Not being a car audio guy myself, I haven't a clue what "free air" means. However, literally, it means the lowest resonance the speaker produces while literally hanging by a "string" with no baffle board.
A baffle is needed to separate the front of the speaker cone from the rear. A speaker is a piston that moves air. However, without separating front from rear, frequencies from both combine, canceling and comb filtering - in other words, sound like crap. The baffle eliminates most of that problem.
If you can picture positive pressure from the front of the speaker, and negative pressure from the back, that is what the baffle separates. Perfect airtight seals aren't required for good bass response however. The worst problem in a car is rattling and/or whistling...
Then there is the matter of infinite baffle (sealed, airtight cabinet), which rolls off the lows very smoothly (depending on the size of the box), and ported (folded horn) cabinet design, which is (and should be) tuned to the speaker.
As a bassist, I prefer sealed. But you get better lows (or perceived lows anyway) from the ported design. I design and build my own bass cabinets using various CAD programs. The principles are the same. Ported can be tuned for the best kick, while sealed (or nearly sealed) will provide smoother lows, and stronger mids, and is easier, lighter weight and cheaper to build. I use tuned port ed cabinets on stage...
True "free-air" speakers would be very small and tin-y sounding. So, what does the slang "free-air" really mean?
[ 02-08-2007, 06:08 AM: Message edited by: Great SScott ]