I just want to gloat. : )

[Art J.]

The Cornwall has a large duct (shelf) vent to get a low tuning at a relatively high efficiency similar to the Jensen Ultraflex, a crude stepped TL in essence. The double bass reflex (DBR) is two separate vented chambers in series, so if your duct vent came back close enough to the rear wall to air mass load this restriction, then it was a DBR.

GM

Hi GM, I was hoping you would chime in. I meant to say DBR not MBR.

I kept my shelf to a minimum trying not to effect cabinet volume but still being
effective at controlling turbulence. Less than half the depth.

The un-even loading was very apparent with the dual woofer design like
the Altec Malibu that we talked about before. The woofer closer to the port
had more excursion than the other. The shelf would help to even the loading.

I smile and always knew this was crude experiments, living on a shoe string in Engineering
school. I had more ambition than resources. It was a great time.

[GM]

Greets!

Right, the closer driver is tuned higher if a large duct vent isn't used and if specs aren't closely matched it can be the difference between just added non-linear distortion and driver damage.

I learned all this the hard way with my dual 515 'subs' when I started tuning them lower once I got a RTR system and a few pre-recorded wide BW tapes. A steam train tape moved one of the bottom ones out of the gap so hard it hung up on the pole piece, tearing free and spitting the hot VC through the DC, yet the top drivers barely moved.

Ultimately, I re-did the vent to be a long shelf vent that goes all the way back along the bottom and up much of the rear wall to create a stepped TL and tuned it to ~16 Hz by mass loading it with a simple baffle thickness reflex vent that's ~critically damped with tightly stretched double knit material, making it basically an IB with gain.

GM

[cradeldorf]

I just noticed something strange, the speakers are not in the center on these? they appear to be off to one side? I wonder why?

[GM]

I meant to ask if this was the case Vs a skewed view from a too close-up shot since this is another design tweak that historically has only been known by pros and advanced DIYers and even then is often ignored to keep the baffle symmetrical for marketing and/or cosmetic reasons.

Anyway, the offset is to average out the eigenmodes (standing waves) that occur between the driver and any hard enough parallel surface beginning at a half WL of the distance. Inside, the insulation will damp its higher modes and if the cab's not too wide, all of them if a dense enough insulation is used, but the front of the cab is a different matter altogether since the only way to damp them would be to mount the damping pads on the sides and extended forward enough to damp all of its eigenmodes BW. This is the reason behind a foam surround on a horn's mouth, felt pads or foam rings around tweeters, etc., but become unacceptably large/ugly for the lower frequencies if not a separate free standing cylindrical or similar absorber placed near it on each side that some folks market.

Due to the way we perceive sound, our brain will sum/average out a multiplicity of peaks, nulls, so offsetting the driver is one of the technically elegant acoustical solutions to an acoustical problem. Another is the shape of the cab with contour shaping of the mouth, i.e. tractrix, Le Cleac'h, Geddes, et al being the horn analogue.

GM

[cradeldorf]

Your pretty smart there GM. ; ) Thanks again. :2thumbsup:
All I know is I can't wait till Tuesday!!! I'm hoping my Marantz will make them more awesome sounding than they already are!!!!

[gortnipper]

GM - How much of an offset is necessary to achieve what you are describing? In say, either a MLTL or 620 cabinet application?

[voice of the theater]

Your pretty smart there GM. ; ) Thanks again. :2thumbsup:
All I know is I can't wait till Tuesday!!! I'm hoping my Marantz will make them more awesome sounding than they already are!!!!

Are you replacing the Optimus? Nice! Which Marantz are you getting on Tuesday?

[cradeldorf]

Are you replacing the Optimus? Nice! Which Marantz are you getting on Tuesday?

Its a perfect condition 2216 but not a 2216B and after finding the loose wire on the crossover inside of the speaker with the 604E in it let me just tell you.....the Marantz is Sawheet!!!!!!!!!
absolutely awesome sounding. I'm very happy. all is well I just need an FM antenna and I'm all set. :2thumbsup: