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

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#201
Radiation of course does go both ways..... if both items are at the same temperature, then as much comes back from piece #2 to piece #1 as goes to #2 from piece #1, and there is no particular effect.

However, the magnet is colder than the coil. Therefore, whenever the voice coil is close to the magnet structure, it is better cooled by highly effective radiation to the magnet. The magnet may radiate some back, but it is minimal in terms of total energy.... it can't radiate back heat that was carried off into the metal, and being cooler, it has less to radiate back anyway. The equilibrium point is with the coil close to the same temperature of the magnet, so the largest flow of heat is away from the coil and to the magnet, until and unless the magnet heats up due to all this dissipation going into it.

When the coil is farther away, radiation is less effective, because nothing cold is close by, and the equilibrium point is with the voice coil hotter.....not as well cooled.

Look at it this way.... if the door on a large freezer (such as is at the food store) is open, 10 meters from you, you may notice that there is something cold over there. But walk right up to within 1 meter of the open freezer, and you will be wanting a coat if you stay there for long, because you very effectively radiate heat to it.

The closer the cold magnet structure is to the voice coil (and/or the colder it is), the more effective it is in cooling the voice coil. The closest it gets is when it is actually in the gap.

The gap isn't required for cooling, it is just the best place for cooling the voice coil....with "cold" metal on both sides of the coil, perhaps a half mm away from it, it is pretty darn well cooled, compared with being outside the gap.

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The point about DC probably comes from the fact that some "ill-behaved" amplifiers do develop a DC offset in the output when they are clipped hard..... it usually isn't very much, a few volts, 10% of the power supply voltage would be a lot..... But it is true.... Whether it is enough to have a bad effect, that I don't know without getting into a specific case.
 
#203
The thing is though, a DC offset is a far, far different thing from all the other babble about the cones "stopping" when they hit the "DC part" (clipped top) of an AC wave. Most of the comments about DC are founded on a notion that the flat tops of a clipped wave are DC, and therefore it's just like plugging a car battery in to the speaker, and the speaker slams to a halt. :rolleyes:

An offset might be a real possibility, and it might even be harmful in its own way, but it's a totally different argument because the speaker keeps moving and is not assumed to overheat from extreme starts and stops.
 
#204
It's been so long since I studied thermodynamics. I thought an object radiates proportionate to its own surface temperature. In your cold freezer analogy I'm feeling cold because nothing is bouncing back.

Conversely if I stand next to a polished aluminium plate I feel warm, yet the plate is an efficient conductor. I don't like feeling fundamentally wrong about laws of nature.
 
#205
Downunderwonder said:
This is where I get confused. Free air is cooler than the metal, radiating back nothing. Or not...
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You're thinking of net equilibriums. The air is cooler than the metal, thus the net radiation is from the metal to the air.

But the air does have energy, which it does radiate in every direction. Some of that energy is directed towards the metal, and is absorbed thus heating the metal. Even if the net exchange is such that the metal is cooling.

It's a little bit like pointing a flashlight at the sun.
 
#206
how would "free air" heated by the coil be cooler than the metal?

the metal of the magnet would obviously be cooler, and would thus draw heat away from that air in the gap.
 
#208
Freight Train said:
Mech, you're spinning your wheels. You're not going to convince them of anything, no matter how much you know or what you've seen and experienced yourself. You're trying to help the guy not to get shot, but instead people want to argue that it's not the bullet that kills you, it's the hole. I'm an EE myself, I've been down this road, and I'm telling you you're wasting your time. Let it go.
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I am an Electronic Engineer too. I was going to wade in on Mech's behalf but perhaps we will leave it there. Inertia, AC/DC, sine waves, square waves (I have never seen a wave that shape in the real world, try surfing that mother). All these ailments are cured, or at least given symptomatic relief by snake oil, output transformers, valves (sorry tubes, I am a limey).

The truth is that too much power kills speakers AC/DC square/sine/anyother wave will kill a speaker if the duration is long enough. A speaker rated at 100W RMS should be OK with an amp delivering 100W RMS. However RMS measurement implies a sine wave. RMS gives the DC equivalent power of an AC waveform. Therefore a 100W RMS amp (power supply permitting) could put 200W RMS through your 100W speakers. That is why speakers were always recommended to be rated at twice amp power.
 
#209
If a temperature differential doesn't exist, no heat will be gained or lost, assuming there's nothing causing the temperature of one object to become warmer or colder e.g., the amplifier is turned off or the speaker is disconnected. If one object is colder than the other, their temperatures will eventually equalize. Heat flows from a warmer body to a cooler one, regardless of whether it's in air, a liquid, in contact with another solid or in a vacuum unless something that can reflect radiated heat is present. If the voice coil can't lose enough heat to avoid damage, it will eventually fail. Proximity to another object does matter.
 
#211
Mr. Foxen said:
Free air allows convection, which will tend to be a more significant factor to cooling than radiation, confined air acts as an insulator compared to fee air, that is how clothes work.
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Similar to putting a lid over a pot of stew but if a more massive metal cover were laid on top, it would lose heat much faster. For conduction, yes- air acts as an insulator because is doesn't transfer heat as efficiently as a solid or liquid and that's why some speaker drivers are cooled using ferro-fluid (also for damping the driver's motion- usually tweeters).
 
#212
Mr. Foxen said:
Speakers will reach their mechanical limit way before their thermal limit is reached, rendering any higher power handling spec a bit irrelevant.
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Seems legit when it comes to a driver dealing with low frequency signals at volume . . .

I doubt the engineers spend all the time developing cooling fins and the like just to spec it to cook way below the limiting excursion - when (AFAIK) the excursion costs more in a design, beyond a small limit.

I guess what I'm saying is they'll generally seek a balance. And thermal limits are probably a cheapish way of getting a big number associated with a driver. I'm no expert though.
 
#215
Well, prior to retraction, I was just gonna say I think you've arrived at more or less the same page I've been on, with that description.

Though I admit the finest details of the transfer from coil to metalare beyond me, the main thing is that the large metal mass doesn't reflect or radiate much back, and transfers enough heat out of the immediate vicinity of the coil via conduction (within the metal itself) to provide an adequate cooling effect.

If it can't shed that heat fast enough to keep up the coil will cook (unlikely?). Or if the coil can't transfer heat to it fast enough, it will cook (thermal limit?). And if the coil spends too much time too far away from it (outside the gap), it will cook.


Something like that.
 
#216
Something like that, yes. I need to bone-up on the radiation part. You and Jerrold have a point in that a cooler nearby object will reflect less energy back at the source. I need to study this a bit.

By the way, I apologize for completely and utterly hijacking the thread.......
 
#217
Heat a cast iron skillet, put it on a piece of wood and hold your hand close to it- you'll definitely feel the heat and if the temperature difference is sufficient and you leave it there for long enough, you'll cook your hand. It's how an oven works and it's how a voice coil heats a magnet assembly. In either case, the heat crosses from one object to another and raises its temperature. In my HVAC and physics classes, we were taught that "heat finds cold" but once the colder object reaches the same temperature as the hotter one, no heat transfer will occur but they could lose heat to their surroundings.

As far as a speaker failing, it can do that in several ways. I have seen over-excursion tear the surround, voice coils that hammered the magnet because they didn't have the recess at the bottom to allow the coil to move without hitting, voice coil windings that released from the former, stretched and broke, windings that fused, formers that warped from extreme heat and fast cooling and in the case of my Altec 417B, the cement that held the voice coil to the former melted and flowed to the bottom, slowing it's motion and allowing the winding to scrape the magnet/break.

I know the amp that was used when the Altec failed would occasionally oscillate at a high, but audible, frequency. It may have done this when I loaned it that day but I wasn't there, so I can't be sure. I looked at the power handling spec for the Altec earlier this morning and saw that it was 60W. A 3" copper coil on an aluminum former and it's only rated for 60W from 60Hz- 8KHz? Must have been using the old "long-term power handling" rating. OTOH, the amp being used is rated at 250W into a 4 Ohm load, so,.....
 
#220
dincz said:
This explains many of the points covered in this thread and is worth a read:

Link Removed
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+1
There is a lot of great information at this site. And from the reference material he references.

Another useful site:
"Basic Car Audio Electronics"
http://www.bcae1.com/2ltlpwr.htm
  • "If your speakers are capable of handling significantly more than your amplifier can produce, driving them with a clipped signal will not likely hurt them."
  • "If the speakers can handle 3 or 4 times the power that your amplifier can produce, there's virtually no way to damage your speakers (no matter how clipped the signal is)."
  • "If your speakers are rated for the same power handling as your amplifier is capable of producing cleanly, driving them with a clipped signal for extended periods of time may cause speaker damage and/or premature failure."
  • "If your speakers are rated for the same power handling as your amplifier is capable of producing cleanly, driving them with a square wave signal for extended periods of time will likely cause speaker damage."
And a final point from this site:


"You should remember:
1.Idiots blow speakers, not low power."
 
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