This became a destructive test for the woofer without a cooling system.

The woofer with the heat exchanger worked flawlessly and stayed cool. I ran it for 2 hours and measured the outside of the magnet at 114º Fahrenheit. The inside pole piece was 138º at the front, its hottest point. The woofer was working well, without strain and sounded good.

I noticed the woofer without a heat exchanger began to make buzzing sounds at an hour and a half. I was pretty certain it had probably been damaged, so I reduced power and inserted the heat exchanger, hoping for the best. But it was already gone, and I terminated the test prematurely, at 1.5 hours. The magnet had reached an external temperature of 131º Fahrenheit and the inside pole piece was a scorching 195º. Looking into the cooling vent, I could see part of the voice coil wire, unwound and hanging behind the cone. The motor still functioned, so the voice coil was not open. It simply had started to come unglued at the edge and had separated from the former. The buzzing sound was made when it vibrated against the pole piece.

I conclude from this that the heat exchanger has proven to be successful at reducing heat when the speaker is used for extended periods of time, and also successful at preventing thermal failures. To tell the truth, I was somewhat surprised at just how effective it is. I knew there would be a difference when using the heat exchanger, and the temperature measurements showed it too. But more than that, from a user's perspective, this is really significant. If you're running these kinds of speakers wide open for a few hours at a time, the addition of a heat exchanger makes a huge difference.

Certainly, the air in very small cabinets will heat more than larger ones. But even a free flowing vent (or preferably set of cooling vents on both sides of the gap) exposed to a free open area of cool air is only able to do so much. A lot of heat still saturates the magnet and metal surrounding the voice coil. As the power levels are increased, the amount of heat radiated is substantial. This makes a hellish environment for the voice coil, and the glue that binds the voice coil to the former can't take it.

Think about how hot a 25 or 35 watt soldering iron gets. Or think about a light bulb. After you turn it on, the glass quickly becomes too hot to touch. 40 watt, 60 watt, 100 watt, they all get hot. Now think about surrounding the light bulb with 1/4" steel, and then surround that with a couple inches of ceramic. This really holds the heat inside. Sure, venting it and blowing some air back and forth helps, and it helps quite a bit. But the little box of steel and ceramic still holds a lot of heat, even when vented. That's pretty close to what a loudspeaker motor has to deal with, except that the wire that generates the heat is glued onto a coil former. It doesn't glow white hot, but it does get hot enough to radiate. And the power levels are often ten times that of a light bulb or a soldering iron.

Think about how many speakers you've heard that were blown. Most buzz. When a speaker buzzes after high power use, it is usually because the voice coil has come unglued and part of the winding is whipping against the pole piece as it moves, or is dragging inside the gap. Those are very common symptoms of a blown speaker. After a while, the mechanical stresses on the wire bending and scraping will eventually cause it to open. But it may work for quite a while, buzzing, before the wire breaks. The cause is a failure of the glue that holds the voice coil on the former, due to excessive heat.

If the rush of current were extremely high, it would fuse the wire and open immediately. It would cause an open circuit condition that happens very rapidly, almost instantly. Those kinds of failures do happen, but it has been my experience that they are rare. They are usually the result of a defective amp sending high voltage out on the speaker line, or some kind of accident that causes AC power or some other extreme over-voltage condition.

Thermal failure creeps up on you. The speaker in danger of thermal failure is being pushed pretty hard, so it is defintely taxed. Its distortion and compression levels are obviously higher than if it were being used at lower levels. But it doesn't usually give any obvious indication that the voice coil is coming undone, it sounds reasonably normal. In prosound environments, the speaker may see most of its use at these levels, or just under. But the heat generated under these conditions is incredible, and it's always working on that voice coil glue. One day, push it a little too hard and the voice coil rubs.

Of all the blown speakers I've run across, the vast majority have fit this description. They've had rubbing voice coils. So I think it is probably pretty important to do whatever is possible to get the heat out, especially on speakers used at levels more than 100 watts. I don't care if it's a horn, a box or an open baffle, it can't hurt to remove heat from the motor.

So it occurs to me as I consider today's test that a heat exchanger would probably be good on just about everything. Basshorns and small sealed cabinets aren't the only ones that will benefit. They're just the obvious canidates. But even bass-reflex and open baffled speakers would benefit, if power levels are moderate to high.

Even just a 8" to 10" disk fastened to the heat tube will provide effective cooling. It doesn't have to be placed outside the cabinet either, although that will help. One could place a heat exchanger inside a cabinet, if it couldn't be easily installed to radiate outside. That won't get the heat out of the box, but it will get it out of the motor, where it's doing the most harm. Anything to get those temperatures down, and prevent voice coil glue failure.

When the temperature inside the speaker magnet is cooking at 150º to 200º, the voice coil is even hotter. Temperature changes cause the voice coil to expand and contract, and when it's hottest, it expands the most, like a coil spring. That's also when the glue is the weakest, because it is being overheated. So it is really important to get the heat out, and I think it would be beneficial to use a heat exchanger like this in all high-power loudspeaker cabinets, not just basshorns with small sealed chambers.

My first thought while reading this was if you just ran a speaker for a short period of time like 15 minutes what temperature does the pole piece go to?

That is important because if the pole piece would normally go to 138 in a short time this would mean there had been no build up of heat after an hour and a half. That would suggest you could just keep pumping at that level with no additional heat buildup.

The addition of the heat exchangers, crossovers with low end cutoffs and overload protection would make for a long lived speaker that would survive during concerts and from what I've read many don't make it now. Making "kits" to retrofit the most commonly used commercial speakers could be a new sideline in your spare time.

Grant.

Great work!

This heat spreader is the real deal.

Thomas

• Initial test results

What I realized yesterday, is that temperature wasn't done rising after 15 minutes, it just slowed down. When the test was run for a longer period, the temperature continued to rise until the voice coil separated from the former and it began to buzz.

I didn't expect this to happen. I didn't think the woofer would fail at this signal level. What I expected was that temperature would hold constant. I was planning to confirm this with temperature measurements taken at two hours for both the stock woofer and the one with the heat exchanger installed. But the stock woofer didn't make it that long.

I think there is a power level below which the woofer can be run indefinitely without a thermal failure. I expect there is a point which you run into a power verses time curve, and over that power level, the more power you apply, the less time it takes to reach a point of thermal failure. In other words, maybe the woofer will take 35 volts forever without a thermal failure, but by 38 volts, maybe it will fail after 10 hours of continuous use. Then maybe 39 volts damages it in 5 hours and 40 volts kills it in two. That's the kind of power rating curve I think you can expect.

The thing is, I expected that the 40 volt level was safe to run indefinitely. I didn't think this woofer would fail until it reached 42 or 45 volts. I thought maybe if I sent a continuous signal, it might be too much, and the voice coil might eventually fail. Then again, when the signal is removed, pumping action stops and there is no cooling airflow, so the temperature surges momentarily before it starts to drop. So perhaps a continuous signal would actually cause less thermal stress than one cycled 15 seconds on and 15 seconds off.

It might be helpful to make more measurements, and I plan to do so as time permits. It would probably be good to plot temperatures by the minute, maybe reducing measurement intervals to every five minutes after a while. This could be done in ambient cool air, and perhaps in a small sealed box for comparison. Each of these will be done with and without the heat exchanger, in order to study the limits more closely.

One thing is certain. The speaker with the heat exchanger stayed considerably cooler than the one without. There was a 57º difference at the pole piece, and this probably wasn't the most severe test that could have been performed. Putting the speaker inside a small box instead of a nice cool 72º room would probably have made the difference even greater.

Then again, since the speaker failed, I'd say 190º or 195º pole piece temperature probably marks its red line. Without the heat exchanger, we crossed this line in less than an hour and a half. With the heat exchanger installed, it ran two hours at a relatively cool 138º inside at its hottest point. The back of the magnet was barely warm, at under 115º. So the heat exchanger did a great job of holding motor temperatures down and preventing voice coil failure.

I'm thinking there should be a LARGE market for this one that doesn't all belong to Pi speakers. Your comments about all speakers, not just subs benefiting from this suggests you're thinking the same way.

It will be interesting to watch where this one goes.

Grant.

It also fits other horns, and even other cabinet types. At most, all that is required to use it in loudspeaker is a few extra support braces. If the box is small, then the radiating panel of the heat exchanger can be mounted to the rear of the box. But if the cabinet is large, then braces can be added to the interior of the cabinet, which can then be used to mount the heat exchanger and hold it in place.

Internally mounted heat exchanger

I'll be working on heat tubes to fit several vent sizes so I'll have heat exchangers available for many speaker motors. It really works well, so I want to make them available for as many high power speakers as possible.

1) the exchanger was messing with rear sound waves.
2) it wouldn't be as effective as external.

Grant.