Do Ported boxes have a Q factor?

Ron E

2002-06-27 10:41 pm
THere is no Q in vented box design that is analogous to Qtc in closed box design. Qtc exists in closed box design even when losses are ignored and it is the only factor in response shape for a lossless sealed box. THe real analogs to Qtc would be alpha=Vas/Vb and h=Fb/Fs. From there it gets more complicated and it would be best for the original poster to consult a book like the loudspeaker design cookbook, one of the Weems books, or the Ray Alden book from radio shack.

AndrewT said:
vented box design is full of Qs.
Go & read all about it.

Sure, and a sealed box is just a vented box with a Qp of 0.0 - or if you wanted you could call it a passive radiator box with a Vap of 0, or depending on what model you are using, a vented box with a Fb of 0.. Vented/PR boxes in general have three loss paths and sealed boxes have two. The vast majority of DIYers do not account for sealed or vented box losses and of those who do, most are not based on actual measurements, rather on the assumption in a program or a rule of thumb. Bringing them up (apparently just to be contrary) is of no value to the original poster at all.

Box design ignoring losses is perhaps 95% accurate, where design with losses is perhaps 95.5% accurate. Quadrupling complexity for a minimal increase in accuracy may make one feel like they have a handle on the major variables, yet they have ignored the fact that most of the T/S parameters are not constant, their lumped parameter relations are only approximate because they actually vary with frequency and drive level. Yet still somehow boxes get designed and they function....