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Passive radiator vs port theory
Passive radiator vs port theory
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Old 4th September 2016, 03:44 PM   #1
bcodemz is offline bcodemz  Canada
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Default Passive radiator vs port theory

So my previous understanding was that passive radiator and ported enclosures were very similar. I thought the diameter of the PR is the equivalent as the diameter of the port. The mass of the passive radiator is equivalent of the mass of air within the port. Therefore, changing the mass of the PR changes the tuning.

However, it turns out the tuning of the PR actually affected by the Fs of the driver and not just the Sd and Mms. Therefore, this means the tuning is affected by the Cms of the PR since Fs is determined by Cms and Mms.

So, there are 3 major things that determine the tuning of a ported enclosure: enclosure volume, port diameter, and port length. But it seems like there are 4 major things that determine the tuning of a passive radiator enclosure: enclosure volume, Sd, Mms, and Cms.

So what is the equivalent of Cms in a ported enclosure? Or is either port diameter or port length a combination of 2 passive radiator parameters?

Last edited by bcodemz; 4th September 2016 at 05:17 PM.
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Old 4th September 2016, 04:57 PM   #2
Arty is offline Arty
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both of them are defined by a tuned volume.
and from then on nothing is common in between the 2.
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Old 4th September 2016, 05:01 PM   #3
djn is offline djn  United States
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I use use at least twice the cone area of the woof as the only measure. It's always work for me. I just built a pair od small speakers with a 5" driver and used two 5" PRs. It sounds great.
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Old 4th September 2016, 05:46 PM   #4
tb46 is offline tb46  United States
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Default General Links on Passive Radiators.

Hi bcodemz,

Post #1: "...my previous understanding was that passive radiator and ported enclosures were very similar. I thought the diameter of the PR is the equivalent as the diameter of the port. The mass of the passive radiator is equivalent of the mass of air within the port. Therefore, changing the mass of the PR changes the tuning."

That is basically correct. I'll re-post the link I posted in your other thread, maybe you didn't see it:

AE Speakers --- Superb Quality, Unforgettable Performance, Definitely.

Also, at PE's website you can find that PR specifications include Fs/Vas/Cms/Mms/Rms/Sd. These are the raw specifications prior to adding tuning mass, e.g.:

Peerless by Tymphany 830878 3-1/2" Passive Radiator

Brian Steele has a nice write-up on his
Subwoofer DIY Page (with further links):

The Subwoofer DIY Page - Passive Radiator Systems

Hope this helps,
Regards,
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Old 4th September 2016, 05:53 PM   #5
chris661 is offline chris661  United Kingdom
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A PR has its own inherent springiness. When you put it into a cabinet, this lumps with the air compliance, to give a new resonance for the PR.

In a ported box, the air inside the box is the only spring acting.

Chris
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Old 4th September 2016, 06:11 PM   #6
Charles Darwin is offline Charles Darwin  United Kingdom
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They are similar in the principle on which they work:
In both cases the bass driver is coupled to a mass via the internal air within the box.
Tuned correctly both will resonate in sympathy with the bass driver.

The main difference is that with a port the mass in question is a few grams while with a PR it is a few hundred grams. Tight bass is not a PRs strength.
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Old 4th September 2016, 06:51 PM   #7
TBTL is offline TBTL  Germany
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When you ignore the suspension compliance and losses (of the PR), PR and ported are identical. Therefore bass tightness is also identical, even tough a PR has a mass of a few hundred grams.
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Old 4th September 2016, 06:51 PM   #8
Arty is offline Arty
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but we can get rid of port rezonance.
in exchange we get more cone rezonance errors, and as the PR moves we have a less linear spring load on the woofer.
bracing is yet a nother problem for some cases, as the passive radiators take away space needed for bracing. accoustic coupling between the woofer and a port may be better in some cases, we can choose the port exit loaction better in many cases than the passive radiator, summing of the 2 might happen better with ports.

i don't dislike PR sound, and it does have advantages in a few areas, but for me the drawbacks outweight the gains.
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Old 5th September 2016, 06:09 PM   #9
Ron E is offline Ron E  United States
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Quote:
Originally Posted by bcodemz View Post
However, it turns out the tuning of the PR actually affected by the Fs of the driver and not just the Sd and Mms. Therefore, this means the tuning is affected by the Cms of the PR since Fs is determined by Cms and Mms.

So what is the equivalent of Cms in a ported enclosure? Or is either port diameter or port length a combination of 2 passive radiator parameters?
You seem to confuse your terminology here. You say that PR tuning depends on the Fs of the driver - that is false.

Here is a picture of a 12" driver in a 150Lbox tuned to 20Hz, ported and with a loose and stiff 15" PR. The loose PR has a Vap of 2200L, the stiff one 225L.
Total mass is 434grams for the 15" diameter port (for comparison), 464 grams for the loose PR and 723g for the stiff PR.

You can see the loose PR has a response very similar to the ported box. I actually changed the leakage losses so you could see it better near cutoff. The main effect of PR compliance is to make a null at the PR's free air resonance frequency. Basically you want to keep the PR resonance as far away from the passband as possible.

Woofers don't make good PR's generally because their compliance is too low and the resultant notch will mess up your curves....

Ports don't have a compliance...it's a really funny question
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Old 5th September 2016, 07:25 PM   #10
bcodemz is offline bcodemz  Canada
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Quote:
Originally Posted by Ron E View Post
You seem to confuse your terminology here. You say that PR tuning depends on the Fs of the driver - that is false.

Here is a picture of a 12" driver in a 150Lbox tuned to 20Hz, ported and with a loose and stiff 15" PR. The loose PR has a Vap of 2200L, the stiff one 225L.
Total mass is 434grams for the 15" diameter port (for comparison), 464 grams for the loose PR and 723g for the stiff PR.

You can see the loose PR has a response very similar to the ported box. I actually changed the leakage losses so you could see it better near cutoff. The main effect of PR compliance is to make a null at the PR's free air resonance frequency. Basically you want to keep the PR resonance as far away from the passband as possible.

Woofers don't make good PR's generally because their compliance is too low and the resultant notch will mess up your curves....

Ports don't have a compliance...it's a really funny question
Thank you for the insightful post.

Haha yeah I was really confused because I couldn't imagine ports having compliance.

What program are you using to get those models? WinISD is not accurate for PR modelling and Jeff Bagby's excel sheet does not account for parameters that cause PR losses. I'd be very interested to work with a program like that.

Even in your example, the stiff PR needed almost twice the mass to achieve the same tuning. Wouldn't that imply that it's not just mass and surface area that determine's a passive radiator's tuning?

I agree most woofers don't make good PRs for the reason. Unfortunately the number of 5.25'' passive radiators on the market is exactly one, not exactly a lot of choices, hence my attempts at making a DIY PR.

Last edited by bcodemz; 5th September 2016 at 07:45 PM.
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