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Old 17th August 2010, 05:14 AM   #1
jaybee is offline jaybee  United States
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Default passive radiator excursion

Hi, 'nother newb here. I'm trying to figure out the excursion induced in a passive radiator. I'm trying to select a PR for my design, but all I can find are various, not-very-reassuring rules of thumb. Maybe someone could tell me, if I'm totally of the mark.

I've seen the method proposed, where the required PR mass is calculated by designing a port with the same effective diameter as the PR - it's simply the mass of the air enclosed in the volume of the port. Sounds good, although I'm somewhat concerned that it doesn't seem to agree with the datasheets Seas publishes for their Prestige line PRs.

I thought, why not also go ahead to calculate the air speed in that virtual port, then from that, figure the acceleration (using the period of a half cycle at resonance), then... multiplying that times the mass of the air "slug", you get the force acting on the PR cone. The suspension compliance is given as displacement per unit force, so it seems not too hard to work out the total excursion in this way.

Using this method it turns out that hardly any PRs exist that fit the bill. Also, doubling up on PRs doesn't necessarily help because while the air speed goes down, the mass goes up. OK wait, I'm still thinking about the proportionality of those changes....me slow...

Finally, maybe it just doesn't translate to "real life" that well, because of et cetera goblins my newbie mind cannot see. What thinkst?
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Old 17th August 2010, 10:22 AM   #2
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Download Basta! and see for yourself.
http://www.tolvan.com/basta/

Remember that excursion (and I guess velocity) is RMS.
The users guide is a good place to start.
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Old 17th August 2010, 10:43 AM   #3
AndrewT is offline AndrewT  Scotland
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is the volume displacement of the PR in any way related to the volume displacement of the active driver?
i.e. is there any significant change in the volume of air in the sealed speaker?
Are the pressures big enough to compress or expand the gas in the speaker?
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Old 17th August 2010, 02:09 PM   #4
jaybee is offline jaybee  United States
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The Basta software looks really useful; mac OSX version please!

AndrewT, I cant say I have definitive answers to those questions, but my own thinking was initially over-simplistic. From what I've been reading (which has all been way over my head!), there is definitely a relationship between driver excursion and PR excursion but it is more complex than I thought at first. The excursion of the driver is minimal at system resonance, but energy transfer to the PR (and therefore PR excursion) peaks there, otherwise it wouldn't do what it does for bass extension I think. Also, and contrary to intuition, the PR motion is in-phase with the driver at resonance. If this is true, it would seem that the air is indeed compressed and rarified by the action of the system, and total volume does not stay the same as I originally assumed. I found a really interesting analogous visualization, I'll try to find it....

Essay

about halfway down there's interesting animations. I can't speak to the veracity of the information there, but it looks pretty anyhow.
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Old 17th August 2010, 04:45 PM   #5
taloyd is offline taloyd  United States
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Post Unibox to the rescue!

Hello,

Unibox does a really nice job modeling both driver and PR excursion, and you can adjust the pertinent parameters and see the results very quickly.

Description linky:
UniBox - Unified Box Model for Loudspeaker Design - Kristian Ougaard

Download:
UniBox - Unified Box Model for Loudspeaker Design - Kristian Ougaard

It's Excel, so hopefully it would work on a Mac OS.

cheers,
-Tal
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Old 18th August 2010, 01:42 AM   #6
jaybee is offline jaybee  United States
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Thanks Tal, that one looks pretty cool, but doesn't seem to go so good on the latest version of OpenOffice. to wit:

Message: The following Basic script could not be found:
Library: 'Standard'
module: 'CalVB_SPLvb'
method: ''
location: 'document'

Last edited by jaybee; 18th August 2010 at 02:02 AM.
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Old 18th August 2010, 10:47 AM   #7
djk is offline djk
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"is the volume displacement of the PR in any way related to the volume displacement of the active driver?"

In general, depending on the alignment, the PR needs to be able to displace 2x~3x the active woofer.

Small 6th order vented systems need closer to 3x, large 4th order vented systems need more like 2x.

BassBox v6.0 seems accurate in its excursion estimates for the PR.

2.5 cu ft, two 9mm 12s, one 18 PR 21mm, 42hz tuning, 2KW input to hit x-max, 124dB at 42hz, 127dB at 60hz, 129dB at 80hz.
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Last edited by djk; 18th August 2010 at 11:17 AM.
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Old 18th August 2010, 11:21 AM   #8
djk is offline djk
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27mm on PR at 37hz, 9mm on woofers, 3:1 ratio.

It was always fun to watch the 400 gram 12" PR on the EV Interface A make large excursions, while the 8" barely moved. The Interface A was turned to around 32hz in a box that looked to be less than 1 cu ft. It had a Q=2 high pass filter in the 32hz region, making it a 6th order vented system.

http://archives.telex.com/archives/E...s%20Manual.pdf
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Last edited by djk; 18th August 2010 at 11:31 AM.
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Old 18th August 2010, 04:05 PM   #9
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You can keep it pretty simple - two 'sizes' up from the active driver. So a 12" active, use an 18" passive... of course a bit depends on the max excursion possible from the PR and the active driver...

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Old 18th August 2010, 05:18 PM   #10
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No need for calcs, just use WinISD pro and it will work it out for you.

I've written a tute on using WinISD here:
Red Spade Audio: How to use WinISD pro

although it's a little different for passive radiators.
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