Vented box alignments

I have understood there are three common alignments when building a vented box; SBB4, SC4, and QB3. I'm wondering if the sound will be degraded in any way if you just mix with the box parameters until you get a nice-looking curve? I have read that the SBB4 alignment has the best transient response, but how do you know if you get a good transient response if you don't use any of the alignments, and how big is the difference?
I am thinking of building a vented box with the S-S 8545 driver. I have seen a lot of different versions of boxes with this driver, has anyone tested different volumes and Fb:s or constructed a good box for it i'd be happy if you let me know!
 
All the alignments are are known and tested formulas for a predicted response. I feel free to mess with them all I want.

Imagine a given box with volume x and tuning y.
Lower y, and your transient response is better and bass is (as I call it) weaker. Raise y and your bass is stronger but is not as good at transient attacks.
Make x bigger, and you'll get more deep bass. Make x smaller, and the bass you have will be more pronounced (hump). Similar to raising and lowering Q of a closed box system.
have you read Vance Dickason's book? If you did I can see why you are confused.
-andy
 

CHRIS8

Disabled Account
2001-12-12 8:47 am
VA, USA
"but how do you know if you get a good transient response if you don't use any of the alignments, and how big is the difference? "

Transient decay response is directly related to frequency response. The smoother, more gradual rolloff will always have better transient 'response' than a steeper rolloff. How much? well, if you want specific softwar simulation to view step response/tone burst simulation....you can use the unibox excel spreadsheet from the frd consortium(IF i remember corectly :). Of course, the sim accuracy is dependant on the accuracy of the t/s paramaters.

-Chris
 
The classic vented box alignments are an artifact of a time when we didn't have personal computer programs -- and in the early days not even any calculators.

A vented alignment is a continuum of possibilities, but back then it was a long tedious process to calculate out even one alignment. So they picked a few spots on the curve and generated tables & graphs so that anyone could easily generate those boxes. Now days with a good computer program (and there are lots) you can explore all the possibilities.

The output of a vent is somewhat time-delayed & a bit out of sync with the front wave. This causes the group delay & the generally inferior transient response of a BR. A sealed box does not have this problem -- it does thou have other issues. Personally i like boxes where i can reduce the pressure in the box and get a tuneable set of compromises between a sealed & BR. TLs, Voigt pipes, aperiodic enclosure. The 1st 2 also have a 1/4 wave resonant mode that can be used to damp the resonant peak of the driver & extend the bass response in a way BR boxes are unable to.

dave
 

CHRIS8

Disabled Account
2001-12-12 8:47 am
VA, USA
"The output of a vent is somewhat time-delayed & a bit out of sync with the front wave. This causes the group delay & the generally inferior transient response of a BR"

the time delay you describe(group dealy), is an effect that builds to a maximum value linearly at tuning in any resonant system. Group delay, though certainly worse than that of a sealed box, is arguably inaudible at the tiny values, proportionate to the wavelengths it occurs. The acoustical output of the port, is in phase with the cone as this is a resonant system, and by fundamental laws the spring system must operate this way. Of course, this also is relevant to PR systems. The transient response, excepting room interferences, a product of the alignment's frequency response. As you said, 'generally' inferiror transient response. A wisely chosen alignment can produce bass as refined as a moderate Qtc sealed alignment. Though, of course a very low Qtc alignment can not be replicated with a ported system. Of course, TLs are vastly superior to either ported or sealed boxes, when efficiency, frequency response and transient response are all factors.

"Personally i like boxes where i can reduce the pressure in the box and get a tuneable set of compromises between a sealed & BR. TLs, Voigt pipes, aperiodic enclosure"

I see your attraction to such alignments, they ARE superior in gerneral. Unfortunately, TLs are 'generally' impractical for ME to build, as well as for many others, when real estate is the deciding factor.

Of course, the final bass quality is dependant on the room also, in non-dipole woofer applications. In some rooms bass quality will always be of low quality, unless heavy treatment is used to reduce standing waves.

-Chris