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Old 8th February 2005, 12:57 AM   #11
Ron E is offline Ron E  United States
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Quote:
Originally posted by simon5
I tested it in WinISD, and for an unknown reason, John is right.

I guess there's something behind that, probably not totally connected to displacement, but when you give more displacement, the frequency response curve looks much much better.

I used an Ascendant Audio Avalanche 18", to get a good FR curve I used three 18" PRs with 38 mm Xmax.

If you look at the excursion curves, it is quite obvious that you need more displacement for a PR than for the driver. 2X is usually fairly safe, but it really depends on the alignment.

In terms of FR shape, if you add more PR's, you need to add a lot more mass (typically) and this drives the PR Null (at the PR free air resonance) down in frequency - which will change the shape of the stop band. If WinISD predicts response shape changes other than the shift in the null, you may have found a bug. If so, it won't be the last in that program
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Old 8th February 2005, 01:18 AM   #12
hofffam is offline hofffam  United States
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xstephanx - I don't believe there is any such thing as the "efficiency" of a PR. No electrical power is supplied of course. The "output" of a PR should be 100% displacement, which is controlled by the PR parameters and the box tuning.
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Old 8th February 2005, 01:19 AM   #13
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ahh thought I might be wrong. I cant remember where i heard that the port is more "efficient" than the driver. meaning that the port could displace more air than the driver could.
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Old 8th February 2005, 04:47 AM   #14
simon5 is offline simon5  Canada
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Quote:
Originally posted by Ron E
If WinISD predicts response shape changes other than the shift in the null, you may have found a bug. If so, it won't be the last in that program
Can you tell me if the FR showed by WinISD seems to be right?

Ascendant Audio Avalanche 18" in a 500 liters box

Stryke Audio 18" PR
FS 4hz
Mms 1600 grams
Qms 85
SD 1200cm^2
Vd 9L
Vas 1840L
Xmax 76mm p-p
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Old 8th February 2005, 03:12 PM   #15
Ron E is offline Ron E  United States
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Quote:
Originally posted by simon5


Can you tell me if the FR showed by WinISD seems to be right?

Ascendant Audio Avalanche 18" in a 500 liters box

Stryke Audio 18" PR
FS 4hz
Mms 1600 grams
Qms 85
SD 1200cm^2
Vd 9L
Vas 1840L
Xmax 76mm p-p

I can't run the numbers right now, but it looks as if I don't have to. It appears the PR is too heavy unless you have multiples. You could use oneor two PR's, but you would need to subtract weight to get the yellow curve.

The tuning frequency of the box is Fb=Fp*sqrt((N*Vap)/Vb-1), where N is the number of passive radiators, Fp is the PR free-air resonance and Vap is the individual PR "Vas".

You can see that the tuning frequency rises as more PR's are added . In your case you needed multiples to get the proper tuning.
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Old 8th February 2005, 03:52 PM   #16
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To me it seems that displacing air does not take as much power as displacing driver cones.
As only the positive movement of the active driver is effective, the energy of the backward movement of the driver cone needs to be transferred to the passive radiator.

As impuls equals mass * speed, i should think the product of the mass and speed of the active cone should equal mass of the passive radiator times its speed.

As speeds of both need to be equal to be phase coherent, the passive radiator can have a larger dimension.
The passive radiator then moves more air, and thus is more efficient.
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Old 8th February 2005, 05:14 PM   #17
hofffam is offline hofffam  United States
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I don't think it works that way. The "energy" of the driver does not simply transfer to the PR. The PR is a resonant device. It is not "driven" by the woofer. The PR's displacement is not directly related to the displacement of the woofer. BassBox can plot the excursion of the woofer and the PR. For my XLS sub and PR system, the PR excursion is smaller than the woofer above 38 hz but larger below that.
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Old 8th February 2005, 06:05 PM   #18
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Air seems like a gas to me.
A gas needs to behave to Bernouilli's gas law.
Unless the active driver generates a large portion of heat or the speaker enclosure heats up the energy law should still apply.

It is a resonant system so the resonance needs to be calculated, but that should be necessary for energy transferance through the enclosed air.
Seems to me that even if the theory is resonance it is still the active driver that supplies the energy for the passive radiator to move.
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Old 8th February 2005, 08:54 PM   #19
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The driver does generate heat, inside the box the air is warm, thats why polyfill works.
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Old 8th February 2005, 09:27 PM   #20
hofffam is offline hofffam  United States
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I don't remember chemistry nor physics as well as I'd like. But I understand that it seems like the air captured in the box SHOULD directly couple the PR to the woofer. If you push the woofer down - the PR goes up and equal amount (displacement). In my XLS system - the PR looks identical to the woofer so the displacement is the same when I PUSH the cone. When actually listening to music or watching movies however - I can see the displacement of these two can be very different. The XLS example is good because the Sd is identical, the cone is identical, and the frame is identical. During the helicopter takeoff scenes of Black Hawk Down the PR moves significantly more than the woofer. The air is not simply pushing the PR in direct relationship to the woofer cone displacement.
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