Openminded--An Open Back Speaker Project

[Jim Griffin]

During the MWAF speaker contest on Saturday some attendees asked questions about my “Openminded” project. I had high expectations of success with this project based upon Martin J. King’s excellent earlier work on two-way open back designs that use a wide range driver on top of a H-frame mounted woofer.

Concerning MWAF: First, I am humbled by placing first in the open/unlimited class. Second, this project is my first open back design but will not be the last one.

For my project the top panel holds a Mark Audio Alpair 10.2 (second generation) driver which covers the 200 Hz and up portion of the frequency band in my project. The A10.2 is a full range driver that Mark Fenlon evolved from his experience with E. J. Jordan. Currently, my ears tell me that the A10.2 sounds better than the similar sized Jordan JX92S that I have used in previous projects. The bottom H-frame, which covers below 200 Hz, houses a 15” Eminence Kappalite 3015LF Neo woofer.

The active crossover for the A10.2 and the 3015LF is accomplished by the dBx DriveRack PA+ which features a 28 band equalizer function. For this open back system EQ (via either the use of a high Q woofer or gain boosts) adapts the frequency response to achieve acceptable performance.
I’m pleased with the music that these speakers produce. Music is clean and tuneful with a natural air of openness that I wanted to attain. The Alpair 10.2 produces exceptional mids and the highs are spot-on without the need for a separate tweeter for my listening.

Furthermore, I’ve become a devotee of dipolar bass. While dipolar bass may not be what you want for deep home theater experiences but for music it is nirvana. If you normally listen to closed or vented box speakers, you owe yourself a chance to hear open box bass. No muddy bass, no boxy sound, exceptional transient response, good integration with the room, etc. are just some of the benefits.

I have a more detailed write-up on this project on DIYAudio at:


http://www.diyaudio.com/forums/full-...e-project.html

https://picasaweb.google.com/1026835...80330646289442

[Brewski]

Congrats on the win Jim. The open minded's sounded great during the meet and greet and during the sample tracks on Sat.

Take it easy
Jay

[GranteedEV]

The thing that makes dipole bass great is that the horizontal and vertical room modes aren't as excited... I think it is a good idea.

[martin]

Very nice looking speakers. Your description of the dipole bass is right on the money. A couple of questions.

1) It is hard to tell from the pictures, did you offset the Alpair 10 in the OB or is it centered?

2) Any measurements to check the accuracy of the simulation?

3) Have you considered trying a couple of the other Eminence 15" woofers with different Qts values? They should be drop in replacements, Alpha 15A or Beta 15A for example. I have found that swapping drivers in my OB systems is half the fun, with the dBx it is easy to adjust the crossover and boost levels to match just about anything.

Thanks,

[blair]

I'm curious about Martin's question as well regarding measurements to validate the simulations. A few additional questions:

1) how much attenuation was needed on the top end of the Alpair? I'm still looking for "the driver" for my dipoles. Currently eyeballing the PS220-8 crossed to a dipole neo3, but I'm always open to ideas.

2) how do you like that Neo Eminence driver? I was looking at the 10" as a mid. Just wondering how close to the published specs they are.

Thanks Jim!

Blair

[Jim Griffin]

Thanks for the responses guys!

Brewski: I appreciate your comments on the sound in Dayton. The Friday night setup was meant to be a dry run to eliminate issues on Saturday.

Martin: 1. Driver Offset: As I didn't have enough time before I committed to cut wood, an off set study was not completed. I note that Dr. Linkwitz does not offset his open baffle drivers though.
2. Measurements: I did some in-room runs to checkout the total speaker and the responses for each driver as well. Confirmed the crossover point and the roll-off of each driver. Overall performance was within a plus/minus 5 dB range from below 35 Hz to 20kHz. My room has furniture and wood floors which create issues for achieving adequate measurements.
3. Eminence woofer selection: I had my eye on their line given your work with them. My choice was also helped by the $48 3015LF Neo (likely a returned unit) I grabbed at the P-E tent sale last year. I did buy the second unit later at the catalog price of $180. Recent neo material price increases have jacked these woofers up to $325 each at P-E. I brought the crossover at the 2010 tent sale for $145 as well. My lucky day.

Blair: The Alpair 10.2 sounds very good without baffle step correction in my MLTL enclosures so I think that any upper frequency response issue is likely a factor of the EQ applied for the open baffle configuration. I like the 15" driver for an open baffle sub woofer and the T/S specs were close to the Eminence values on my WT2 test.

[Jeff B.]

Thanks for the responses guys!


Martin: 1. Driver Offset: As I didn't have enough time before I committed to cut wood, an off set study was not completed. I note that Dr. Linkwitz does not offset his open baffle drivers though.

I discussed this with SL some time back. He keeps the driver centered (as does John K) due to the symmetrical off-axis behavior. Although off-setting may help diffraction on the main axis, it causes some problems off-axis, especially for an open baffle driver. I kept the PHL mid centered in my Archos design for this reason as well.

[augerpro]

Jim when you measured the in room response, how sloped was the response?

[Jim Griffin]

I need to take a final configuration set of data to really answer your question. The H-baffle takes quite a dipole dip beyond 200 Hz. Below 40 Hz it should have an 18 dB per octave roll-off. The top mounted open baffle panel is going south about the 200 Hz region so it is a great place for a crossover.

[martin]

I discussed this with SL some time back. He keeps the driver centered (as does John K) due to the symmetrical off-axis behavior. Although off-setting may help diffraction on the main axis, it causes some problems off-axis, especially for an open baffle driver. I kept the PHL mid centered in my Archos design for this reason as well.

You trade diffraction against off axis response. For a full range driver and relatively wide baffle style of OB design, not a Linkwitz or John K multi driver narrow OB style of design, I think off-setting the driver has some real advantages.

[Jeff B.]

You trade diffraction against off axis response. For a full range driver and relatively wide baffle style of OB design, not a Linkwitz or John K multi driver narrow OB style of design, I think off-setting the driver has some real advantages.

Yes, it's a trade-off, and you have to see what works best in each application. You may be right, and it may be advantageous in an application like Jim's. However, as the middle judge I had the best seat in the house, and Jim's speaker sounded excellent as it was.

Jeff

[augerpro]

I need to take a final configuration set of data to really answer your question. The H-baffle takes quite a dipole dip beyond 200 Hz. Below 40 Hz it should have an 18 dB per octave roll-off. The top mounted open baffle panel is going south about the 200 Hz region so it is a great place for a crossover.

Actually I was more interested in the overall response, like 200-10000hz. Does it tilt at all (in room, with ungated measurement)?