Since you're a pretty experienced individua in the industryl, I just wanted you to clarify:
500 watt dynamic peaks, in a home environment, listening to dynamic content IE uncompressed music, or did you mean 500 watts in large environments, with continuous program material and even louder peaks?
I completely see where you're coming from, but I just want to know how much of an issue is, in a home environment, where the average power level even at higher SPLs won't be 500w. For example, if our average listening level is around 80db, for these speakers, with some 105db bursts, how much heating can we expect? For reference, the driver we're discussing, is going to be the little 6.5" unit intended as a surround, and is high passed around 50hz such that it'll never exceed its 12.5mm xmax with a 500w input. Le = .84mH
Cheers.
This is a good question that likely would require a chapter in a good text book to answer completely. First of all it is a surround, so it should be about 12db less than your mains, and sensitivity is likely a non issue. Now, in general talking about mains, which is what the picture of the Salks posted surely looks like.
Music definitely has peaks, and averages, but they move about so quickly without a quantifiable definition we are waving at flies with our hands. Let us assume a peak with a duration of 0.1 second. That is a reasonable amount of time to call a peak a peak. We could integrate it longer or less, but I would argue integrating less than 33 milliseconds is likely useless, as that seems to be the approximate integration time of our ear brain mechanism. So, regardless of the duration of the peak, the VC will eventually get hot. What you find in a speaker is two time constants, one for the VC, and another for the motor surrounding it. You could count the box as a third, but this is a tertiary effect. The VC time constant of a small woofer like this is probably on the order of 3 seconds. Now, if you look at music which is typically played or recorded today, you will find the actual dynamic range QUITE limited. This means the difference between peaks and Rms is now very small. That makes a difference. Todays recordings have a dynamic range which is very limited relative to for example, a recording of a symphony orchestra. So, you pick a product which gives you 85 db SPL for 1 watt. Now, how far away from it are you? How loud do you listen? If your average is 75 db, and you have a live room, and sit 2 meters from your speakers, the answer is quite different than if you sit 4 meters away in a room with lots of drapes, a high ceiling, shag carpeting and two big couches. So, if it is loud enough, then there is NO issue. If not, then you have to look at why the designer chose to make his or her product in this fashion. Efficiency is your friend, not always a compromise to play louder because we don't care about accuracy. We certainly have to weigh the moving mass into the consideration of what we want the product to do. But to say that a speaker of a given size needs to go down to 50 or 60 hz, and then you find without sufficient Vd (Volume displacement = Sd (area) times Xmax (excursion)) it means you have decided what you will limit the output abilities of the device.
The real question people want to know is was this a good compromise. I say probably not based on my experience in driver design. If one has access to a sub, put the six in a small box, or if you are crossing the thing over to a mid at several hundred Hz, then you are using the wrong size driver. It is simply too small for the job you gave it.
The problem is that this is not a pure engineering discussion, as Marketing will want to talk about product size, and sex appeal. If you have a three way box, and you STILL need a sub, it begs the question why are you using this small of a driver to begin with? From a purely engineering problem standpoint, it makes little sense to me as a solution. Why not use a 8 or 10 inch driver instead? (You could side mount it if baffle size is an issue).
As for heating and compression, this is not difficult to measure. ANYONE who can come up with $15 and find their way to Radio shack can buy a meter that will measure DCR. Measure the speaker at the terminal with the amp disconnected, and the woofers cold. Play your speakers for an hour, as loud as you want to, then quickly disconnect the amp, and measure the speaker again. As long as there is no loud noise after you turn off the speaker, or as long as you don't have a fan or wind blowing at the cone, you should be able to get a somewhat stable DCR reading. That will tell you just how far off center the heat has pushed the device. I can measure it other ways, but I've thousands of dollars worth of gear to do that. You can calculate the heat in the VC based on the difference between the starting and stopping DCR (assuming any series choke presents a negligible amount of DCR in addition to your VC). All speakers heat up. My point was if I want 105 db EITHER peak or Rms in the room (RMS would be DAMN loud too) then having 8-10 db more sensitivity to begin with is a big advantage. I cannot agree that all light cones sound bad, and all heavy ones sound good. I can confirm what a lot of designers attribute to damping is often more a function of cone geometry than materials used. (Not always, but frequently). I can tell you if you double your moving mass and change nothing else, you have quartered your efficiency. (That is a fact that cannot be in dispute). At some point, making the cone heavier becomes a BAD thing because even IF the power is available, we have to remember VC's work better at a watt than at 500 watts (2/3rd of a horsepower). Dropping too much mass causes problems. Adding too much mass causes other problems. This is not a situation where if some is good more is better. There needs to be a proper balance struck. I think Jim listens at modest SPL's and makes a product he wants to live with. A high efficiency speaker can make low SPLs. A lower efficiency speaker often cannot make the higher ones. Striking a balance is what engineering is about.
To your question. Is it loud enough for you? If yes, then nothing I say matters. If no, then consider a more efficient loudspeaker rather than simply trying to force more current through the VC. Heat is natures way of saying NO MORE CURRENT PLEASE! Typically we can heat up pro Voice coils to over 300 degrees F. At that point we have doubled the DCR, and have lost 3db of available amp power. The bigger the VC, the longer the time lag will be. There is no one right answer to this. It depends on everything!
