How is energy dissipated in a voice coil?

[Eaulive]

We all know that energy is not created or lost, but transformed.

Suppose a sine wave of 100Hz in a woofer, how is the energy dissipated?
If we take the voice coil out of the magnet for example, no mechanical work is created and I can assume the electrical energy will be dissipated in form of heat.

However, if the VC is into the gap and attached to the cone, some work is produced... is the heat dissipation reduced by the same amount?

I've been crunching on this problem for a couple of nights and I can't find an answer.

On a normally functionning woofer, assuming an impedance of 8 ohms and a applied voltage of 20VAC, what amount of power (W) is dissipated as heat in the coil? the whole 50W!?
Obviously it has to be less, but what are the mechanics involved? How do we measure or calculate the mechanical (then accoustical) energy VS the heat dissipation?

Any amount of electrical current that passes through a circuit, resistive or inductive has to be dissipated in one form or the other so what is happening in a loudspeaker?

I'm asking the question because these days I'm reconing a lot of woofers and for fun I passed 13.8VDC in a E140-8 woofer voice coil just for fun (the cone was smashed but the coil was OK)

The coil was obviously out of the gap and with 13.8V DC at around 6 ohms it should (and did) dissipate a little more than 30W which caused it to be untoucheable after a couple of seconds and started smoking a while after.

I don't see how this coil could dissipate 200W or more even with heavy venting, hence my question.... where does the energy go?

Thanks

[jcrobso]

You know the answer, you are right on top of it.
Most of the energy is converted into cone motion! How much depends on the efficiency of magnetic structure and the coil design. Some is converted in to heat! As your experiment demonstrated DC voltages are death to voice coils, they are designed for AC voltages. The power ratings are for AC voltages(audio) not DC voltages. The speaker is a complex impedance load, thus a 4 ohm DC voice coil will have an impedance of 8 Ohms or way more at cone resonance.
So relax and get a good nights sleep. John

[ratitifb]

Unfortunately very little cone motion ernergy is converted into SPL!

[Mr. Widget]

I don't see how this coil could dissipate 200W or more even with heavy venting, hence my question.... where does the energy go?

If you want to burn up more coils put DC on an AC motor or pull the armature out of an AC motor and plug it in to the AC source.
While thermal capability does come into play, the real issue is in the vector analysis.

For additional fun reading, follow the current wars of Edison and Westinghouse/Tesla as they fought over DC vs. AC.

http://en.wikipedia.org/wiki/War_of_Currents


Widget

[Mr. Widget]

Unfortunately very little cone motion ernergy is converted into SPL!

Of course that is where the acoustic transformer or HORN comes in.


Widget

[Eaulive]

You know the answer, you are right on top of it.
Most of the energy is converted into cone motion! How much depends on the efficiency of magnetic structure and the coil design. Some is converted in to heat! As your experiment demonstrated DC voltages are death to voice coils, they are designed for AC voltages. The power ratings are for AC voltages(audio) not DC voltages. The speaker is a complex impedance load, thus a 4 ohm DC voice coil will have an impedance of 8 Ohms or way more at cone resonance.
So relax and get a good nights sleep. John

Thanks!

I felt it had to be, but I just could not (and still can't) explain why.
The relation between a magnetic field and mechanical work is puzzling. I can understand that at resonnant frequency since the impedance rises dramatically the dissipated power is less.
I also understand that because of this peak of impedance the woofer resembles a resonant paralell circuit that will become inductive above and capacitive below fs, and that the voltage/current relationship will be shifted accordingly, the load stays at or around 8 ohms and this is expected since it becomes basically R+/-j.
But how in heaven is the energy transmitted.

I'm a ham and am proficient with antennas, RF and the like and the electrical portion of these circuits is no mystery to me but when mechanics are involved I'm lost
We don't have this problem with antennas since the conductors are usually way too big for any current to damage them, however we have to worry about resistive losses with some designs.

Is there a rule of thumb? How efficient is the magnetic motor? 50%? more? is there a way to tell the heat that the bobbin will have to dissipate at a given frequency?

So many questions, thanks for the link BTW.

[Eaulive]

If you want to burn up more coils put DC on an AC motor or pull the armature out of an AC motor and plug it in to the AC source.

Widget

Interesting analogy, I know that an AC motor that is not turning will burn in no time, but if you measure the current draw there's a huge difference between the free running state and the "jammed" state.
In a loudspeaker the DC resistance of the coil out of the magnet and its impedance when working properly is basically the same.
Also a motor runs at one specific frequency...

Argh

[Mr. Widget]

Also a motor runs at one specific frequency...

Not all of them... many AC motor designs can be speed controlled by varying the line frequency.

My point was that while DC is easy to figure out with simple mathematics, you need advanced algebra and calculus to fully understand AC.


Widget

[boputnam]

Unfortunately very little cone motion energy is converted into SPL!

Thak you for that. These acoustic transducers can get very hot...

Suppose a sine wave of 100Hz in a woofer, how is the energy dissipated?

There is another component of your question. For a simple case, consider a pulse at 100Hz - the cone moves (inward, of course! ). But after the pulse, the cone settles back to it's "at rest" position, generating backward EMF directed toward the amp.

[Eaulive]

Thak you for that. These acoustic transducers can get very hot...

There is another component of your question. For a simple case, consider a pulse at 100Hz - the cone moves (inward, of course! ). But after the pulse, the cone settles back to it's "at rest" position, generating backward EMF directed toward the amp.

Right, this is why at resonance the impedance is higher, since this back EMF is in phase with the supply, away from this frequency the current is shifted away from the voltage, as in any resonant circuit.
This part is clear to me but my problem remains, if a woofer resonates at 50Hz and a frequency of 500Hz is apply, we're far from the resonance and there's little cone motion....

My other question is: how efficient... 50% more, less?

Sorry, I just have to know

[Robh3606]

My other question is: how efficient... 50% more, less?

It's right in the T/S tables under efficiency. 50% not even close. Compression drivers rule as far as efficiency and they are lucky to get around 25%

Rob


http://www.lansingheritage.org/html/...s/tech1-3a.htm

http://www.trueaudio.com/downloads/spl_eff.xls

[Eaulive]

It's right in the T/S tables under efficiency. 50% not even close. Compression drivers rule as far as efficiency and they are lucky to get around 25%

Rob


http://www.lansingheritage.org/html/...s/tech1-3a.htm

http://www.trueaudio.com/downloads/spl_eff.xls

Oh right, thanks for the link.

So my e140-8 at 4.9% efficiency has 95.1% of the power dissipated as heat!
That is a lot, considering a program power of 400W or a pure sinewave of 100W.

[jcrobso]

The output impedance of an audio amp is very low, about .25 Ohms or even less. It is this low impedance that absorbs the back EMF voltage. We used to call this damping factor.

[Hoerninger]

... has 95.1% of the power dissipated as heat!
That is a lot, ...

Yes. The temperature can raise up to about 280 degrees Celsius (536 degrees Fahrenheit my calculator tells), it depends on the glue.
There are some prof speaker systems which have an air cooling for the voice coil of the woofer.
____________
Peter

[Tim Rinkerman]

Much of the heat is dissapated by the speaker pumping air through the hole in the magnet and around the voice coil.