simon wrote: ↑Thu Dec 31, 2020 12:07 am
1. I wonder if some of the low end roll off of the 208s in the plots is because of baffle size. Assuming a diameter of 250mm for my ~450mm square baffle gives a corner frequency of around 425Hz, additional effect of the H frame it's standing on notwithstanding. The Quasar baffle is much larger of course and the 208s will go a lot lower in it.
2. Perhaps I should try adding some series R to the 285s to mimic the effect of the Quasar's series crossover, which will raise Qts, and hopefully lower the roll off?
1. A lot of the 208's roll off is just due to them being around Qts = 0.2. A smaller baffle won't help, as you say. So a bigger baffle helps them to reach lower, but they are always going to lack lower-midrange, on their own.
2. You can try by all means, but this is the area I was struggling with, on my Quasars with the Fx120 pair - I was adding a few ohms ( up to 3.3R ) to pad down the 285 to match the lower efficiency of the FX120's. However, in the bass area where you are using the 285's, because they have a high Qms, they have a big sharp resonance peak with a high resistance - so adding series resistance onto that, causes an uneven and peaked amplitude response, with the phase shifts associated with that too.
If you stick with just the 208's on OB, then you need a bass solution that goes up to 400 or 500Hz, and is stereo. If you also want it to go down to 30Hz or so, then it's a tough job to find a driver that is also good enough at the top end of that range. There is a known solution that's been written up, the one Jono built, which is Dick Olsher's ( Black Dahlia ? ) where he used 15" Eminence definimax drivers on big ( 200l ? ) ported boxes. You might want to read up on that. The ported effects will be pretty low down on something that big, so it wouldn't sound like a small ported box system.
"Once you find out ... the Circumstances ; then you can go out"
