Adding weight to passive radiators

Thanks guys. I was just curious. They seem to be making the rounds more often these days. From what I read, if a ported box is designed right and will not be really cranked up, they sound the same. When cranked the port noise is said to be noticeable, but if it is cranked up, how can you hear it. The only reason I just installed them in my project is because I don't have room for a tube.
 
Your real concern is not the sound... if done "properly" I think it is an improvement over a port. Leaks ("QL") are your enemy.

The concern is the suspension.

For most standard PRs the suspension is merely a driver without a magnet. It's not designed for two things: A) extra weight B) extra moving mass and C) extra pressure in the cabinet. Did I say two?

The extra weight, IF it is substantial will create a problem - slightly different depending on the orientation of the PR. Vertical is the only one that is viable with a stock PR.

You will get "sag" over time, and depending on the suspension that may be sooner rather than later. A PR with ONLY a surround = NG.

A PR made from a standard basket with no magnet = suspension slap at high SPLs.
(the lower the Fpr, the more likely)

The extra internal cabinet pressure has been known to rip diaphragms, if not immediately then over time, sometimes not that long a period of time.

_-_-
 
yeah... but last time I looked there is no "foam" or "rubber" suspension in a port??
No mechanical limit due to a surround and suspension??

What are we talking about??

the word 'extra'? ok, by extra I meant to say "exceeding the physical limitations of the surround" - the word 'extra' coming because it would be in excess of what the PR would likely see if it was the same cone & suspension with a VC and magnet in an enclosure... clear as mud, I am...
 

Ron E

Member
2002-06-27 10:41 pm
USA, MN
yeah... but last time I looked there is no "foam" or "rubber" suspension in a port??
No mechanical limit due to a surround and suspension??

What are we talking about??

You said extra pressure could rip diaphragms.

I simply point out there is no extra pressure.

Next you say it rips surrounds. Then you say it is the mechanical limitations that rip surrounds. Maybe you should answer your final question in the quote above.
 
Take a deep breath. Breathe. Good.

You are absolutely correct, I did go back and see that I wrote "diaphragms".
That was an error. I meant to say "surrounds". Why I typed diaphragms? Dunno.

Are you feeling ok now? Better?

Just for the sake of curiosity, have you ever seen a PR with a surround that has been ripped due to the pressure in the cabinet causing a failure??

_-_-
 
ok, mechanical limitations.

there is just so much Xmax available, once you reach the limit, then the only thing that can happen inside the box is for the air to compress... a lot depends on the ability of the driver to move air and produce SPL. In the case of a typical consumer woofer it may not produce enough pressure to cause damage. But the pressure on the diaphragm and the surround is not planar. The role of the moving part of the PR is to make a linear motion in one axis out of the nodes that occur inside the cabinet. This can be observed easily by mounting a diaphragm with a single suspension member (a surround) and driving that through a range of frequencies. You will see all sorts of not in a single axis motion by the PR in this set up.

Those forces are still acting on the diaphragm (which is stiff) and the surround (which is not particularly stiff in the case of foam or rubber) so these nodes can add extra flexure on the surround - it may not be observable to the eye in normal operation, but could cause fatigue and failure on and along the surround.

Whatever...

Small increases in PR mass to "tweak" the tuning lower won't have much negative effect, but attempting to take a typical factory supplied PR and drop the frequency by even 1/2 octave will take more and more mass the lower in frequency one tries to tune. That's when the mass loading becomes a real design consideration.

All imho - subject to various typos and minor mis-statements.
 
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