Question about a Cambridge Audio sub

[Nanaea]

Hey all,

I was looking at sub woofers and I came across this Cambridge Audio X500 and I noticed it has 2 drives unites (I think) but they say one is active and the other is passive, what exactly do they mean? Only one drive is being powered while the other sits still??

Link is here: X500

 

[theJman]

"Sits still"? That's a good one...

A passive radiator is a bass reflex style of subwoofer without an ports. The inactive driver utilized the backwave pressure from the active driver to create sound. A pretty good explanation can be found here. The concept has been in use for many years because it allows a smaller enclosure to produce deeper bass than would be possible if it were acoustic suspension.

The CA subs don't strike me as being cost competitive, but I would imagine they're good.

 

[KEW]

The inactive driver is commonly called a passive radiator. The Jman gave good info, but I find it convenient to think of a passive radiator design as splitting the difference between ported and sealed sub designs.

 

[Steve81]

Just for a little further explanation (at least to the best of my (pitifully limited) knowledge):

A passive radiator works exactly as a port would. The PR has a resonant frequency, just like a port, where it will make its contribution. This resonant frequency is controlled via adding or removing mass to the passive driver.

The advantages over a standard ported bass reflex design are A: no port noise B: a smaller enclosure. The smaller enclosure is possible given that you don't have to route a (occasionally lengthy) port through the enclosure. The primary disadvantage of a passive radiator is cost.

 

[theJman]

Just for a little further explanation (at least to the best of my (pitifully limited) knowledge):

A passive radiator works exactly as a port would. The PR has a resonant frequency, just like a port, where it will make its contribution. This resonant frequency is controlled via adding or removing mass to the passive driver.

The advantages over a standard ported bass reflex design are A: no port noise B: a smaller enclosure. The smaller enclosure is possible given that you don't have to route a (occasionally lengthy) port through the enclosure. The primary disadvantage of a passive radiator is cost.

For someone who has "pitifully limited knowledge" that was a pretty darn lucid explanation. Perhaps you know more than you think...

 

[Steve81]

Thanks Jim! Of course, it seems like a lucid explanation until someone a bit smarter comes along and says "Steve81 is full of BS. This is how it really works."

 

[theJman]

Thanks Jim! Of course, it seems like a lucid explanation until someone a bit smarter comes along and says "Steve81 is full of BS. This is how it really works."

Don't sweat it this time. You're good.

 

[alphaiii]

The primary disadvantage of a passive radiator is cost.

And steeper rolloff below tuning, IIRC.

 

[Steve81]

And steeper rolloff below tuning, IIRC.

Can't claim to have read much about that. A cursory search of the Klipsch SW-311 review at AH yields

Klipsch SW-311 Subwoofer Review Measurements and Analysis — Reviews and News from Audioholics

As you can see from all of the measurements, at about 28 or 29Hz it starts what appears to ultimately be a 48dB/octave roll off, which is a function of the standard 24dB/octave roll off of a bass reflex alignment below tuning, plus what is likely a 24db octave rumble filter applied just below tune as well. This type of thing is standard with most bass reflex alignments to prevent unwanted cone excursion below the system tuning

I did find some mention elsewhere though that a PR would have a slightly steeper roll off due to frictional losses associated with the surround versus air just traveling through a port. In any case, I'm not sure how big of a tradeoff/problem that would be. Ideally, you're filtering out content below tuning anyway to protect the driver, as mentioned in the last part of the quote.

 

[Nanaea]

Thanks for the help guys .

Sorry about the "sits still" what i meant was that it didn't get any power and any movement it had wouldn't be caused on its own.

So i guess this sub turns out to be more interesting that what i had imagined at first