Help with my first 2-way OB project

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Joined 2011
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I am a new guy in speakers building and maybe my questions will be a little bit stupid, but you have to start somewhere, right... :)

I'd like to start with a high efficiency 2-was speakers in an OB. I found and study this doc: It looks like a nice first project!
However, I'd really like to change a full-range driver in there for something bigger than 4" FE103. I wanted to use AN 8" or 6" or bigger Fostex, but as I understand from the article above, it should be 90db or a bit less sensitive, so Alpha 15 can provide enough bass. Another, from what I see many bigger drivers have problems after 10kHz.
Can you help me to chose a driver? Or maybe I don't need to do it and EF103 is the best choice(I am afraid that I will not get enough "weight" from 4" driver) ?
High eff. is not right, as a woofer in OB 'consumes' most of the power to
overcome the acoustical short circuit.
The diameter of the cone is in direct relationship with the wavelenght of the frequencies to be reproduced, so the littler the better when seeking very good HF reproduction; the fact is that the larger the cone, the more the 'sound image' :eek: is spread by the it mainly depends on the geometry of the cone and its stiffness, and a little cone is more rigid, has less mass so much more ability to move fast .
What pico posted is definitely true - Open baffles are definitely not efficient (at least in the bass region) - but of course that can be fought with cheap power :)

I have seen many designs on here that have used any where from a 12" full range down to a 3" - in the end, I think most people choose drivers that they have experience with, what they have on hand, or based on others' impressions and recommendations.

Have you read through this yet?
Build it as described. It works very well as designed. Do use a chipamp or
solid state amp though. the weight does not come from the 4 incher.
then if you want to install a larger driver (which is probably more efficient),
just go active or line level passive with the bass. Efficiency problem solved.
If you have purchased MJK's worksheets, look at the default design in the OB 3 drivers sheet. He used the Fostex FE167E (nominally 94 dB/1W), and two Alpha 15's per side to match the efficiency. I did the same, but with the FE166E and two 15" Jamo woofers that were PE buyouts (no longer available). Not efficient? No attenuation was needed on the Fostexes, so a watt or two could drive them. If you want to do a passive system with a higher-efficiency driver, you need more woofers!

That said, I have since bought a MiniDSP and bi-amped the system. With two amps a lot of efficiency concerns disappear.

Most 6" or larger full range drivers have whizzer cones for HF.
So for practical reasons, you may first want to decide whizzer or no whizzer.
Start by listening to some whizzered drivers and see if you like them.
If you don't like whiizers you got to go smaller cone or add tweeter.
Some OB gurus do say that 8" is optimum for OB midrange.
Just to play devil's advocate, how much power do you need for 94db @ 40Hz? :D

I myself am curious as to what I need, but I lack an SPL meter.

My system has around 6 dB of boost @ 40 Hz right now. Probably should drop that a bit, really, but I haven't tinkered with it for a few weeks now.

I suspect Alphas would need less boost, and the big Goldwoods even less.

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