Upon fiddling with passive radiators for a few months now, I have discovered a very easy and effective way to add and remove mass from the cone.  There are many methods for doing this from gluing nickels to the cone to washers to modeling clay or some kind of putty plastered to the PR diaphragm.  But the one major disadvantage to all of these methods is the difficulty in changing quickly between different masses.  Let's say you plaster on 200 grams of putty to the cone only to realize that it's too heavy.  How are you going to easily get rid of the necessary putty?  Or let's say you've glued on fourteen 25 gram washers to the cone, you've waited two days for the glue to set real hard, then you test the box only to find out that you need 4 more washers to get the tuning just right.  Well you've got to pull out the PR, glue on 4 more washers and wait two more days before you can do any more testing.  In the meantime your anxiety is killing you to hear this sub enclosure you've been working on for the last two months.  This is the final step.  All you need to do it get the tuning perfect and your done.  But instead your waiting days and days for this blasted glue to dry so you can finally have fun with your new sub.   Well this little invention of mine solves all those problems.  In minutes you can go from a tuning of 29 Hz all the way down to 15 Hz and test every frequency in between if you want.  It's simple it's easy and when you've got the perfect tuning, it's permanent.  Until you build a new box and need a new tuning frequency.  Then once again it can be remodeled to accommodate.  And this is how it works...

What You Need

The premise for this design starts with a bolt, some washers, a reinforcing ring, and a nut.  You take a large bolt and glue it to a piece of MDF cut to fit the inside of  your PR tube.  The center of the MDF must be cut to fit the head of the bolt tightly.  Then you glue this bolt structure to the inside of your PR tube.   That's it.  You're done.  Once the initial glue dries, you can stack and take off as many washers as you need till your tuning is perfect.  No hassle.  Easy and quick.  All you have to wait for is the glue to set just once.  Here is the layout for how the thing is built.  I hope this method works as well for everybody else as it has for me.  As soon as I get the film developed in my camera, I'll have pictures of how I did it to my PR-15.  

A Few Important Notes

Some elemental components that must be considered are:  size and length of bolt to use as well as washer size.  You should kind of already have an idea of the desired tuning frequency of your enclosure and how much mass you will need.  This can be calculated from my site at Tuning PR's.   If you only need 285 grams and you plan on using Avatar's PR-15, obviously it isn't wise to glue in an extra 100 grams which you will not be able to remove ever from your PR.  This method should only be used if you're already sure you will be needing at least 150 grams or more of extra mass.  Because once that glue dries to your MDF ring and bolt, it's in there for good.  I used Liquid Nails For Projects as well as Liquid Nails and basically poured the stuff in there surrounding the ring and a little bit of the bolt itself.  It took almost two weeks to dry completely, but that bolt is in there permanently.  Depending on the size of bolt you use, how much and what kind of glue you use, you can figure the new original mass of your PR.  Mine sits at about 400 grams now.  I'm not sure, I've got to go check that again.   But you get the idea.  The bolt I used was a 5/8" by 2.75"  machine bolt.  It cost $0.44.  The washers are 5/8" standard washers.  They weigh in at 32.5 grams each, give or take a gram or two.  Not all washers of the same size will weigh the same.   So take that into account!  Measure them each individually and add them up or weight them all together to get a more accurate total mass amount.   Now depending upon the size of your enclosure, the mass difference between each available washer weight will affect the tuning frequency by a little or a lot.  A 2 ft^3 box with a 32.5 gram change availability will cause either a 0.3 Hz rise or drop in Fb of enclosure.  Whereas the same design in a 5 ft^3 enclosure will cause a 0.7 Hz rise or drop.  This is the case with the enclosure my Shiva and PR-15 will be going in.  So once again, the bolt size and length as well as washer size is up to you and your design specifications.  Good luck with your new PR Mass Addition Kit.   

Design

Finished Product