I just finished building a new pair of speakers with AE OB12 drivers, so I thought I’d post a little writeup here. I believe this is the first pair of OB12s made. I feel quite special 🙂
I was kicking around ideas for dipole woofers on another forum, and there weren’t too many choices that could go up to 4-500Hz (my desired crossover to the midrange) and have the Xmax to handle dipole EQ down to 60-80Hz. John suggested that he could do something based off the IB15/12 line. The Lambda dipole drivers would of course be better suited to my project, but they’re out of my budget. As far as I know, John took the IB12, reduced some mass, switched to a softer dustcap, and changed how the cone is doped. He might have done more to it that I don’t know.
Anyway… T/S parameters from John (I wouldn’t trust my setup with anything more than simple frequency response measurements):
Re: 5.5 ohm
John’s measured response:
He can fill in the details, I think the red curve is nearfield, while the yellow curve is from a little bit further out.
My rough design for my speakers looked something like this:
When the drivers arrived, I was really impressed. First off, they look really sexy. And I was very impressed with the packing job. The plastic/foam inserts that held the driver in place were hot-melt-glued to the box. I was a little nervous when one of the boxes had a boot print on top, but the drivers were fine.
My test baffle to measure the OB12:
Note the box of nails propping up the magnet 🙂
Predicted response using MJK’s worksheet:
And my measurements. All measurements are smoothed to 1/16th octave to make the graphs easier to read.
Blue = Nearfield, mic roughly at the plane of the surround
Black = Calculated OB response (invert, delay by 1/2 baffle width, combine with nearfield)
Pink = Ungated ground plane measurement from ~1m
Purple = ~10ms gated ground plane measurement
This convinced me to stick with a flat baffle, and not try U or H frames, because I figured I’d get cavity resonance problems well below 500Hz.
I have an active crossover/EQ between the woofer and midrange, and the woofer EQ’s schematic and transfer function are shown below. It’s basically a shelving low-pass filter to counter the dipole rolloff, and a notch filter to handle the peak.
Responses. Blue is calculated – I took the calculated dipole response from before, and applied the EQ transfer function. Red is an ungated/farfield ground plane measurement of the woofer with the EQ applied. Both charts are smoothed to 1/6 octave, and levels adjusted to line them up.
Added to this is an LR4 crossover at 500Hz, but I don’t have graphs of the final response.
The speakers ended up looking like this:
I built this with a jigsaw, so it didn’t turn out too bad, I think 🙂 To me, they sound really really good. My previous speakers (seen in the background) had an OB midrange and a sealed woofer. Compared to that, the bass from these speakers is much more articulate, and ‘textured’. I can hear more of the notes in every song, and hear more differences in the sound of bass guitars from one recording to another.
The remaining drivers are an Audax PR170M0 midrange, and a BMS 4538 compression tweeter on an 18Sound XT1086 waveguide/horn. I’m still working on the crossovers, but they sound good enough that I moved them from the garage to the listening room, and will continue the final tweaking there.
Any questions/comments are welcome.