Originally Posted by dazzdax
Can anyone tell me (maybe Brian or Enrico can give some elucidation) why some competitors (JL Audio, Velodyne) use kilowatt amplifiers instead of a more modest 300 or 600W amplifier?
Both competitors use very heavy drivers with massive magnets. Is that the reason or is it just marketing hype?
Some of that is marketing. The whole "power rating" competition started with Sunfire which came out with their "true" subwoofer models, which I think rated 2,000WRMS for 12" model. Velodyne lost a lot of business and quickly they learnt customers do believe in this wattage number even though this number does not linearly translate into outputs.
Here is the reasoning process why they need such high power (of course JL is not a nonsense company, and some of their stuff is really good). The governing rules of Physics are "Mathematics". You cannot talk Physics without talking math. If we have a driver with say 1" peak to peak excursion. If we want to make it into a 2" peak to peak excursion, most may think we just use the same driver design and give 4x power to it. Now let us keep the power at 4x of and see what we actually get out of it.
The first problem we run into is the voice coil will be rocking if we don't do anything to "stabilize" the voice coil motion. So we need to add a second spider. In sealed subs, the power amplifier spent its power fighting air spring of the sealed enclosure plus the spring on the driver. By using a double spider, we make the spring on the driver even harder to overcome. In addition, the surround was originally a soft flexible material. When the pressure inside the enclosure is high, they can distort and cause a behavior called "inversion" which is literally a standing wave on the surround. So the solution is harden the surround which adds even more stiffness to the spring of driver. The result is the Vas value of the driver become so small with horrible low end bass efficiciency, there is also no reason to use larger enclosures anymore. Second, in order to achieve the 2x excursion, the voice is also increased in length and that also causes a loss of efficiency. The original size magnet is no longer sufficient. In order to get the same efficiency, the size of magnet need to increase 2-3x or the motor suffers a loss of efficiency. Next is weight of the voice coil. Since now the power is 4x of what it was before, the temperature rise of the voice is 4x if we don't do anything. We can increase the voice coil diameter so that there is more air flow for the heat. But it is limited in effectiveness. A more powerful trick is increase the voice coil weight because the temperature rise is inversely proportional to the voice coil weight. But that has a downside impact on mid-bass efficiency. If the voice coil weight increase is so much that the cone assembly is twice as heavy, we wipe out all the gain that we are supposed to get with 4x power. It takes that much of power to move a cone assembly twice as heavy to same 1" excursion.
In short, there is a heavy penalty going to larger excursion on just one driver. The power amplifier number does not really reflect how much more the sub's output will be from one sub to another. The common things you can see from the driver with thousand watts power is 1) very very stiff spring of the driver (hard to press even in free air), 2) large magnet (to make up the loss of efficiency), and 3) heavy voice coil and therefore heavy Mmd. Our approach is pick the sweet spot before all these go into spiral. Our drivers have pretty light spring in order to get the best efficiency (avoid being a burden to the amplifier). Our enclosure size is also larger and yet reasonable in order to improve efficiency. It just works for most customers. I talk to customers. Some of them told me, they just need sufficient bass. Think of it as a distribution curve. We are not in the business to win the top 2% customers. But we are good enough for the remaining 98%.
-Edited by Rythmik - 8/13/13 at 9:48pm