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Building a passive radiator


Bret

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I started a crazy experimental enclosure back in March of 2017 and am picking the project back up again. I initially was building a sealed enclosure but changed my mind. I tried to design a port but the box is too small so I’m going to try a passive radiator setup. I’m using 2 sundown sa 8’s with three 8” pr’s.

I can build the radiators for next to nothing. I did the math and figured out the weight the cones need to be to tune to 30hz. What I don’t understand is where spider stiffness comes into the equation, or if I even need spiders at all. It seems the spider would affect tuning more than the cone weight. 

If anyone has any thoughts on this I’d like to hear them. Thanks.

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You do need the spiders. 

As far as trying to do calculations, the easiest method to get the tuning you desire is just trial and error.  Once you have everything mounted in the box you can play sine wave tones while watching/feeling lightly the cone movement of the subs (NOT the PRs).  The frequency where you get the least cone movement is the tuning frequency.  From there you can add weight to the PRs to lower the tuning frequency to where you want it to be. 

"Nothing prevents people from knowing the truth more than the belief they already know it."
"Making bass is easy, making music is the hard part."

Builds:

U7qkMTL.jpg  LgPgE9w.jpg  Od2G3u1.jpg  xMyLoO1.jpg  9pAlXUK.jpg

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Ok. I’ll use my calculations to get in the ballpark and then fine tune like you say. Thanks for the feedback.

Why do speakers move the least at the tuning frequency? I have never understood this. I know the port or radiator is doing all the work but how does the port do anything if the speakers aren’t moving?

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A ported enclosure is a resonant system, it has a frequency it rings at like a tuning fork.  When you input energy at the resonant frequency it takes very little actual movement to get things going.  Its like cracking a whip, a small movement at the handle end makes the other end move a lot.  

For me info, this guy explains ported enclosures better than I ever will be able to:
https://hometheaterhifi.com/volume_5_2/cmilleressayporting.html

There are some GIFs about 2/3 down the page that do a good job of showing what the driver cone and port/PR do relative to each other above, at, and below tuning.  

"Nothing prevents people from knowing the truth more than the belief they already know it."
"Making bass is easy, making music is the hard part."

Builds:

U7qkMTL.jpg  LgPgE9w.jpg  Od2G3u1.jpg  xMyLoO1.jpg  9pAlXUK.jpg

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