In the car audio world, it's pretty simple. For the most part you have 3.5", 4", 5.25", 6.5", 8", 10", 12", 15", and 18" as the standards. These are based roughly upon frame size, not radiating surface. For the most part, it seems to me that the manufacturers of DIY drivers as well have done pretty close to the same in the past.

I for one, feel frame measurement should be the standard. I say this based upon a couple of things. One, I don't feel a driver should be called a 5" driver when I need 6-1/8" of baffle to mount the thing properly. Of course this is on the extreme of things but even calling a particular driver a 4" driver when I need 5" bugs me at times. The second reason is driver spacing, which can be a pretty important thing to consider when desiging a mulitiple driver speaker.

For instance, back in the day, Advent made a venture into the car audio circles. They had what they called a 5-1/4" driver. It was called this because that was it's approximate frame size. But when all the rags did their reviews they would point out that since the radiating surface is so small, they should be compared with 4" drivers. Well I said, and still do say...poppycock my man, pure poppycock. When it came down to people buying the drivers, they would base much of their buying comaprison on the competition's 4" stuff. Just not fair to the competition, or the people buying the product in the end. Fact was, to mount the drivers properly as "drop-in" speakers as they were being marketed as, they needed that 5-1/4" of space. So, in all fairness to the comp and the buyer, they should have been compared to drivers in their frame-size category, in which they usually failed to "reach the bar" so to speak.

So, in closing here, it is my opinion that a particular speaker driver's size rating should be based upon it's frame size. Now I know there are MANY different frame sizes out there, but I think there should be a bit more stringent standard than is in place today...whaddayathunk folks?

Noidster