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Discussion Starter · #1 ·
Several threads have been about passive radiators and I was wondering how the amount of weight on the PR affects the response time of the individual PR. Also, since the PR is going into the box when the active driver is pushing out of the box and vise-versa, isn't the contribution to the SPL out of phase? I know it's not but I can't figure out why. I haven't been able to find any articles to the theory of PRs.

Thanks.

Bob
 

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Hi Bob,

I will partially address your questions for now :). If you understand tuning using a port, then it is pretty similar with a PR. (Box size kept unchanged), mass and friction of air inside the port are the main factors that specify the tune value. The more of them the lower the tune.

Now, you can remove the port and replace it by a PR to give you the necessary amount of mass that will characterize the tune. The more mass, the lower the tune value.

Regarding the out of phase issue, you're right, but it is not 180 degree, there is a phase shift but not a cancellation. Around tuning freq. the driver's contribution to SPL is minimal, while the PR will be maximal.

You may also check THIS
Hope this helps.
 

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Discussion Starter · #3 ·
Now, you can remove the port and replace it by a PR to give you the necessary amount of mass that will characterize the tune. The more mass, the lower the tune value
The confusing point is the more mass which (by the laws of physics) is going to be more difficult to move so if you have fast changing low frequencies, does the higher mass have an affect on the response time of the PR? If it does, then I think you will have some phase shifting. Yes?

Bob

And thank you for your explanation. I'm not sure that I understand it 100%, but it helps.
 

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The confusing point is the more mass which (by the laws of physics) is going to be more difficult to move so if you have fast changing low frequencies, does the higher mass have an affect on the response time of the PR? If it does, then I think you will have some phase shifting. Yes?
No, it is not a matter of a response time as much as it is the required wavelength to excite the PR and make it resonate. Nevertheless the time it takes the PR to react to the "driver's signal" will depend on the tuning frequency (you're right). Consequently phase shift occurs and embodies group delay in ported subs.

I don't know if I 100% understand myself either :rant: but it is you who ask hard questions :bigsmile:
 

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The confusing point is the more mass which (by the laws of physics) is going to be more difficult to move so if you have fast changing low frequencies, does the higher mass have an affect on the response time of the PR? If it does, then I think you will have some phase shifting. Yes?
No such thing as "fast changing low frequencies" per se, actually. If you have something that resembles such, e.g., an LF signal with a sharp envelope, it's actually going to be a sum of high frequencies on top of the low frequencies, meaning a low-passed woofer will only see the low parts.

As to the cancellation, yes, there will be a frequency at which PR totally cancels active, but it will hopefully be at a low enough frequency that it will not matter...

Hope this quickie helps...
 

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Discussion Starter · #6 ·
No such thing as "fast changing low frequencies" per se, actually. If you have something that resembles such, e.g., an LF signal with a sharp envelope, it's actually going to be a sum of high frequencies on top of the low frequencies, meaning a low-passed woofer will only see the low parts.
Not sure how you would get high frequencies if you do something like generate a 25Hz signal then jump to 45Hz versus a slow increase to 45Hz.

Bob
 

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Not sure how you would get high frequencies if you do something like generate a 25Hz signal then jump to 45Hz versus a slow increase to 45Hz.

Bob
You said "jump." That means the tweeter will get a workout, especially if the waveform has a sharp edge or other discontinuity to it. The woofer will most likely see stuff faster than 45Hz as well, but only to the extent its lowpass filter allows it.

If you're really interested in this kind of phenomenon, would suggest looking into the concept of Fourier series. Not the math, necessarily, but the general ideas... But really, if the waveform looks "fast" or steep, chances are the tweeter sees it more than the woofer. That's what crossovers are for, to give tweeters the fast stuff, and woofers the slow stuff...

"Slow" bass generally refers to woofer ringing in time, which is peaking in frequency, but nothing to do with speed per se, and everything to do with damping and temporal accuracy.
 

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Discussion Starter · #8 ·
For simplicity sake, let's just assume that a sub is under test, no other driver hooked up to the system. A signal generator is attached to the amp and we run low frequency signals into the sub. My question was, how would the mass of the PR react to the speed of change in the frequency. I'm simply questioning if the mass of the PR affects the response time. Would it be better to have three PR lightly weighted rather than a single or double with more weight.

Bob
 

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Bob I don't really know the answer to your question, but most of the time PR's are tuned down around 20hz or below that and that means that they aren't really contributing much up higher in frequency. I don't think that it's very easy to distinguish much relating to speed or tightness down there.
 

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For simplicity sake, let's just assume that a sub is under test, no other driver hooked up to the system. A signal generator is attached to the amp and we run low frequency signals into the sub. My question was, how would the mass of the PR react to the speed of change in the frequency. I'm simply questioning if the mass of the PR affects the response time. Would it be better to have three PR lightly weighted rather than a single or double with more weight.

Bob
Mass of PR alone is not enough information. You also need to know compliance of PR, and box volume. And in general, you want as many PRs as you can get, because it's not uncommon to be PR-limited...

And just thought of a better way to think of change in frequency. If you go from 25Hz to 45Hz instantly, you're pretty much going to need every frequency from 25Hz up, including infiniteHz. If you go from 25Hz to 45Hz in infinite time, you'll only need frequencies from 25Hz to 45Hz...
 
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