How to tune a passive radiator subwoofer. ???

[ssmokeyy]

So I have 2 12's audio frogs g line in a 2.7 cubic sealed box before drivers. I'm pushing them with old kicker zx1000.1. The low end is not keeping up with my morel hybirds getting pushed with 160 rms. I have ordered a Dayton audio 15 inch passive radiator. I plan to add it to the two 12's. I check specs and the 15 is enough above the 2 12's. Being the 15 pr does 60mm xmax.  The g line only has 13mm xmax. So now how do I find out how much weight I need to install on the back of this passive radiator? Second Q is more weight mean lower tunning? I'm trying to tune for about 38hz 

[Triticum Agricolam]

Yes, more weight will lower the tuning of the PRs.

The most foolproof way to set your tuning is by watching the cone excursion of your subs while playing sine wave test tones.  At the tuning frequency your cone excursion will come nearly to a stop (on your subs, not the PRs).  From there you can keep adding weight until you get your tuning frequency where you want it.

[aculous]

Also you can do some modeling just to check but since you already ordered the Dayton f-it just test and tune it. I watched a video from 123toid which helped, he is using WinISD to do the modeling, may be good to model it when you looking at the power level.

 

[Eggz]

More weight = lower resonance of the PR.  That in turn will lower the resonance of the entire subwoofer enclosure.  Remember that if you go too heavy on the PRs (i.e. a couple pounds or more), then the PRs are essentially just a wall of a sealed enclosure.  

As you add mass, the sub system requires more energy to move the PRs, so you'll lose efficiency.  That means PR setups in smaller enclosures are generally  inefficient and thus are power hungry.  This is because smaller boxes have higher resonance, meaning you need more weight on the PRs to lower the box's starting point.  Moral of this is that you should just get a bigger amp than you expect - an RMS output of at least 50% more than your sub can RMS.  For example, if you sub is rated at 600 RMS at x ohms, your amp should be rated to do at least 900 RMS at x ohms.  

As for WinISD, just select passive radiator as the type of enclosure when prompted.  You'll have to enter the sub specs and the PR specs, which can be annoying to find.  But after you input that information, there is a tab for the passive radiators that will allow you to enter different amounts of added mass.  That will get you very close to tuned if your net box size is actually the size you indicate in WinISD. 

To get your net box size right, be sure to subtract out space that's taken up by anything other than air (i.e. the sub, the PRs, any bracing or supports, polyfill stuffing, etc.).  So if your box is 12x12x12, it's 1 cu/ft only when totally empty.  If your sub, stuffing, support dowels, PRs, etc. take up 0.25 cu/ft, then the net air space of the box is really 0.75 bu/ft (viz. 1.00 - 0.25).  I make a point about this because the modeling WinISD gives you will be totally inaccurate if you forget to use net volume.

As an example, my net volume on a JL TW3 10 d4 is 0.6 cu/ft after subtracting out support dowels, the sub, the PRs, and the polyfill.  With 1 lb (about 450 grams) on each of two Earthquake SLAPS M10 passive radiators, the sub will barely move at about 25 hz, even though the PRs are flopping like crazy, so I consider my tuning to be about 25 hz for the enclosure (It's on about 800 watts).  Landing there is a little tricky because the box itself (when sealed, before butting out the PR holes) resonated at around 50 hz, and the PRs with pound on each are tuned to about 12 hz.  That makes the overall enclosure net out to a resonance of about 25 hz.  

Side note: I suggest getting two passive radiators, not just one.  Having just one PR, especially with additional weight, will cause the box to lose energy because it's trying to walk all over the floor.  Imagine only one side of the box shaking a 1 lb weight back and forth at about 40 times per second.  A lot of that energy will go into shaking the box rather than into the air.  But if you have two PRs facing away from each other on opposite sides of the box, the box-shaking goes away and energy goes into sound rather than shaking your box.  Both PRs will either shoot out away from each other or pull in toward each other at the same time, canceling out the lopsided shaking effect.  That's much more efficient, and you need all the efficiency you can get in a PRs setup because they are inherently inefficient even when done properly.  PR setups just need more power for same loudness as a ported box, but the upside is that they take up only a fraction of the space a ported enclosure requires.