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

[1954bassman]

For me, when I have always heard about blowing a speaker due to "underpowering" it, I did not take that to mean an amp with not enough power to properly drive the speaker would blow the speaker.

My understanding was (is) of underpowering and possibly blowing a speaker? Say you have a really nice 400 watt cabinet, and a not so great 200 watt amp. Now you start playing nice and loud, and as the gig goes down, the overall stage volume of the band keeps going up. So you are thinking this is a 400 watt cab, I will just keep turning up my 200 watt amp. Eventually, your amp starts running out of good clean signal, and becomes more and more distorted, until it finally puts out enough wattage (although distorted) that it blows your speaker.

I did this plenty of times back in the 70s and 80s using Fender Bassman tube heads and Peavey speakers. Try plugging you Fender P into a Bassman 50 - then go from the speaker out into a Peavey Bass 200 and then into two 215 cabs. nice and loud while it lasted.

Maybe I am completely wrong. Wouldn't be the first time.

 

[Derek Balonek]

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.

The air is not needed as the steel is what provides the most cooling. When the coil is in the gap and in closer proximity to the polepieces the heat transfer is greater. Radiation is how the heat moves from the coil to the polepieces. I was incorrect in stating it was conduction.

In a vaccuum, the heat transfer to the polepieces is not affected. It still radiates. If it was a pure vaccuum, I believe the coil would not cool since the heat has nothing to be transferred to. Thankfully a speaker cabinet is not a vaccuum. The process of heat transfer is about the equilibrium of energy, and steel is able to absorb more energy than air; when the coil is closer to the steel the energy transfer is more effective than when it is further away.

At least that's how I think it works. Hopefully someone with a better grasp of it can explain it better. I just know that the polepieces provide most of the cooling for the voice coil.

 

[David Jayne]

In a vaccuum, the heat transfer to the polepieces is not affected. It still radiates. If it was a pure vaccuum, I believe the coil would not cool since the heat has nothing to be transferred to.

That's it. That is incorrect. Radiation radiates into empty space just fine. Nothing physical nearby is required for a radiating body to radiate.

 

[Derek Balonek]

For me, when I have always heard about blowing a speaker due to "underpowering" it, I did not take that to mean an amp with not enough power to properly drive the speaker would blow the speaker.

My understanding was (is) of underpowering and possibly blowing a speaker? Say you have a really nice 400 watt cabinet, and a not so great 200 watt amp. Now you start playing nice and loud, and as the gig goes down, the overall stage volume of the band keeps going up. So you are thinking this is a 400 watt cab, I will just keep turning up my 200 watt amp. Eventually, your amp starts running out of good clean signal, and becomes more and more distorted, until it finally puts out enough wattage (although distorted) that it blows your speaker.

I did this plenty of times back in the 70s and 80s using Fender Bassman tube heads and Peavey speakers. Try plugging you Fender P into a Bassman 50 - then go from the speaker out into a Peavey Bass 200 and then into two 215 cabs. nice and loud while it lasted.

Maybe I am completely wrong. Wouldn't be the first time.

I never really though about it that way, but I'd agree with you on that. Using an underpowered amp often precludes speaker destruction. I don't hear about people blowing speakers with overpowered amps as often. Perhaps one is more cautious once they get to the point that they're comfortable with using an amp that can blow a 410 without breaking a sweat.

 

[David Jayne]

Try plugging you Fender P into a Bassman 50 - then go from the speaker out into a Peavey Bass 200 and then into two 215 cabs. nice and loud while it lasted.

Maybe I am completely wrong. Wouldn't be the first time.

Yes I'm afraid doing what you've described is a colossal mistake. It doesn't make the Peavey louder, but it does tend to blow it up.

 

[Derek Balonek]

That's it. That is incorrect. Radiation radiates into empty space just fine. Nothing physical nearby is required for a radiating body to radiate.

Okay, yeah, I am off on that. I'll let somebody with a better grasp of thermodynamics explain the process. Maybe the convection - conduction was more accurate?

 

[David Jayne]

Maybe the convection - conduction was more accurate?

Not tooting my horn, but I actually have a pretty firm grasp of these concepts, having designed/ proven lots of equipment used in commercial foil stamping. That's all about thermal transfer, at the highest possible speeds, while keeping an eye on the temperatures of all the elements, all the time.
What we are talking about- transferring heat from a coil to a nearby chunk of steel, in a way that actually benefits the coil, absolutely requires air, which we already know is a poor conductor.
I still maintain that if you feed a signal to a voicecoil, enough to get it warm, it will stay cooler if it's kept away from anything else, rather than closer.

 

[fdeck]

For me, when I have always heard about blowing a speaker due to "underpowering" it, I did not take that to mean an amp with not enough power to properly drive the speaker would blow the speaker.

My understanding was (is) of underpowering and possibly blowing a speaker? Say you have a really nice 400 watt cabinet, and a not so great 200 watt amp. Now you start playing nice and loud, and as the gig goes down, the overall stage volume of the band keeps going up. So you are thinking this is a 400 watt cab, I will just keep turning up my 200 watt amp. Eventually, your amp starts running out of good clean signal, and becomes more and more distorted, until it finally puts out enough wattage (although distorted) that it blows your speaker.

I did this plenty of times back in the 70s and 80s using Fender Bassman tube heads and Peavey speakers. Try plugging you Fender P into a Bassman 50 - then go from the speaker out into a Peavey Bass 200 and then into two 215 cabs. nice and loud while it lasted.

Maybe I am completely wrong. Wouldn't be the first time.

A 200-Watt amp would put out somewhere in the ballpark of 280 W when driven with a square wave. Something else could have been going on, namely that the 400-Watt rating of the speaker was exaggerated.

 

[David Jayne]

A 200-Watt amp would put out somewhere in the ballpark of 280 W when driven with a square wave. .

Did you mean to say, "when driven into hard clipping" ? Not the same.

 

[Ewo]

Try plugging you Fender P into a Bassman 50 - then go from the speaker out into a Peavey Bass 200

Well, there's the problem, bolded.

 

[makohund]

The gap isn't what aids the cooling, it's the highly heat-conductive metal on the other side of it.

Radiated heat is absorbed there, and the energy is spread out into a much larger mass and surface area. (Same amount of energy, dispersed... results in lower temperature. Same principles that make chip heatsinks and heatpipes work.)

The material absorbing the heat also gives off that heat. But with greater surface area, it can get rid of it quicker than the coil could have done without its help.

The gap is an unavoidable space... no it doesn't help cooling, it hinders it. You're right the heat would transfer even faster if it actually touched. But then it couldn't move, which is the whole point of the motor.

Outside of the gap is space without a heatsink close enough to "wick" any of that radiated heat away faster. So with enough power and too much time outside the gap, it'll cook.

 

[David Jayne]

Well, there's the problem, bolded.

Yep. Before I knew better I used to run my car stereo speaker outs into a power 'booster' amp, thinking the wattage would 'add up'. Wrong.

 

[Mehve]

For me, when I have always heard about blowing a speaker due to "underpowering" it, I did not take that to mean an amp with not enough power to properly drive the speaker would blow the speaker.

My understanding was (is) of underpowering and possibly blowing a speaker? Say you have a really nice 400 watt cabinet, and a not so great 200 watt amp. Now you start playing nice and loud, and as the gig goes down, the overall stage volume of the band keeps going up. So you are thinking this is a 400 watt cab, I will just keep turning up my 200 watt amp. Eventually, your amp starts running out of good clean signal, and becomes more and more distorted, until it finally puts out enough wattage (although distorted) that it blows your speaker.

Yes and no.

You're correct that the clipping can make the amp put out more than it's rated power, definitely. And it's certainly possible that the increase in power could wind up exceeding the cab's limits, even though you thought you had headroom to play with. But this danger is somewhat exaggerated. People who claim 4, 5, or 10 times the rated output power are pulling numbers out of nowhere.

The theoretical, absolute highest level of power that an amplifier pushed into clipping could put out is twice the rated power (this can be demonstrated with some basic calculus, fyi). That's what you'd get if it was managing a perfect square wave. Google a square wave sound file and give it a quick listen - even heavily distorted, badly clipped audio doesn't come close to that extreme. So it's a safe bet that even a fairly clipped output is still putting out much less than twice the rated power.

As mentioned earlier, the more common issue is when you have a crossover in the cab, which directs all that higher-frequency distortion away from the woofer and into the smaller extra speakers which were never meant for it.

 

[fdeck]

Did you mean to say, "when driven into hard clipping" ? Not the same.

Ah, good point. Er, um, I meant whatever is correct.

 

[Mr. Foxen]

Something else could have been going on, namely that the 400-Watt rating of the speaker was exaggerated.

More likely the 400w rating is thermal, but the damage done was mechanical, since mechanical limits of drivers are loads lower than the thermal ones usually.

 

[Downunderwonder]

Read the first line then scoot to his sig. Most of you amateur thermodynamics professors wouldn't get a job sweeping the floor in a lab. Give it up.

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.

 

[makohund]

Read the first line then scoot to his sig. Most of you amateur thermodynamics professors wouldn't get a job sweeping the floor in a lab. Give it up.

Yeah, no point in trying to learn or understand anything, or even explain what little one does think they might know as best they can to anyone else. All pointless wastes of time, better to go sit on the couch and gaze at the tube.

Now gimme that broom.

 

[Downunderwonder]

It's the tone of guys spouting nonsense as if they had actual knowledge that bugs me. Particularly when an actual speaker guy lays it out and they carry on with the nonsense like he was just some other guy expressing an opinion.

 

[David Jayne]

...

 

[Downunderwonder]

My limited grasp of thermodynamics says that the magnet should act as an insulator to the radiation of heat by the coil since it back radiates to the coil. Since you guys say the coil scorches where it overhangs, obviously I'm wrong about that. Carry on....