As a long time audio enthusiast one of my goals has always been to build a passive radiator subwoofer for my home audio system.
This is the speaker I used to hack my own passive radiator.
So what is a passive radiator subwoofer design? Basically its an alternative to a standard ported subwoofer box. Instead of tuning to a specific post size and length, you tune using a passive speaker or more specifically a speaker with no magnet or driver.
Radio Shack Subwoofer
I’m using radio shack subwoofers I bought up to 12 years ago. I’m not sure you can even purchase them in the store anymore. However they we’re inexpensive at the time and at least provided me with some feasible test speakers.
I used MDF for the box. Luckily the local hardware store has a $10 2 ft X 4 ft sheet.
I tacked the box together with a little finishing nail gun just to hold the pieces together. I then pre-drilled the many holes for the screws to pull it all tight. If you’ve ever seen one of my speaker boxes I typically go a little overboard with screws, but if you’ve ever worked with MDF is notoriously soft. I set the torque on my drill to 5 out of 20 when I driver in the screws to keep them from stripping out.
The final job is pretty unpolished at the moment. But as a proof of concept I think it’s pretty good. If you’re thinking of trying to build on yourself, let me give you some pointers I’ve learned so far.
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Box Interior Close up
Make grooves on the edges of your pieces where they fit together and use glue. The groove provides a place for lots of glue, and in the end the box will be super strong and more air tight.
- Silicone the inside joints on the box to ensure air tightness. Also allow plenty of drying time(24 hours at least). The chemical reaction of the silicone drying can destroy certain types of speaker surrounds, better safe than sorry.
- Use t-nut fasteners for the speakers. Using the t-nuts will allow you to change the configuration without destroying the precious MDF box. I typically use 1/4 20 bolts of some type. Look in the parts bin at your local hardware store and you’ll find them.
- Always solder the interior wires. Its a better connection that won’t accidentally move and cause crackles when the bass is thumpin
- Most terminal connectors are not air tight at all, and once installed they can compromise the air tightness of your box. Even worse your at the last moment about to install speakers and now you have to silicone the back of the plate! AHHHH!!!
Hot Glued Terminal Plate
Fear not! Grab a hot glue gun instead. Its really easy to completely backfill the connector plate with hot glue, and create a great air tight seal in less than 15 minutes.
- To create a post for your passive reciever use a carriage bolt with a wing nut. It makes changing the weights a snap.
- For weights I used fender washers from the local hardware store. I bought a bunch and then tested with different weights till I was happy with the sound.
- I use 1 inch camper foam on the back side of the speaker lip to help create a good air tight seal with the box. Its not perfect, but its easy and replaceable whenever I want to switch the tuning again.
In the near future I hope to veneer the outside with beech or poplar to stain. I’ll then add legs to create a downward/upward facing subwoofer. I’ll probably add legs to both sides of the box so I can add a table top as well. If I use basic ikea legs on both sides I can easily switch the configuration in th future if needed. I still have to find out which way will sound best with the passive radiator toward the floor or the ceiling? Most likely the addition of a table top also provides an opportunity for change. When you add up the pieces you can see why I wanted a box I can change the tuning on.
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