Using PRs to extend the Rythmik servo's FR? - diyAudio
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Old 17th December 2005, 11:26 PM   #1
dscline is offline dscline  United States
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Question Using PRs to extend the Rythmik servo's FR?

I am in the process of building a sub using two Rythmik servos in a sealed configuration. I wanted to go sealed, because I wanted the accuracy/transient response that a sealed enclosure offers. I chose to use two of them (in a single dual chamber cabinet) to make up for the output that is lost when choosing sealed over ported. I have, however, begun to wonder if I can't have the best of both worlds by using PRs tuned VERY low. Sealed, the servo kits have good extension, only 3db down @ 14hz:

Click the image to open in full size.

At such low frequencies, I'm sure they are excursion limited. I can't help but wonder if it'd be possible to throw a couple of 18" PRs in the cabinet, and mass load them enough to tune them around 8-10hz, and cause that curve to to remain close to the same, but just extend out a little bit at 14hz and below. It seems to me that if they are tuned low enough, the downsides of using a PR (group delay, etc.) would be kept well out of the audible range. Is this possible while keeping the cabinet at a reasonable size? I don't know how you'd even model this considering the effects the servo circuit has. Thoughts?
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Old 18th December 2005, 06:25 AM   #2
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This isn't the "best of both worlds!"

It's really just a vented subwoofer, but in place of moving air in vents you are moving a mechanical PR. Ideally if this is what you want to do then a pair of 18" PRs should be placed on opposing sides.

How about a vented sub designed using the approach suggested by Dan Wiggins? ie. low tuning and drooping response with low GD.

Have you considered push pull mounting?

Try vented first. If you don't like it, go back to sealed. If you don't like passive radiators, the price of entry is more and you have wasted money if you don't like it. If you like what you get vented, then you can always get PRs but I don't see much advantage unless you have trouble building a port that doesn't chuff.

As to how to model it ... start in WinISD and model Brian's suggestions on the website in a vented box. Then do a model in the same size box with a PR and experiment until the PR box is almost identical. Then you have your design. If you can't allow for the impact of the filters included in the kit, at least copy the vented design as I'm sure Brian will have already accounted for that. I'm sure as well that he will help you out with it.
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Old 18th December 2005, 01:50 PM   #3
dscline is offline dscline  United States
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Quote:
Originally posted by paulspencer
This isn't the "best of both worlds!"

It's really just a vented subwoofer, but in place of moving air in vents you are moving a mechanical PR. Ideally if this is what you want to do then a pair of 18" PRs should be placed on opposing sides.
Yes, that much I understand. By the "best of both worlds", I was referring to trying to maintain a sealed system reponse in the audible range (20hz+), while using the PRs to help only in the infrasonics. The reason I was thinking about PRs was because it's nearly impossible to get single digit tuning using vents in a reasonable size aesthetic enclosure (6-8 cu ft).

Quote:
How about a vented sub designed using the approach suggested by Dan Wiggins? ie. low tuning and drooping response with low GD.
And that's exactly what I was hoping for, except using PRs instead for the size advantage. But I did some playing with winISD last night, and it appears that even with PRs tuned below 10hz, their "effects" are still present well above 20hz. I believe dual sealed servos will have plenty of output for me even below 20hz, I was simply hoping to extend that a little using PRs, without giving up the sealed sound in the rest of the range. It seems physics are against me though... their effect doesn't roll off in the high end as quickly as I'd hoped.

Quote:
Have you considered push pull mounting?
Yes, my sealed box is using a push-pull configuration. In fact, all the panels are cut, and I just need to assemble it, I was just having some second thoughts yesterday, and wanted to make sure I was headed down the right path. After tinkering with winISD, I think sticking with the pure sealed design is best for what I want, so I can continue putting it all together.

Quote:
As to how to model it ... start in WinISD and model Brian's suggestions on the website in a vented box. Then do a model in the same size box with a PR and experiment until the PR box is almost identical. Then you have your design. If you can't allow for the impact of the filters included in the kit, at least copy the vented design as I'm sure Brian will have already accounted for that. I'm sure as well that he will help you out with it.
Yes, but the problem was that the ported design on his site is tuned to 20-22hz. I did email him yesterday, but I was just throwing the question out here to get some input while I waited on his reply.

Thanks!
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Old 18th December 2005, 03:12 PM   #4
BAM is offline BAM
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Take a look at the AE Speakers passive radiators, which are designed to use very high moving masses. 18" PRs sound about right for what you're trying to do, but I wouldn't tune in the single digits. The Rhythmik setup's subsonic filter only sets as low as 14 Hz or thereabouts, if I remember correctly. So your tuning in the single digits wouldn't do much good.
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Old 18th December 2005, 03:53 PM   #5
dscline is offline dscline  United States
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Quote:
Originally posted by BAM
Take a look at the AE Speakers passive radiators, which are designed to use very high moving masses. 18" PRs sound about right for what you're trying to do, but I wouldn't tune in the single digits. The Rhythmik setup's subsonic filter only sets as low as 14 Hz or thereabouts, if I remember correctly. So your tuning in the single digits wouldn't do much good.
Those are the ones I was using. With the maximum mass you can load them with (3kg total), in a 6cu ft. box, a pair of them ended up tuning around 10hz. While the amp may roll off below 14hz, it still has output down that low (as can be seen from the above FR plot). I'm happy with the sealed response down to 14hz, so I didn't want to boost response above that. But even with a 10hz tuning, the PRs were contributing well above 20hz.
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Old 19th December 2005, 12:23 AM   #6
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Ok I misunderstood a little regarding the "best of both worlds." Sounds like an interesting idea, although I wouldn't take it too far, nor would I be concerned that there is output useful at 20 Hz from the PRs. In fact I'd still suggest trying this out. I think you are getting into territory where experimentation wins over simulation.

How about making a big test box - say 120L per driver and tuning ~ 12 Hz. Because the tuning is so low the vent velocity is quite low and you can use a 90mm vent. In fact in that size box such a vent will be ~1m long and at xmax, velocity is 7m/s @ 20 Hz and peaks at 17m/s @ 11 Hz before the 14 hz filter - in other words you could throw in a 1m PVC vent @ 90 mm and test it out. If you like the results you get you can then go with a PR.

How do you know your sealed version is actually better than a vented design?
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Old 19th December 2005, 11:35 PM   #7
dscline is offline dscline  United States
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Well, part of my impatience Saturday was simply second guessing whether or not I could improve it before I actually spent more time building the cabinet. I've since convinced myself I'm heading in the right direction, and almost have the cabinet done. It's all I can do to get this project done, so I probably shouldn't sidetrack myself to satisfy my curiosity.

As far as the last question, without opening up any of those old sealed vs. ported debates, lets just say that I believe that a good sealed system has a better chance of being closer to the characteristics that are important to me at this point than a good ported system.
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