket science.

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While you're at it, I suggest opening up a chart recorder. Most laboratory type chart recorders -- going back even to the tube era -- used a feedback servo to move the heavy pen with reasonable speed and damping behavior. There was also a brand of chart recorder that used a conventional meter movement and snapped the meter needle against pressure-sensitive paper, that was widely used in industrial control rooms. It had pretty slow response.

 

[fdeck]

If you do it, make sure you track the voltage input and the pencil output over time. You'll find the pencil output lags the voltage input level.

Alternatively, you can explain how voltage applied to a coil is supposed to translate into a linear distance, instead of a force like every physics textbook seems to think.

Actually, a DC voltage will translate into a roughly linear distance after a sufficient period of time for the cone to settle.

Still, your point is well taken. There are a couple of factors that are easy to overlook in any simplistic explanation of speakers. First, it's current and not voltage that generates the driving force. While voltage and current are related, they do not track one another instantaneously.

Second, there are a number of restoring forces acting on the cone, including inertia, the suspension, and back pressure, all of which have their own dynamics.

 

[levis76]

Can anyone show me that there is no DC component to the two waveforms in this pic or that a speaker will act other than described? This is a decent 100Hz square wave on the top with over 99% DC duration and a clipped 100Hz sine wave with about 50% DC duration on the bottom.

mech

It's 2012, get a digital o'scope already.

I read the right answer to this post within the first page, if underpowering could damage a speaker then volume knobs wouldn't exist and we'd all have tinnitus.

 

[Mehve]

Actually, a DC voltage will translate into a roughly linear distance after a sufficient period of time for the cone to settle.

Still, your point is well taken. There are a couple of factors that are easy to overlook in any simplistic explanation of speakers. First, it's current and not voltage that generates the driving force. While voltage and current are related, they do not track one another instantaneously.

Second, there are a number of restoring forces acting on the cone, including inertia, the suspension, and back pressure, all of which have their own dynamics.

Point. Just emphasizing that even on an electrical level alone, expecting the voltage and the "pencil" to match is a lost cause, even before you start factoring in all those physical issues.

 

[makohund]

Yes, but I'm saying it doesn't matter. In use, the magnet assembly will rapidly reach a temperature greater than ambient, thus the ambient air temperature will always be lower than the air temperature in the gap. And that means that the coil would be cooled better if it were just hanging out in the open, ambient air. The magnet/ gap assembly can't 'draw' heat from the coil simply by being near it. Thermodynamics doesn't work that way.

Uh... yes it does. See Fourier's law of heat conduction.

It is cooler than the air in the gap, so it does. And being a better conductor than air, it transfers more of it away from there (to the cab air, over much greater surface area) much faster than hanging out in air alone.

Basically a big heat sink.

 

[Mr. Foxen]

And that is why power compression doesn't exist in a speaker run within its thermal limit, apparently.

 

[Jerrold Tiers]

RADIATION is the primary method of heat removal..... the air is not needed, and the cooling (but not the speaker) would work just as well in a vacuum as in air. There is NO NEED for the heat to be transferred to air and then to the steel polepieces.
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A12V battery MIGHT kill a speaker..... it depends on the suspension, and the voice coil. if the 12V, as it might, moves the cone enough that it causes part of the coil to be outside the gap, then it could possibly have enough dissipation in the "uncooled" part to burn the coil and kill the speaker.

Then also, many large excursion speakers have coils longer than the "top plate" of the magnet structure. In that case, some of the coil is ALWAYS outside the gap. Those speakers depend on the power being enough to have cone motion that will move the coil into the gap sufficiently to cool it. At lower powers, the coil is safe, at higher, it is cooled.

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Power compression will ALWAYS exist.... but it may not be significant. As soon as the coil heats at all, its resistance goes up, and the total power that can be forced into it is reduced (assuming constant maximum voltage).
Since the net opposition to current flow is partly resistance, and partly back EMF*, the net effect may be rather small unless the coil heats up a lot. Since the coil may well get to a fairly high temperature at max power, there will be at least some effect. if the speaker is well designed, it will not be a big problem.

* the voltage generated in the coil due to coil motion in a magnetic field.

 

[Derek Balonek]

Thank you Jerrold and makohund. I don't think I was explaining it very well.

And that is why power compression doesn't exist in a speaker run within its thermal limit, apparently.

Power compression is still there, within thermal limits. It's not like the coil is room temperature. It just stays cooler in the gap than out. Power compression is the result of the wire's resistance going up with temperature and the temperature does indeed go up.

 

[Mr. Foxen]

Point being, it heats up regardless of the position of the coil.

 

[seamonkey]

Why not just use the equations at my web page? Or consider my argument, given above, about the filtering behavior of a speaker. This is not rocket science.

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While you're at it, I suggest opening up a chart recorder. Most laboratory type chart recorders -- going back even to the tube era -- used a feedback servo to move the heavy pen with reasonable speed and damping behavior. There was also a brand of chart recorder that used a conventional meter movement and snapped the meter needle against pressure-sensitive paper, that was widely used in industrial control rooms. It had pretty slow response.

That's not how it works around here. Nobody uses science.

I'll check the equations out later.

Yes servo's are used in audio drivers. Common in sub-woofers. They are a feedback mechanism, used to track the position of the cone compared to the input voltage.

 

[B-string]

Point being, it heats up regardless of the position of the coil.

Very true. If it didn't that would be MUCH closer to 100% efficiency.

 

[beans-on-toast]

If you want to make a speaker blow, connect a positive voltage. If you want to make a speaker suck, connect a negative voltage.

It is my experience that most speakers blow because of a blown coil. The temperature raises due to various reasons, the efficiency of the driver goes down, the amp is turned up to compensate which makes matters even worse, the coil eventually blows.

Can an amp blow speakers rated higher than the output of the amp? Yes, I see it it happen with amps that are pushed as hard as they can go. Strange things happen at the output when the amp is pushed well beyond its design limits. If you want to kill a speaker, send it an impulse.

 

[David Jayne]

Uh... yes it does. See Fourier's law of heat conduction.

It is cooler than the air in the gap, so it does.

No, sorry, it does not. The heat is not 'attracted to' the cooler body. The cooler body absorbs the heat simply because it can, not because it 'wants' the heat.

RADIATION is the primary method of heat removal..... the air is not needed, and the cooling (but not the speaker) would work just as well in a vacuum as in air. There is NO NEED for the heat to be transferred to air and then to the steel polepieces.
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That's kind of where I was going, yes.
But since radiation is the primary means of cooling, please explain how 'the gap' is actually able to assist in cooling the coil? I'm really missing something here.

 

[makohund]

No, sorry, it does not. The heat is not 'attracted to' the cooler body. The cooler body absorbs the heat simply because it can, not because it 'wants' the heat.


That's kind of where I was going, yes.

OK, I think I must've just not understood what you were saying. Sounded like "would cool better in just air than inside the gap assembly", and that the cooler assembly won't absorb heat despite close proximity, neither of which is true.

Thanks to Jerrold for clarifying that radiation is how most of the heat transfer from the coil happens. (Though conduction is still how it moves from the smaller area near the coil to the larger mass and surface area of the "heat sink", right? Which in turn mostly radiates?)

Not sure of the practical difference between "absorbs heat because it wants it" vs "absorbs heat because physics says it has to". Other than the former being a silly anthropomorphization that somewhat describes the effect of the latter. If someone said heat was "attracted to it" (like what, a magnet?) I missed it.

 

[David Jayne]

We're still not on the same page. You say the gap aids cooling. I say it doesn't. I can think of no way that it could, unless the coil is actually touching it. The coil will radiate heat just fine in a vacuum, as Jerrold says, with no gap in sight.

 

[David Jayne]

RADIATION is the primary method of heat removal..... the air is not needed, and the cooling (but not the speaker) would work just as well in a vacuum as in air. There is NO NEED for the heat to be transferred to air and then to the steel polepieces.

If there is no need for air to conduct heat to the steel parts, why have the steel parts as part of the cooling equation at all, if pure radiation will cool the coil? I really don't see how the steel gap is able to help at all.

 

[David Jayne]

........ Those speakers depend on the power being enough to have cone motion that will move the coil into the gap sufficiently to cool it.

HOW does this cooling take place?!?
I don't understand.