OB with Alpair 12P and Eminence Beta 15

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Max N
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#1 OB with Alpair 12P and Eminence Beta 15

Post by Max N »

The last thing this place needs is another OB thread, but what the hey...
I'm actually pretty happy with my existing sealed cabinet speakers (after a fair bit of development). But we have re-arranged things in the living room, and the speakers are now on one of the short walls, firing down the length of the room.The room has a very strong 40Hz mode, and moving the speakers to one end means that they excite this mode more strongly.
40Hz has a wavelength of 8.6m, so pulling the speakers forwards so that the woofers are about 2.15m from the back wall should produce a null at 40Hz. Sure enough I have tried this and it works really well. The speakers sound very good. Unfortunately this is not a domestically acceptable solution. Back to the drawing board.

I got to thinking that an OB speaker would not excite the room mode in the same way. And (to my surprise) I have been given the OK for a pair of OBs from an aesthetic POV. Quasar types are a non-starter apparently (even though I think that James D's are gorgeous), but something along the lines of the Pure Audio Project Trio 15s have met with favour.
So that is the design brief:
  • Similar looks to the PAP Trio 15
    Placed near to the short wall of a long room - one of them near to a corner
    Not excite the 40 Hz room mode
The sound I'm looking for is tight, lively, fast good timing. But above all good tone and no distortion.

Drivers (at least for iteration 1) will be Alpair 12P and a pair of Eminence Beta 15 per side. The 12Ps have been sourced (thanks Colin). I need to save up for the Betas. I've started work on some modular baffles for experimentation, using some 19mm mdf I've had knocking around in the garage for years.

I used to use LSP CAD software, I shall have to see if the old PC with that on still works. LSP CAD appears to be still available, but no mention on the website of win 10 support. Maybe I should look for something more modern.
MJK
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#2 Re: OB with Alpair 12P and Eminence Beta 15

Post by MJK »

Try the BASTA program to simulate OB speakers.

http://www.tolvan.com/index.php?page=/basta/basta.php

It is free, easy to use, and will handle baffle size, shape, driver placement, and passive or active crossovers.
Martin
Max N
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#3 Re: OB with Alpair 12P and Eminence Beta 15

Post by Max N »

MJK wrote: Thu May 18, 2017 12:58 am Try the BASTA program to simulate OB speakers.

http://www.tolvan.com/index.php?page=/basta/basta.php

It is free, easy to use, and will handle baffle size, shape, driver placement, and passive or active crossovers.
Thanks Martin, I'll take a look :)
Max N
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#4 Re: OB with Alpair 12P and Eminence Beta 15

Post by Max N »

I've been experimenting with the Basta software, it's impressive, but I have a couple of questions for anyone who is more familiar with it.
As far as I can see, for a 2-way design, you model 2 systems, and then sum the response, which is fair enough. But the software seems to assume that the 2 systems will always be connected in parallel, which means it can't simulate the series connected crossovers which James D, Nick and most people on here have used. Is that correct or is there an option somewhere for series connection of the 2 systems?
Second question - can Basta model the impulse response. Or more generally, can it present results in the time domain rather than the frequency domain?
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#5 Re: OB with Alpair 12P and Eminence Beta 15

Post by MJK »

I do not see options in BASTA to do a passive series crossover or to convert the frequency domain response into the time domain.
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Ray P
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#6 Re: OB with Alpair 12P and Eminence Beta 15

Post by Ray P »

MJK wrote: Sun May 28, 2017 5:34 pm I do not see options in BASTA to do a passive series crossover or to convert the frequency domain response into the time domain.
That largely explains why I've been struggling to understand Basta for my own OB project...

I think I'll just build mine as per Nick's drive unit complement/crossover and take it from there.
Sorry, I couldn't resist!
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#7 Re: OB with Alpair 12P and Eminence Beta 15

Post by MJK »

There is no magic in a series crossover. I have modeled many series and parallel crossovers and the parallel crossover has always given excellent results. Smooth linear SPL response both for magnitude and phase. Measurements have confirmed the modeled results. Once you have the baffles built and the drivers installed try both. Design the parallel crossover with BASTA, use the recommended series crossover, build both, and see which sounds better. To cut cost build one of each, listen, and then build the second with the winner of the testing.
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#8 Re: OB with Alpair 12P and Eminence Beta 15

Post by Ray P »

MJK wrote: Sun May 28, 2017 7:40 pm There is no magic in a series crossover. I have modeled many series and parallel crossovers and the parallel crossover has always given excellent results. Smooth linear SPL response both for magnitude and phase. Measurements have confirmed the modeled results. Once you have the baffles built and the drivers installed try both. Design the parallel crossover with BASTA, use the recommended series crossover, build both, and see which sounds better. To cut cost build one of each, listen, and then build the second with the winner of the testing.
Thanks Martin.

That's more or less what I meant with my comment...
Sorry, I couldn't resist!
Max N
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#9 Re: OB with Alpair 12P and Eminence Beta 15

Post by Max N »

Thanks for the comments, Martin & Ray.
I shall press on with Basta. I'm away from home for a few days, so don't have access to it at the moment, but I already have quite a promising first iteration, which I'll share here when I get home.
Max N
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#10 Re: OB with Alpair 12P and Eminence Beta 15

Post by Max N »

Not much progress, but I have converted a sheet of tatty veneered mdf (moisture-damaged left-overs from a previous speaker project) into a first iteration baffle. Enough to mount the Alpairs and run them in at least.
I have made the holes for the Eminence Betas, but too skint to buy them at the moment. Anyone have any they want to sell?
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#11 Re: OB with Alpair 12P and Eminence Beta 15

Post by IslandPink »

Good effort !
Now, if these are not going to have side-wings, I'm just wondering how you're going the drive the Betas, because they will tend to roll off sooner than on eg. Nick's-style-Quasar-style baffle because the side-wings effectively increase the width of the baffle there. The Alphas... well, I still think they are too high a Qts and loose, but depends really. I wonder what one of each, on each baffle would sound like ? Couldn't be sure, might sound dreadful :D hmmm...
Anyway what I mean is the Betas might need some EQ, but on the other hand the ear may accept the slow roll-off. James did after all suggest the same principle when he promoted the FX120 sealed-box (30L) speakers he built for his son.
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Max N
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#12 Re: OB with Alpair 12P and Eminence Beta 15

Post by Max N »

Actually, based on the bit of simulation I've done, it looks like it's going to work surprisingly well.
I keep meaning to post the the sim results, will do when I can find them again, but I think I'll get away with it because the Betas are higher sensitivity than the Alpairs, so I can passively EQ them to give a good FR - actually rising a bit in the LF. That's without allowing for room gain or anything. Having 2 per side helps. I was actually really surprised how good the sim looked.
Obviously I need to take some measurements and see if the sim stands up....
But first I need some Betas
I want to stick with the Betas because I want tight bass. Any time I listen to a speaker in this room that's relying on a bit of resonance to fill in the lowest frequencies, its just boom time - one note bass.
Oh, and a calibrated measurement mic would be handy. I have a Behringer from back when I was playing with speakers before, but it wasn't calibrated.....
Anyone got any recommendation for a calibrated measurement mic?
Max N
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#13 Re: OB with Alpair 12P and Eminence Beta 15

Post by Max N »

Ah, just realised, you meant one Alpha and one Beta per side?
I thought you meant just one Alpha per side.....
Interesting idea.....
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Ali Tait
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#14 Re: OB with Alpair 12P and Eminence Beta 15

Post by Ali Tait »

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Nick
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#15 Re: OB with Alpair 12P and Eminence Beta 15

Post by Nick »

I don;t know if the umk is absolutely calibrated, or just frequency response. Depends if you want to measure sensitivity of the finished article.

This is another option http://www.cross-spectrum.com/measureme ... ayton.html
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