Passive Radiators

An 8" Passive Radiator on the Back Side of a Small BoomBox

 

What exactly is a passive radiator, when and why would you use one?

I remember my first encounter with a passive radiator.  I was in high school and I was browsing a catalog (way back in the stone age before we had the internet).  I was smitten with some very sexy tower speakers that had multiple 6.5" drivers.  Reading through the stats I noticed that 2 of the 3 drivers were passive radiators.  That same catalog had pre-loaded car audio subwoofer enclosures designed to fit under the front seat of a car, these designs used passive radiators, also called drone cones.  Several manufactures have made these over the years.  The modern version is the JBL Fuse, pictured below.  My initial gut reaction was that this was a rip-off.  A speaker with no magnet or voice coil that gave the illusion of multiple drivers.  I could not have been more wrong.

JBL Fuse Subwoofer (Click on Image to View on Amazon)

It turns out that these drone cones have some interesting uses and are a great way to get some extra bass out of small enclosures.  Back in the 1970's when Kicker started building subwoofer cabinets for cars they used passive radiators.  Why?  Because passive radiators shine when you are trying to build small enclosures designed to fit into tight spaces.  Kicker has even brought back a line of enclosures that utilize passive radiators.  Heck they will even sell you a passive radiator that you can use in your own custom enclosure.  

Kicker 8" Passive Radiator (Click on the Image to View on Amazon)

What is it and when do you use one?

For all practical purposes a passive radiator is nothing more than a very complex port.  That sounds strange.  I have a blog post on how a port works.  Here is the tl;dr version.  When a subwoofer compresses and rarefies the air in a ported enclosure it will cause the air in the port to resonate. The port air has mass, so it takes some effort to move it, so it only resonates at a very specific band of frequencies.  When you do move it it will cause sound waves in the exact same way that a speaker cone causes sound waves.  You can adjust the port tuning by changing the opening (the cross section) and the length of the port. 

Passive radiators have mass, you tune them by increasing they size (or number) and by adding additional mass to them.  They are not as effective as ports, and they can be a lot more expensive.  But they have one specific situation where they really shine.  Small enclosures.

As I discuss in my port tuning series on YouTube as the enclosure gets smaller the port has to get longer in order to maintain the same tuning frequency.  

 

So if you want to fit a ported enclosures behind the seat of a regular cab truck, under the seat of a crew cab truck, under the front seat of a car, or ANY other tight spot you will need to make the enclosure very small. So then you need an absurdly long port.  You can't get the port to fit in the required space, so you use a passive radiator.  There really is no other good reason to use one.  I will say that again just so we are clear.  A port will out-perform a passive radiator at a lower cost in all but one case, when you are trying to make a very small enclosure.

JBL Bluetooth Speaker (click on image to view on Amazon)

 

Passives also work very well in boomboxes and "Pill" shaped Bluetooth speakers.  Parts express offers several compact Bluetooth boomboxes that make use of a passive radiator.  

 

I own a quad-cab dodge truck and I desperately need some upgraded bass.  So I have been experimenting with pervasive radiators.  My eventual plan is to create a custom fiberglass enclosure and make use of passive radiators to kick my performance up to the next level.  I have already experimented with some test boxes and have learned a lot about them.  Stay tuned so you can see what happens when I figure out how to work with fiberglass!  

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