Thermal Management of Subwoofer Drivers

[Oborous]

Hi All,

Again, I'm looking at things that distort the sound of a subwoofer from it's 'calculated' values.

One of the things I figure will be the effect of heat on the voice coil, magnet structure and the rest of the driver as a sub is driven moderately hard for a long period of time.

I saw a thread a while back, sorry no link, can't seem to find it, talking about a high performance subwoofer and someone commented that in a small sealed enclosure you should almost build the cabinet out of heatsink material.


I'm wondering does 'heatsinking' the subwoofer driver improve long term sound??

Then I was looking at S.L. Linkwitz's review of mounting a driver,
Linkwitz review of mountnig scheme, and other decoupling techniques.

Would there be benefit to having a metal mounting bracket to decouple a subwoofer, as well as providing a path to transfer heat? I was thinking a large, heavier plate of aluminum with a tight tolerance hole drilled to fit the subwoofer magnet assembly. Maybe also have an aluminum plate for the top face of the sub cabinet, and weld the mounting bracket to that plate, large external surface area.

Thx

[noodle_snacks]

take my opinion with a grain of salt on this matter, but personally i don't think there would be much point, by the time heat sinking hte magnet has had any effect, the voice coil would probably be damaged due to over-heating.

[moray james]

Wayne over at the Audio Round Table did a study o this and aslo presents some practicle ways to deal with thermal overload on drivers. In most situations at home a good driver ought to get the job done without much concern under normal hoe listening. Tiny bass boxes that are sealed might get kind of hot if you pound them but that should only really eat up some of your amps output. However for those that want it loud for a long time might be a good idea to try some heat sinking. Regards.

http://audioroundtable.com/ProSpeakers/

[Oborous]

Thanks Moray,

Interesting reading. and different implementation than what I was thinking.

The only thing is, everyone seemed to agree that thermal management would be a good idea, but not why it would be a good idea. Longevity of driver was commonly mentioned.

I'm wondering if a 'hot' subwoofer is less efficient and has more distortion than a 'cold' subwoofer. Dealing with the lengths/gauge of wire that a voice coil is, it seems like the effect of temperature can cause a minimal change if you go from 25°C to 100°C; but it's still there.

What I really should do is take a subwoofer and experiment with it, determine its t/s specs at 25°C and at 100°C.

Was looking at
Vas is one of the trickiest parameters to measure because air pressure changes relative to humidity and temperature — a precisely controlled lab environment is essential.

If one were to change Vas vs. Vb, then you'd be changing Q, which would change the sound...

So, wouldn't that affect things??

[moray james]

it's my understanding that a driver will start to experience thermal compression when it is run beyond about 1/3 of its rated power. This means you are increasing the DCR of the vioce coil as a result of built up heat in the coil and will then need more power to get more output and the problem has begun where you run out of power or usually thermal capacity and the voice coil fries. You can probably find much information on this topic at any of the pro driver home sites.
Transfer of heat is probably the biggest issue in pro drivers. There are a number of makers that have voice coils wound both on the inside and outside of the coil former. This means that your coil can effectively sink heat to the pole piece which is your biggest chunk of metal in a speaker as well as to the top plate. 18 Sound and Beyma come to mind but there are others. Thermally conductive non electrically conductive voice coil formers would be a nice trick. Aluminum has been used for this purpose as have other metals including metal cones to sink heat into. Large high efficiency drivers will go the longest way to avoiding such problems. Regards Moray James.