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

[wcriley]

...the voltage applied across the driver is not positional, but accelerative.

How often do you think you'll need to repeat this before someone listens to you?
You must have the patience of Job.

 

[Mark Reccord]

How often do you think you'll need to repeat this before someone listens to you?
You must have the patience of Job.

Bob clearly has no idea what he's talking about even though he's an engineer working for a highly respected company in the industry and a member of the AES executive.

 

[rdk]

If you have a 400 cab and a 50 watt head the cab will try to draw 400 watts out of the head and blow up

Electronic devices don't "draw anything" they simply "allow" it.

Thus in speaker cabinets, they cannot "allow" anymore than the wattage rating. So as in a 50 watt head on a 400 watt cab, that happens all the time. Or close to it. Old school Marshall electric guitar amps. Say a 50 watt plexi paired with a traditional 1960 (300 watt) cabinet) or how about a full stack (600 watt) power rating.

 

[Derek Balonek]

Where does this idea that the heat changes if the coil isn't moving come from? Most energy fed to a speaker is wasted as heat whether its making sound from that energy or not.

When the voice coil is in the gap it dissipates more heat through the gap into the magnets. If the coil is forced beyond the gap it dissipates that heat into the air which a much worse heatsink than the magnet. The coil cannot dissipate the heat as well and the total heat of the coil increases.

If it's sitting there in the gap it will dissipate more heat than outside, but the motion of the cone itself also dissipates heat. As the cone moves and the air moves with it, air is circulated away from the coil. It gets cooled by convection.

On a (kind of) related note, has anyone ever tried to attach a heatsink to the coil to increase the thermal handling? I realize it that the added weight would affect driver performance as well, but I'm sure it would be more effective than using the magnets for cooling.

 

[walterw]

On a (kind of) related note, has anyone ever tried to attach a heatsink to the coil to increase the thermal handling? I realize it that the added weight would affect driver performance as well, but I'm sure it would be more effective than using the magnets for cooling.

i think that's part of the idea behind the hartke hydrive speakers; the aluminum on the cone around the center acts as a heat sink, cooling the voice coil a little more.

 

[seamonkey]

The force does but the position doesn't. Apply an AC signal of a constant voltage but varying frequency across a loudspeaker, and you'll see that the displacement varies inversely with the square of the frequency, all else being equal.

That's because although the accelerative force on the loudspeaker is the same at all those frequencies, the force vector will of course reverse direction more frequently at higher frequencies.

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

 

[Derek Balonek]

i think that's part of the idea behind the hartke hydrive speakers; the aluminum on the cone around the center acts as a heat sink, cooling the voice coil a little more.

That's a clever setup. It adds less weight than a conventional heatsink (I was thinking of the big vaned aluminum blocks) and still absorbs a good bit of heat from the coil.

 

[David Jayne]

When the voice coil is in the gap it dissipates more heat through the gap into the magnets. If the coil is forced beyond the gap it dissipates that heat into the air which a much worse heatsink than the magnet. The coil cannot dissipate the heat as well and the total heat of the coil increases.

.

I'm not so sure. No matter where the coil is, it has to dump it's heat into air. It's never in direct contact with the magnet structure, so what's the difference? Air is air.

 

[walterw]

I'm not so sure. No matter where the coil is, it has to dump it's heat into air. It's never in direct contact with the magnet structure, so what's the difference? Air is air.

yeah, but the air in question is also right next to the magnet structure, which can suck up its heat.

 

[Calaverasgrande]

Honestly, I was TAUGHT this when I went to college!
The prevailing wisdom was that overdriving an amp creates square waves, which melt the voice coils. I even asked why it doesnt melt the VC when they amplify distorted guitar, or square wave synths?
I got shouted down essentially because I had funny hair at the time.
Year later I read that this IS true of some tweeters, but not of all drivers.

 

[Derek Balonek]

I'm not so sure. No matter where the coil is, it has to dump it's heat into air. It's never in direct contact with the magnet structure, so what's the difference? Air is air.

In the gap the air that is contacting the coil is contacting the magnets, which more readily transfers heat away from the VC. When the coil (really just part of it) is outside the gap, less heat is transferred to the magnet structure. Compare putting your hand directly over a pot of boiling water to putting your hand above and to the side.

 

[Mr. Foxen]

Probably more like having a lid on the pot or not, the coil in free air can be subject to convection, whereas surrounded by the impermeable magnet, the air trapped in the gap acts as an insulator, since it can't move much.

 

[David Jayne]

In the gap the air that is contacting the coil is contacting the magnets, which more readily transfers heat away from the VC. When the coil (really just part of it) is outside the gap, less heat is transferred to the magnet structure. Compare putting your hand directly over a pot of boiling water to putting your hand above and to the side.

I get it, it's an interesting idea, but I don't buy it. The heat still has to transfer to air first. The voice coil doesn't care where it goes from there.

 

[Derek Balonek]

I get it, it's an interesting idea, but I don't buy it. The heat still has to transfer to air first. The voice coil doesn't care where it goes from there.

It does, and air is not too good at taking in heat. The magnet can absorb more heat than the air though. The coil doesn't care where the heat goes, but it cares how much heat goes away. The magnet can remove more heat than open air.

 

[1958Bassman]

It does, and air is not too good at taking in heat. The magnet can absorb more heat than the air though. The coil doesn't care where the heat goes, but it cares how much heat goes away. The magnet can remove more heat than open air.

Heat from the voice coil is lost to the air, assuming the air's temperature is lower. If the coil was in open air, convection would cool it, but not if it was in a narrow gap with a non-absorptive material nearby. Assuming the magnet's temperature is lower than the temperature of the air, heat from the air in the gap will be absorbed by the magnet. Heat from the coil will be lost through all three methods- conduction (direct contact with the air, radiation and convection (air will move in/around the voice coil) and carry the warmer air away.

 

[Korladis]

If you have a 400 cab and a 50 watt head the cab will try to draw 400 watts out of the head and blow up

No. It won't.

I have a 180 watt head and a 400 watt cab. Despite repeated gigging use, they are both fine.

 

[12bass]

No, here's how it works: If you have a 400W cabinet and a 50W amp, the cabinet gets so incredibly hungry from lack of power that it dies of starvation. Feed me, Seymour!

 

[Derek Balonek]

Heat from the voice coil is lost to the air, assuming the air's temperature is lower. If the coil was in open air, convection would cool it, but not if it was in a narrow gap with a non-absorptive material nearby. Assuming the magnet's temperature is lower than the temperature of the air, heat from the air in the gap will be absorbed by the magnet. Heat from the coil will be lost through all three methods- conduction (direct contact with the air, radiation and convection (air will move in/around the voice coil) and carry the warmer air away.

Convection cools it a bit, depending on construction. If there is a vented polepiece, air circulates more easily and cools it better. Without a vented polepiece the air is still circulated from the gap, as small as it is. Air is also circulated around the magnet.structure and the polepieces which aids in cooling. The majority is conduction through the magnet assembly and the polepieces. Steel has better heat transfer than air.

 

[Bob Lee (QSC)]

Some years ago Community--mid 90s, IIRC--showed a PA loudspeaker system with forced air going into the back of the magnet to ventilate the gap.

 

[Baer]

[Little voices inside my head saying “Do it … no, don’t do it … yeah, go ahead and do it …no, I don’t think it’s a good idea … hey, what, you got no balls? … you know what happened last time! … come on, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it, do it …]











I can’t believe this thread went any further than Post # 3, errr, ummm, I mean Post #123.