i was just wandering something last night. i was looking at a pair of old speakers pioneer i think. and they had the passive radiator. and you know to be 30 years old they sounded really good. had alot of bass and everything. i was just wandering though. does anyone know why p rads never really became a big thing? and if i could find a good set of new ones you think theyd make for car boxes?

 

I'm not too familiar with passive radiators, but one reason they may not be very popular is cost. They essentially replace the port on a ported box. As I understand, a passive radiator is basically a speaker that has no voice coil or magnet. It still has a cone, basket, surround, etc which probably adds up to a lot more then a plasic tube.

I'm sure you could use them in your car, but i think they would probably take up more space then an equivalent ported box.

 

Passive radiators and ports both work as delay devices...

The goal theoretically is to cause a delay of exactly 1/2 of a wave, because the sound energy inside of an enclosure is exactly 180 degrees out of phase with respect to the sound energy coming off the front of the subwoofer cone... since when a subwoofer moves out, it pressurizes the air in front of the cone, and depressurized the air behind the cone... and vice versa when the cone moves inwards...

That's what an enclosure does, in fact!
It's fundamental goal is to prevent the pressure nodes from the front of the cone from dissipating into the pressure anti-nodes behind the cone.. that's cancellation!

These delay devices - ports or passive radiators - act to delay the sound exiting the box, so that by the time they exit the box they are nearly 1/2 wave delayed... aligning the peaks with the peaks, and the dips with the dips...

Only thing is that it IS somewhat frequency dependent...
Near the "tuning frequency", you will be close (but never exactly perfect) to in-phase with respect to the sound coming off the front of the cone, but the further away you travel from that tuning frequency, the less in-phase you become, and the less advantage you have.


With passive radiators vs. ports, effeciency is the key!
With ports, it's just air moving.. there's no moving mass, per se. It's inherently efficient.
But with passive radiators, the sub stimulates the air in the box, which in turn stimulates a passive radiator... which in turn stimulates the air outside the box!
And the passive radiator has mass.. and it has a suspension, which has compliance...
Simply put, it's not as efficient as a port.
When you factor in that 10' of 4" PVC can be had for under $10, compared to about $50-$150 for a passive radiator...
And the vast knowledge out there on how to tune ported enclosures, compared to those with experience tuning passive radiator enclosures...
It does help explain the popularity of ports.

But... that being said..
passive radiators make GREAT sense in a truck!

You guys are typically running tiny enclosures to fit behind seats and things...
And in tiny boxes, port lengths can become prohibitively long (again, they are delay devices.. and their length is dictated by the port diameter you choose, and the size of the enclosure).
Passive radiators make GREAT sense here!
Don't hesitate to use one... it's mostly out of lack of knowledge and typical ease of construction and cost savings that ported enclosures rule the market.

 

it just seems like with some more knowledge in this type of loading the cone that there is the chance to have more efficincy with a p rad over a port. personally i run cloded boxes because of the tuning prob. i need acracy in myne. but unless i want to use multiple amps with a xover tuned for each box (to much stuff to break for me) i need to stay seeled. i just wonderd that the other night and didn't have an answer that made sense to me. cause im always looking for the extra Db.

 

It's not a lack-of-knowledge thing... It's tied to fundamentals.
You won't ever be able to acheive higher SPL with a passive radiator than you will with a standard port of equivalent tuning...
because of laws of physics, which dictate that there is less resistance in moving air (a low-mass gas) than there is moving a rigid panel/cone (a high-mass solid).

Or, worded another way...
if you were to achieve big SPL with a passive radiator setup, you will be able to find a port that offers you even higher SPL.

Think of it this way.
With a port, you have air moving through the port, nothing else.
The actual pressure fluctuations inside the box are allowed to escape, in an inherently slightly delayed fashion, no real interferance.
With a passive radiator, you have the pressure fluctuations inside the box that have to act on moving a passive radiator cone/panel.. there are inherent losses there (both in just moving the mass, and in the lack-of-compliance of the PR's suspension)... which in turn acts on the air outside the enclosure.
It's very much inherently less efficient.

Read my post above, I believe I explained it a little better above..