I have a question about passive radiators. I heard that in a box that has one driver and one passive radiator the PASSIVE Radiator should move more than the driver. Is that correct?
I just tryed to make one of these with an 8" driver and a 10" PR and the driver seems to be moving more than the PR. I feel like that is wrong. Is it?
I just tryed to make one of these with an 8" driver and a 10" PR and the driver seems to be moving more than the PR. I feel like that is wrong. Is it?
I think that would be better put that the passive radiator should be able to move more air than the woofer. The 8 inch moves a certain amount of air. The PR being that much larger only has to move about 1/2 - 2/3 as far to have moved the same amount of air.
To check the sealing of your cabinet, push the radiator all the way in. As you do this, the woofer will move out. Hold the PR in place and watch to see if your woofer returns to it's postion or stays in the out position. If it stays ( for 10 seconds or so), your cabinet is sealed. If it slowly returns to center (less than 5 seconds), then your cabinet is leaking. That is if your drivers are fully sealed and are suited for a PR system.
Cal
To check the sealing of your cabinet, push the radiator all the way in. As you do this, the woofer will move out. Hold the PR in place and watch to see if your woofer returns to it's postion or stays in the out position. If it stays ( for 10 seconds or so), your cabinet is sealed. If it slowly returns to center (less than 5 seconds), then your cabinet is leaking. That is if your drivers are fully sealed and are suited for a PR system.
Cal
To do a PR system correctly, your PR's need to be able to displace 2x-4x the volume that the active driver does. A good general rule is to use a pair of 15" PR's for a single 12" woofer, pair of 12" PR's for a single 10", etc.
Now in your design, the PR essentially works like a port. At and around Fb, the tuning frequency of the box, the PR will do most of the work and the woofer will barely move. At 1/3 octave above Fb, the woofer will be at it's peak in excursion.
A PR system is the most critcal of any to have well sealed. If it is not well sealed you kill your bottom impedance peak, and the added output from the PR's is greatly decreased.
For some general PR info you can look at the FAQ I put up a long time ago: http://www.stryke.com/prfaq.htm
John
Now in your design, the PR essentially works like a port. At and around Fb, the tuning frequency of the box, the PR will do most of the work and the woofer will barely move. At 1/3 octave above Fb, the woofer will be at it's peak in excursion.
A PR system is the most critcal of any to have well sealed. If it is not well sealed you kill your bottom impedance peak, and the added output from the PR's is greatly decreased.
For some general PR info you can look at the FAQ I put up a long time ago: http://www.stryke.com/prfaq.htm
John
The only time that you'd notice the PR moving further than the driver cone is at the system resonant frequency, and you'd have to be sending it a sine wave signal at that frequency. Throughout the rest of the pass band the PR and cone will displace about the same amount of air, and since your PR is larger than your cone it has to move less than the cone does to do so.
becuase a PR is more efficent than a woofer( I believe, but I'm probably wrong). and will displace mroe than the woofer at tuning. assuming they have the same amount of displacement, if the woofers excursion is reduced by 50% at the Fb, then that means that you can only use half that woofers output at Fb, which is slightly limiting IMO. Plus you want the PRs movement to be as linear as possable.
simon5 said:
In days of old we didn't. The first passive system I owned was a JBL LE8T with an identical, purpose made drone. Perhaps as we went along, the need for a larger area was realized. Perhaps it will continue. Perhaps John was right a couple of posts back when he gave the example of close to four times the area. ie: one 12 and two 15's
Cal
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.
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.
simon5 said:
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 Ron E said:If WinISD predicts response shape changes other than the shift in the null, you may have found a bug.
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
Attachments
simon5 said:
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.
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.
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.
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.
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.
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.
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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