Speaker crossovers

I want to dip my toe in making a pair of DIY open baffle speakers using one 15 inch coaxial driver per speaker. Can anyone explain (at the 4th grade level  :)  why I need crossovers for coaxial drivers? Seems to me if I need crossovers, I might as well use separate drivers. I ask because I'd like to use a couple of these: 


To complicate things the specs recommend XO-4 crossovers which I've found only to be manufactured in Italy. I thought the whole advantage to using coaxial drivers was so one wouldn't have to bother with crossovers. Guess I'm wrong. Please explain why and how one would hook up the crossovers. Also, what would happen if I didn't use crossovers (decrease the sound quality of the drivers or amp? Destroy or damage one or both)? Are there any high quality 15 inch coaxial drivers with "built-in" crossovers? I need "simple"  :(
Mew, it is not that complicated. The speaker you chose is a speaker mounted within another speaker, two separate speakers mounted coaxially. If you send bass to the high frequency driver you will likely damage it. You can design your own crossover. There are tools on line to do this but I would just get the recommended crossover. 
Check out Parts Express and see what they have for drivers.
the crossover is there for protection to prevent bass going into the tweeter and highs going into the woofer. This is very basic knowledge
When it gets to where guys who know less than kenjit are building them you can be pretty sure the open baffle fad has just about run its course.
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I looked up the specs. That driver is not a good driver to start with in an OB set up. Total Q is way to low (.21) needs to be over .50 or more.
99 and 105 on the sensitivity, there is a footnote on that measurement too, a "2" and a "5" for whatever reason. Probably different distance.
Anything to confuse you...and get a better measurement... Tannoy uses drivers that are coaxial. Cures a lot of problems in some cases...

It says in the information you need the aforementioned XO..
Gives every spec you need.
TO BUILD A BOX.. sorry had to say that... :-) No mention of OB unless I missed something.

millercarbon, how (besides reading) do we learn if we don't ask? Even students have teachers. Nobody starts out an expert  :)  oldhvymec, sounds like you might've built speakers. What kind? Never knew there were specific drivers for open baffle. Once I get a grasp on crossovers I'll get to work. One things for sure, coaxials take up less space...

Thanks to you all pointing me in the right direction, I've spent some time doing due diligence. I now have Lii Crystal 10 full range drivers burning in (almost 200 hours) on open baffles and am expecting a pair of Acoustic Elegance dipole 15 woofers within the next week or so. These speakers are recommended specifically for OB. Slowly but surely learning about and how to build crossovers specific to my speakers. Hopefully, @millercarbon, when this old gal (who knows less than kenjit) is done, I'll let you know how they sound and post a picture or 2  :)
Mewsickbuff, the open baffle speaker I first encountered as a teen were the ones in the musical instrument amplifiers. Many lower priced ones are open back. Some of the more pricey ones have sealed boxes, or ported cabinets (bass-reflex).

In theory, the infinite baffle would be ideal - the back wave never meets the front wave of the driver. But typical open-back cabinets are finite in size, and the back wave comes around and cancels the front wave, cutting off the low bass at a frequency determined by how large the baffle is. 

I built an open baffle cabinet with two 12" guitar speakers (drivers). Each driver had a low frequency resonance of around 80 Hz. The driver surround was stiff enough not to respond to very low bass, so "safe" for open baffle. For guitar, 80 Hz was O.K. For wide-band music reproduction, not so.

Then I tried to make an music speaker using guitar drivers. That's when I really had to read about the interplay between tradeoffs of extended bass vs. efficiency, vs. cabinet size.

Eventually, I switched to a better quality 12" driver for the bass, and split the outputs to mid-range and tweeter through a crossover. Then the speaker went through upgrades and improvements as I learned more. Currently the sealed, bass cabinet is about 1.5 cubic feet internal volume, has a Q of about 0.8, and low frequency cutoff around 41 Hz. The tweeter and mid-range drivers are in a separate enclosure, sitting on top of the bass enclosure with vibration isolation between the two. Learned even more since then. 

Let us know how it turns out for you.


Here's my system with DIY open baffles (Yes, I made them). Not saying they're the best, but they're the most open, detailed speakers I've ever owned.