• Magnet Structures

JBL uses shorting rings in their prosound woofers, and Eminence does in their Magnum series. I think B&C and Eighteensound use them in some of their top models as well. Nick McKinney did in his Lambda woofers, sadly no longer available. Do you know of any other high-efficiency woofer manufacturers that use shorting rings?

On the subject of power compression, I recall reading a paper written by JBL engineers in which they compared the power compression of several different 15" woofers under very high input power. The JBL design won, which was hardly shocking. What was, at first glance, shocking was how poorly the Alnico magnet woofer did - it compressed by something like 7 dB at the tested power levels, while the JBL woofer's compression was maybe 2 or 3 dB.

But, what they did was overheat the Alnico magnet and force it into partial permanent demagnetization! Basically, they chose an input power level high enough to ruin the Alnico woofer, and then compared it with their woofer. I though that was unethicial, and couldn't help but wonder why they chose to do that. Could it be that at lower (i.e. sane) input power levels their woofer didn't compare as well with the Alnico unit?

Hmmmmm....

I think that most home hifi systems probably don't require as much power, but I'm working on a cooling system to reduce compression and increase power handling for systems that are pushed hard. There are some illustrations at the link below for proposed ways to cool the voice coil and magnet.

Basically, I propose a system that makes the speaker more effective as a pump. Right now, most speaker vents basically just move the same hot air back and forth, hoping for some turbulence and convection to carry heat away. It works a lot better than earlier unvented speakers, but it isn't very efficient. What I'm hoping to do is to make the speaker a better pump, and to use this to carry heat away.

The trick is introducing unidirectional flow without introducing pneumatic asymmetry. I'm woking with an ME, and he has come up with excellent schemes that make he speaker a very good pump. The problem is that the obvious pump configurations basically use single-cycle valves that introduce uneven pressure. That makes 'em great pumps, but increases even harmonics so that's not a good solution. One way to get around that involves reworking of the speaker motor itself to seal the vent and voice coil gap from the rear of the speaker cone.

But I'm interested in making a device that can be press fit into existing vents and ducted to an intercooler. We have a proposed solution, and we're working on the details now. I am confident it will provide improved cooling over a non-ducted vent and that it will not introduce pneumatic asymmetry. I am making this an open project and posting my results so that any interested DIY builder can do the same thing.

Wayne

• Loudspeaker Venting and Cooling Techniques

Thanks for the question.

You are absolutely right to ask about "scaling the effect". So many of these discussions go on and on about thgings that don't amtter at all. Like arguing over the differences in second haramonic for different motor structures - and then finding out that we can't hear second harmonic distortion. In my book I put audio BS into two categories 1) there are just wrong 2) they are absolutely correct, but it is irrelavent. Another example of the second is the skin effect in wire. Sure it occurs but "So what?"

I thknk that a lot of arguments would not get as heated if we always classified the significance of the effect.

Now back to your original question. Yes would lean towards your being correct that pro sound drivers in homes would rarely see enough energy to heat them up. But, in some cases the speakers are on for a very long time so the magnet will heat.

I read on some of the other answers a discussion of power compression. I think it worth noting that the vast majority of power compression comes from the voice coils resistance change with temperature and not the magnets. Further, since the voice coil heats almost immediately - it has a very low thermal time constant - it will heat almost independent of how good the structure is cooled. There are two types of power compression - the long term and the shot term. We usually see data only on the long term one and tend to forget about the short term one.

Incidentally it is an easy matter to elliminate the short term power compression. Simply wind the voice coil with copper wire with about 5% nickel in it (also called Constantin) because it has almsot no change in resitance with temperature. Unfortunately it also has a higher resistivity to begin with. A double edged sword.

Ciao

I think that you're probably right that the voice coil heats pretty quickly after power is applied. But I also think that anything that removes heat will help. I imagine that the whole idea of venting was to carry some of the heat away, and as inefficient as it is, I expect it probably provides some benefit. I also imagine that if the pumping action of the vent were improved, the thermal performance would also be improved, compression reduced and power handling increased.

What say you?

Correct, anything that helps cool the voice coil helps the situation. I was mostly refering to the fact that it is the wire heating that accounts for most of the compression.

Vents help a lot esspecially when high power is an issue. But as Duke points out, heat in a home system (using pro speakers) is not really an issue. Few people realize however, how much difference there is in the effect of heating between a smaller wide band driver (like the typical 6 1/2 inch woofer) and a large band limited pro driver. Under normal conditions heating of the pro driver is not an issue,but it is a big issue in the small driver even at normal listening levels.

There is a very good paper on driver heating in the JAES some months back. If I find it, I'll let you know.