Ziptree said:
Well I figure I would go with Morel MDM-55 midrange and Dayton ST385-8 15" or ST305-8 12". The mid goes from 500 to 6500 Hz. The 15" woofer goes up to 1000 and the 12" goes to 2000Hz. I'm thinking I'd be better off with the 12" and crossover around 1200. I won't need a handtruck to move it either.
Tweeter selection may be the least of your problems. Before you proceed, forget the claim of a frequency response of 27-2000 Hz for that Dayton 12" woofer, and have a look at the frequency response curve. Pay close attention to the region between 700 and 3000 Hz where the worst of the cone breakup noise is. If you center your crossover at 1200 Hz, you’ll be right in the middle of that. Your woofer-mid crossover must be able to filter that out. There are several ways to do that:
1) Choose a lower crossover point, about 300-400 Hz (no higher than 500 Hz) that puts your crossover at least one octave below the beginning of the breakup noise.
2) Use crossover with a steep roll off slope such as a 4th order crossover that rolls off 24 dB/octave.
3) Use a different woofer that allows a higher crossover point. A 10" or 8" woofer might be able give you that and still produce plenty of bass.
Do the same for the Morel midrange, forget the claimed frequency response of 500-6500 Hz, and look at the frequency response curve. While there is no breakup noise on the high end (that's good), the response at 500 Hz it is about 5 dB lower than across the plateau beginning at 1500 and ending about 6000 Hz. Above 6000 Hz the response falls rapidly at about 25 dB/octave. To get the best response from the mid, I'd guess that woofer-mid should be crossed at about 1500 Hz. Now you see the problem. While that Dayton 12" is a decent woofer, there is no way it can be combined with that Morel mid without having a lot of breakup noise from the woofer or a large dip in response because the mid can't go low enough.
Ziptree, is this is your first DIY speaker? I really don't want to pour cold water on your enthusiasm, but I strongly recommend that you find some tried and tested plans for a DIY 3-way. There are quite a few available on the internet that have been designed by experienced DIY designer who have tested their designs with computer design software and with acoustic measurements of the speaker. Look at it this way, if you were starting out as a cook, would you invent a new recipe on your first effort or would you follow a tried and tested recipe from a cookbook? Just because you can choose among all the numerous speakers available through Madisound, Parts Express, or some other vendor doesn't mean you know how to design a good crossover. Trust me, I’ve been there. A good crossover can make even cheap drivers sound good, and a poor crossover can make expensive drivers a disappointing waste of money.
If you are interested in building 3-way speakers, have a look at these two designs:
http://www.*****************/projectshowcase/jubilee/index.htm
http://murphyblaster.com/content.php?f=MBOW1_3-WAY.html
I haven't heard the 1st one (the Jubilee), but the designer is widely respected. For some reason that link doesn't work. The Jubilee can be found at the Parts Express project showcase website (or substitute "parts-express.com" for all the stars and leave out the hyphen after parts). Better yet, Google on Jubilees and Parts Express. I've heard the MBOW1 3-way and it is excellent. While you are at the Murphyblaster web site you should read about the MBOW1 2-way and its less expensive cousins the MB20 and MB27. They all can easily be converted into the 3-way. I've built the MB20 and love it. There are plenty more designs out there, maybe someone else can suggest some.
