That's all academic from my perspective though, a mental excercise, just for fun. I guess the real test would be to build the driver with a 0.75" vent and measure it to see. I'm confident that Eminence is able to assess the problem and come at it squarely. They've got the means and the research tools at their disposal to verify whether the port size is a problem or not, and if so, to provide a different solution.

Full modulation of the atmosphere (plus and minus 1 atmosphere about a 1 atmosphere mean) is 194 dB. Just as a point of reference 50 pounds of TNT detonated 10 feet away will generate 200 dB (and kill you). Eardrums pop instantly at 190 dB. Your body will be physically damaged beginning at 150 dB.

As I indicated before, I dont think any of this is close to happening in the LAB12 vent.

There's also the issue of vent and/or plenum resonance, causing a sort of ram-charge effect that increases pressure/vacuum cycles at resonance. But putting that aside for a moment, I'm wondering where the onset of non-linear distortion happens from compression/rarefaction asymmetry. Do you happen to know of any references to reliable experimental data on this subject? There must be some, 'cause compression non-linearity is discussed in other areas of engineering and science. I expect someone has done a pretty good study on it. Do you know where?

It would be really good if your hunch that 0.75" was large enough at maximum pressure delta. I'll ask Jerry if Eminence has any means to measure pressure in the vent at various frequencies and excursions. That might provide some important information in this matter.

It also looks like Tom Danley is taking delivery of a flux stabilized LAB12 next week. I think it is very likely that the same woofers we've been discussing here might be of interest to the folks at ServoDrive as well. Makes sense, so we'll see.

• Danley says he will be taking delivery of flux stabilized subwoofer prototypes

• Nonlinear thermal modeling

I think this is why manufactures start adding additional air vents rather than simply increasing the size of a single air vent to reduce power compression. Is this possible to do on the MAG12? You could have additional air vents through the back plate which would be aligned with the voice coil or something, although this may also require a redo of the motor layout, just like the option of mounting the shorting ring's in the plates/outside of the VC (what was the plan before if we stick to the 0.75" vent? The shorting ring wasn't inside of the voice coil, was it? If so, I havn't see this sort of implementation used anywhere before, it's always been on the outside surrounding the VC on woofers that I've seen and in textbook diagrams or online).

One thing for sure though is that power compression is bad, and I wouldn't want to play a juggling game with making an air vent smaller. Black anodized voice coils and extra heatsinks is an option to dissipate heat as well; unless some sort of other heat transfer device is implemented, I wouldn't want to see the air vent on the MAG12 reduced. Tom Danley commented on the live-audio forum his results from an experiment to examine power compression. He noted that typical VC woofers average -3db to -9db compression within the first 15 seconds, which also caused quite major resistance and frequency response changes, and T/S parameter shift. I wouldn't want to do anything which could potentially increase power compression and distortion, and the other non-linear affects that come along with it.

I'm not sure if the original LAB12 woofer had any extra heatsinks in addition to the vented pole piece. My assumption though, is that this $150 woofer would have heat transfer or cooling similar to other woofers in its price range, which happens to be nothing special. What also pointed me toward that conclusion is because it uses a Kapton voice coil instead of a black anodized aluminium one, the latter being more effective in removing heat and has higher temperature characteristics. It just so happens that these woofers would have to have a lot more compression than your typical JBL pro woofer, which may have 4db or so compression at full power. I've seen on other websites such as Beyma and other brands, looking at their woofers which don't have all the high tech heat dissipation options like JBL, which DO actaully post power compression graphs which are some 6db compression or greater at full power on some woofers. Not to mention the increase in distortion caused by this. That's why I assume the LAB12 woofer would be similar - by comparison, higher power compression just like these woofers. If this new MAG12 woofer is going to be as great as it's supposed to be, it must have adequate cooling. If the vent size in the pole piece must be decreased, then additional cooling or heat transfer methods should explored really.

If you're worried about motor noise from chuffing in the motors air vent if its size is reduced, why not flare the air vent like is done in the lambda 001 motor.

It's all just my opinion, but thats what I believe and what I would personally like to see happen. I dont even know how big the vent is in the LAB12 myself, the mere fact of reducing its size though, I would say that one cannot expect performance not to suffer. I guess we'll know just how much it will suffer after Eminence do the tests on it.

Adrian