July 30, 2008, Page 1
Initial Test Baffle Construction
I've had a Neo8/Neo3 design in the back of my head for a long time. There are several Neo3PDR designs out there including a couple of mine. I've seen few with the Neo8 other than some line arrays. There's not much out there on how to use the Neo8. A couple of open baffle designs. And the mysterious line in the B&G literature: "It is worthwhile to note that a carefully designed rear enclosure provides additional equalization at the low end of the reproduced frequency spectrum." WTF? I decided to conduct my own investigation.
Although open baffle systems are the current "gotta do" project in the DIY community, I'm not convinced of the practicality for my own home. Arizona is dry & dusty most of the year. With 4 dogs on the property, (two are major shedders), 20 year old carpet, and not much motivation for cleaning, I won't even run without grills -- let alone with open drivers and wires. But I'm open to new ideas so I assumed I might run a Neo8 open. I chose the Neo3PDRW with Back cup for the top end. I wanted everything I could get on the bottom end so I chose the regular (non pdr) Neo8. (The woofers are down to a single RS225-8 or a pair of RS-180-8s. I haven't decided yet.)
I hate to lose the space behind the baffle for woofer volume. So I built a test baffle assuming I would do the RS225. If I used a sealed chamber, I could cut down on the size of the box. Final size is 10.5" Wide by 16" High from 1/4" baltic birch. I cut out a pair of rough, slightly oversize openings in some masonite with a saber saw. Then I attached some scrap pieces the correct size of each driver. (I can't cut fer crap with a saber saw, this way I had 4 straight edges.) I used hot melt to stick the baffle to my template. I then used a trim bit in my router to cut the openings, followed by a 60 degree chamfer bit. I tried a 45 on the verticals of the tweeter that's why it's rough looking.
For the back, I built up 1.25" braces around the Neo 8. I also built a 1.75" deep box to fit over the Neo 8. So I could have a 3" deep chamber, a 1.25" chamber or open baffle. The Neo3 comes with a thick gasket, I stuck it to front of the tweeter, and used #6 X 3/8 screws to mount the drivers. For the Neo8, I used the black PE speaker mounting foam. (260-540) It sealed around the rivets just fine.
Testing Setup and Results
I did a set of measurements with the test baffle clamped to my ancient workmate. This put the baffle about 26 inches off the ground. I set up the mike at about 3 meters, about 5 degrees off axis and the approximate height of the mid between the tweeter and mid. I ran an open baffle measurement. Then I used hot melt & duct tape to add the box for the 3" deep chamber. (I stuffed the chamber with mattress topper.) Next, I inverted the chamber box and taped the lid directly over the Neo8 to make a 1.25" deep chamber. Because these looked a little ragged, I also did a ground plane to confirm findings. RAW MEASUREMENTS ARE HERE
The chart below has the db adjusted for the multiple setups. You can see the comparison. Surprise! open baffle goes the lowest (at least with my tests). The worst was the shallow chamber. I was afraid I would have to crossover to the 8 at 1000 or so. I think I can get it in the 500-700 range without too much strain. It's certainly worth building a complete box to try. At the very bottom of the page, you can see both drivers, from the ground plane measurements. This confirms relative sensitivities of the drivers. At this point, it looks like a doable deal.
