4th order bandpass - 2 questions, and opinions sought

[DeonC]

Hi guys

I have been looking at a 4th order BP design, and it seems to give me the specs I want (ITO freq. response and depth), but now I have a few Qs about this design.

The first question is this- my logic seems to dictate that a box like the 4th order BP does not suffer from baffle-step. Is this correct?

Secondly- can I use aperiodic ports on the sealed volume to reduce the size. There was a question like this previously, but that person wanted to extend the frequency range that the sub covers. I do not. All I want to do is to be able to reduce the total size of the box.

Lastly I would like to hear from people who have built and/or heard 4th order BP designs- how good are they? I will be using this one with a pair of Tannoy DU386 15" dual-concentrics loaded into a sealed (possibly aperiodic) box for the upper bass and upwards. All opinions would be welcome.

Thanks,
Deon

[freddi]

hi Deon - been more than a decade since I tried BP4 with 12" acoustically mated to AR M1

although BP are not in favor (more like hated-lol), had general impression BP4 can be OK if summation, etc are right with mains. (u might look at Don's TH try) Earl Geddes uses BP and think goes for higher BL, lower qts than normal, "pan-pipe" front chamber vent (plus open-cell foam) and LC assistance which can "boost" assuming low source Z.

there should be some careful BP builders including car-sound addicts to give answers to your vent question. normally there be some fill in rear chamber

couple quick sims Lab12 w/o fill (don't have proper Z data at 1K, 10K) - maybe the late & great Bob Bullock's BPBOXMODEL will do LC accurately - I can't get screen shots in XP of old DOS-based programs - thanks microsoff for no backwards compatability (?)

lab12 60l total BP4 vs 40L sealed box
http://img255.imageshack.us/img255/1613/lab1260lsw4.jpg

lab12 53l total BP with LC (100uF/6mH) vs 60L BP4
http://img255.imageshack.us/img255/6...h1000uflv1.jpg

btw - retuned k15 was exemplars sub

[Ron E]

Originally posted by DeonC
The first question is this- my logic seems to dictate that a box like the 4th order BP does not suffer from baffle-step. Is this correct?

No. The only way to avoid baffle effects is with an infinite (literally) baffle. In the case of the bandpass box, basically you are below the baffle "step" so sensitivity is just 3-6dB less than on an infinite baffle

Originally posted by DeonC
Secondly- can I use aperiodic ports on the sealed volume to reduce the size.

Model this as a 6th order box with a damped port - Qp = 2 or so. I haven't tried this so I am not sure what your results would be - but you don't get something for nothing - you will most likely lose LF extension.

Originally posted by DeonC
Lastly I would like to hear from people who have built and/or heard 4th order BP designs- how good are they?

Most people who have heard bandpass have heard the one-size-fits-all boxes and have come to the conclusion they are junk. If designed properly, they are easily as good as sealed or vented boxes. The KEF 107 used a bandpass configuration and was praised for its bass.