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Old 26th September 2007, 03:01 PM   #31
Vix is online now Vix  Yugoslavia
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Quote:
Originally posted by MJK


Instead of doing a trial and error set of experments, why not do the design work up front and build it once.
...
In my opinion, it will take as much effort to design an OB speaker system as it does to design any of the boxed speaker systems.
Thanks Martin! That's where I will need some help from the experienced builders.

The idea was to build a Visaton No-Box: http://www.visaton.com/en/bauvorschl...xbb/index.html

However, there are some changes, but I don't see too much difference:

1. Bass Driver: Visaton uses its BGS40. I will use Eminence Beta 15, because it was recommended to me for OB use, and it is quite cheaper than Visaton. (and doesn't look worse)
2. Visaton went with passive X-overs, I want to go active. It should be at least equally good, or better. But, I 'll need some help here, as I am not that good with software...

Best regards,

Vix
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Old 26th September 2007, 06:20 PM   #32
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Hi Martin,

Great article.

Quick question. Is there a reason the polarity is not reversed on one of the drivers to get it back into phase?

Cheers,
Gio
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Old 26th September 2007, 10:39 PM   #33
MJK is offline MJK  United States
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Hi Gio,

The polarity does not need to be reversed. If this were a textbook case, pure resistance load and coincident point sources, it would need to be reversed. The picture below shows the SPL and phase of the two sources at the listening position.

http://www.quarter-wave.com/Responses.jpg
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Old 28th September 2007, 10:02 PM   #34
BHD is offline BHD  United States
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My parts are ordered... anyone else have them on the way?

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My religion is the golden rule. The only time it's forsaken me is when I've forsaken it first.
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Old 28th September 2007, 11:07 PM   #35
Salas is offline Salas  Greece
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What about hacking out the Alpha 15's dust cap and installing a wacko wooden phase plug? I think that the Eminence's huge mount of midrange ringing might be sabotaging overall clarity even if it is located far from crossover.
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Old 28th September 2007, 11:36 PM   #36
MJK is offline MJK  United States
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What about hacking out the Alpha 15's dust cap and installing a wacko wooden phase plug? I think that the Eminence's huge mount of midrange ringing might be sabotaging overall clarity even if it is located far from crossover.
I think that is a terrible idea. A phase plug will have no impact on the frequencies being produced by the woofer in this design (40 to 200 Hz). I have never heard a ringing problem from the four Alphas I am using in my open baffle system. In my opinion the risk of screwing up a good driver are far greater than any slight potential for improvement. Have you tried the Alpha 15A driver in this type of system and heard a ringing at 2 kHz?
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Old 29th September 2007, 01:05 AM   #37
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Wow, great article. I love it when someone can explain something complex simply. Its a sign of mastery.

I had a question: at the end , "Figure 13: Final Calculated System SPL Response". This graph doesn't take into account the actual drivers frequency response - if it did, the resulting SPL response would no longer be within +/-2dB anymore would it? Some sort of sum of the peaks/dips of fig. 13 and the drivers - but the point of this article was to design the best baffle/xo given the woofers limits on sensitivity, fs and Qts, correct?

Is the inverse of this article true? That if one were to use an active crossover/equalization based on measurements, selection of drivers, baffle shape/size and crossover frequency would not be as critical?
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Old 29th September 2007, 02:24 AM   #38
Salas is offline Salas  Greece
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Quote:
Originally posted by MJK


I think that is a terrible idea. A phase plug will have no impact on the frequencies being produced by the woofer in this design (40 to 200 Hz). I have never heard a ringing problem from the four Alphas I am using in my open baffle system. In my opinion the risk of screwing up a good driver are far greater than any slight potential for improvement. Have you tried the Alpha 15A driver in this type of system and heard a ringing at 2 kHz?
Hmmm, Impulse response will show in its trail edge. You won't hear it clearly ringing but you will hear it by its absence if fixed. A hobby knife taking out a dust cap is nothing to be afraid of on a cheap woofer. It can be easily glued back. No I haven't done it on an Alpha. But you thinking its an outright terrible concern, maybe it isn't.

I quote Tom Danley: "A bad sign (regardless of cone material) for a direct radiator is a large peak(s) associated with its hf roll off. It is assumed that a steep or low crossover fixes this problem but it is not gone. So, picture a driver with a big mound in its response associated with breakup.
Lets say that mound is 10 dB high above the “flat” response zone, this represents an frequency dependent acoustic gain element AFTER the voice coil.
So, here is where it is not fixed even with a brickwall crossover.
The driver’s motor produces harmonic distortion, that acoustic gain amplifies it by 10X (10dB) for harmonics when the fundamental is N fraction of the peak.
In other words, if the mound were at 1200Hz, the third harmonic of a fundamental at 400Hz is amplified by the acoustic gain even if one had a 120,000dB / octave digimatic crossover at 500Hz. Here it is better to fix the source.'' Its taken from
here . In the attachment is the published Alpha 15A chart.
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Old 29th September 2007, 02:31 AM   #39
Salas is offline Salas  Greece
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P.S. That's who is Tom Danley.
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Old 29th September 2007, 02:44 AM   #40
MJK is offline MJK  United States
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Quote:
Wow, great article. I love it when someone can explain something complex simply. Its a sign of mastery.
Thank you very much for the positive feedback.

Quote:
I had a question: at the end , "Figure 13: Final Calculated System SPL Response". This graph doesn't take into account the actual drivers frequency response - if it did, the resulting SPL response would no longer be within +/-2dB anymore would it? Some sort of sum of the peaks/dips of fig. 13 and the drivers - but the point of this article was to design the best baffle/xo given the woofers limits on sensitivity, fs and Qts, correct?
Your understanding is exactly correct. This is the kind of study I typically do when considering a new project for a specific driver. If I decided to pursue the project, the next step would be to aquire the drivers and measure their responses. Then I would make a second pass through the calculations using the actual measured results to see if any tweaking of the design was required. If the driver produced a good listening impression when auditioned on the bench, I would expect the design to be a success.

Quote:
Is the inverse of this article true? That if one were to use an active crossover/equalization based on measurements, selection of drivers, baffle shape/size and crossover frequency would not be as critical?
I think it would be a slightly different set of design challenges. If I were looking to use an active crossover, I would still look for roughly the same driver parameters/relationships and use the crossover for its conveninece and adjustability. I would not use a signficant amount of EQ or boost and would rely on the acoustics of the baffle and drivers. Using a little boost for the bass is fine but if you apply a lot of boosted power to the wrong woofer it will run out of gas. The high Qts Alpha 15A is very efficient and its motions when music is played, even at a loud volume, is very small. I think that this is a big advantage.
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