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Old 12th December 2012, 12:03 AM   #1
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Default Computer doesn't like MathCad, help with MJK worksheets

My computer doesn't like MathCad, it's a lost cause. Would someone be kind enough to help model the betsy full range driver on an open baffle with an eminence Alpha 15 helper woofer? I'm looking to get away from the larger folded baffle into something with a smaller footprint. I was also wondering if I could use the same lowpass passive crossover MJK uses for the Alpha and what the highpass would look like for the betsy. Been playing with Edge and it shows the betsy with a big spike around 600 Hz on the same baffle size as the one used by MJK. Edge shows a compensation circut, but I'm not quite grasping on how it's wired. I still have more reading to do on circut diagrams. Thanks for looking.
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Old 12th December 2012, 02:23 AM   #2
holdent is offline holdent  Canada
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MJK used different baffle sizes depending on the project. I think you may be referring to the study that he did originally using a Fostex FE103E and the Alpha 15A (http://www.quarter-wave.com/OBs/OB_Design.pdf). That baffle was 20" x 38" (WxH) with the FE103E mounted 8" in and 32" up, and Alpha 15A at 10" in (centred vertically) and 10" up.

The Betsy is more sensitive then the Alpha by about 5 dB in this baffle making use of a passive xover problematic. You could a) use an active filter, or b) use a plate amp for the low pass and passive filter for the high pass, or c) use a passive low and high pass filters and attenuate the Betsy. I modelled it both using both active and passive filters.

First I tried an active crossover with a 2nd order Linkwitz-Riley low pass filter at 100 Hz and a 2nd order Bessel filter at 500 Hz. This produced an excellent modelled response.

Next I tried the original passive xover that was optimized for the FE103E - this produced a terrible modelled response with a peak around 1000 Hz. I spent 20 min. or so playing with the high pass filter w/o any better results. Note that I have no real knowledge of passive filters (and that's why I prefer using an active MiniDSP!) and someone else may be able to pitch in here to help.

I can't post the SPL curves right now but would be happy to send the worksheets as .rtf files if you're interested - PM me.

Last edited by holdent; 12th December 2012 at 02:53 AM.
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Old 12th December 2012, 12:43 PM   #3
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Thanks holdent, you have a pm.

I am currently away from home for the next 9 weeks and wanted to get a project together for when I return. After a home theater upgrade a month ago, I decided to keep the old srx608 and use it for my stereo setup. I was planning on bi-amping the alphas and betsys (which is an option on the reciever) and use audyssey for room correction. I have considered minidsp in the past, but I realy don't need more gear at the moment. By using audyssey room correction, would I even need a passive crossover on both drivers?
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Old 12th December 2012, 07:15 PM   #4
GM is offline GM  United States
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Hmm, the Betsy ideally needs a wider baffle [~48-49"] than the Alpha 15, so how much bass response are you willing to give up to get a baffle narrower than MJK's?

GM
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Old 12th December 2012, 09:38 PM   #5
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I do have an NHT W1 with the X1 crossover and A1 amp to run in the system. I'm looking at the alphas to fill in for the betsy due to a narrow baffle, if that makes sense. Seeing this Open Baffles for under $300 got me on the path of finding an optimized baffle for the betsy based on MJK's writeup on his open baffle with the alpha helper woofer. I was going to use the worksheets to drum up the high and lowpass networks then post them here to see if I was on the right track.
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Old 13th December 2012, 01:58 AM   #6
GM is offline GM  United States
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OK, then for example, using its 140 Hz/2nd order low pass = F6 and since a dipole rolls off 1st order @ 6 dB/octave, one octave away from 140 Hz = 280 Hz, which equates to a ~4316.3"/280 Hz = ~15.41" baffle width, barely bigger than the woofer, though ideally it will need to support a 140 Hz WL, so assuming the baffle will be setting on the floor and perpendicular to it, the baffle only needs to be half its area, ergo ~ [13560”/140 Hz = 96.86”]^2/4pi = ~746.54”^2/2 = ~373.27”^2/15.41” = ~24.22” high with the driver down ~40%.

This will allow the Betsy baffle to be ‘gapped’ to the woofer’s or just suspended above it, otherwise make it the full ~48.45" high with the woofer down 40%.

That's as far as my pre-computer rule-of-thumb way to do dipoles takes me and the few I built performed well enough once properly positioned in room, though no doubt could be improved upon with today’s powerful simming programs, especially when near a wall or corner, but should be good enough to get you started.

GM
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Old 14th December 2012, 03:26 AM   #7
holdent is offline holdent  Canada
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GM - is the first baffle that you are proposing 24.22" x 15.41" (HxW)? I think the drivers will just barely fit but their sizes will not allow the placement of the Betsy down 40% (or 9.69") from the top. While I couldn't find the outside diameter of the Betsy, the cutout is 7 3/8" so I'll assume 8". The outside diameter of the Alpha 15A is 15.15". So the two drivers together account for about 23" (15.15 + approx 8"). If the drivers are mounted vertically, they'll barely fit on the baffle with the Betsy mounted 4" down. Yes?
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Old 14th December 2012, 04:14 AM   #8
GM is offline GM  United States
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Again, the baffle size is based on using the floor to effectively double the baffle area for just the Alpha 15 XO’d at 140 Hz/2nd order. Any mids/HF would be on a separate 24.22” x 15.41” baffle suspended above it with some empirically arrived at gap between the two and the driver down 40% down on it. This will also allow you to physically time align them.

If only one baffle is used, then make it ~24.22” x 2 = ~48.45” high with the Alpha 15” down 40% and locate the mid/HF up around ear height.

All dims can be rounded off somewhat, so no need to hold to odd fractions if they waste wood, though again, with the advent of MJK’s and others software, best to just use these dims only as a starting point to fine tune somewhat before ‘burning’ wood.

For instance, vertically tapering the baffles to average out their eigenmodes is a worthwhile tweak, though ideally you want to maintain the same baffle area, so this will make the baffle wider at its base.

GM
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Old 16th December 2012, 01:22 AM   #9
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I'd like to keep both drivers on the same baffle. So a 48.45"H x 15.41"W baffle with the betsy at ear level and the alpha 40% down from the top edge? And a 140Hz/2nd order crossover on the alpha? Do I keep the betsy centered on the baffle or offset by 2"? GM, what do you mean by dims? I would like to be able to run MathCad myself to try to figure out what you are saying.
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Old 16th December 2012, 07:12 AM   #10
GM is offline GM  United States
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Correct.

Since that's what I based the example on, it's the starting point.

Conventional wisdom dictates using a golden or acoustic ratio and a 2" offset doesn't work out to any I know of. Offsetting both drivers ~2.57” and assembling them in mirror images is where I would start in a sim.

'Dims' = dimensions.

I don’t have MJK’s later shareware software or learned AkAbak yet, so looking forward to what it sims like.

GM
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