[Skilly]

I have learned so much by reading the threads here. Thank you to all contributors. I really enjoy Danny's tech talk video as well, although I have to admit, he loses me sometimes.

Anyway, my question is why do we never discuss passive radiators here? What are the benefits and drawbacks to them vs Ports? How do they impact on speaker placement vs. ports?

Thank you in advance for the tutorial.

[JLM]

Passive radiators have no port chuffing and are less sensitive to being placed too close to walls (although I feel that issue is overblown as even sealed speakers suffer from close placement and proper imaging/soundstaging requires placement farther away from walls than port issues would ever be a factor).  In extreme cases the mass of a single passive radiator located on one side of the speaker can cause it to lose a degree of stability.  Of course passive radiators cost more to build than a tube.  It amazes me how speaker manufacturers scrimp on stuff that then spend half their budget on veneers. 

[hawkeyejw]

In terms of GR-Research, Danny has designed speakers with PRs in the past, but I think once he figured out open baffle servo designs he put his focus there, as that is where you're able to get the most performance.

[roscoe65]

I’ve only own a single pair of speakers with passive radiators (Polk SDA2, back in 1986 or so) so don’t have a lot of direct experience, but can offer some thoughts:

A passive radiator is a method of creating a lossy box.  In a typical sealed box, as the driver excursion increases the pressure within the box increases, and as the driver goes to opposite excursion, the air in the box is rarified.  The pressure or vacuum within the box tends to help the driver return to a neutral position.  This may result in some compression at higher power levels as the driver works against increasing air pressure.  If we add a passive radiator, it effectively makes the box bigger.  As pressure within the box increases, the PR moves outward at a rate and magnitude determined by its mass and suspension compliance.  The box becomes leaky above the box pressure that begins to move the PR up to the limit of the PR suspension.  Above that suspension limit, it acts like a slightly larger sealed box.

Within a certain power band, the box behaves similarly to an aperiodic alignment.  I would expect some smoothing of the acoustic impedance in the bass, with a sound that is still reminiscent of a sealed alignment.

One unusual example that I have not heard personally but trust the ears of those who have is a pair of vintage Coral Beta 10 speakers and factory 10” passive radiators in a factory PR enclosure.  These drivers are also used in a BLH enclosure.  In the PR enclosure, the passive radiators effectively double the cone mass of the active driver, allowing it to be used in the sealed box.  The second cone also increases the radiating area, boosting the low frequencies.

[Faralon]

You can make smaller boxes w/ PR's too, as they effectively count as volume for an enclosure.  My Emotiva S8 Flex sub is a good example.  8" down firing driver, 8" front firing PR.  Small little box with lots of boom!

[walkern]

One advantage of using a PR instead of a port is when you wish to use a very compact boxed subwoofer or bookshelf speaker, and you still wish to 'tune' the box to a very low frequency.  The length of the port to get a very low frequency tuning point would be too long to fit inside the box.  The compact Sunfire subwoofers are a classic example of this.  Bob Carver mass loaded the PR, used a very stiff suspension, and got tiny boxes to allow significant cone excursions and produce a LOT of very low bass.  The downside (there's always gotta be at least one) is that the PRs mass tends to want to make it keep moving briefly after the low frequency signal has stopped (inertia)... so it is tricky to get tight and fast sounding bass (there is more overhang... when the bass note stops, the driver and PR keep moving for a while longer).  If you want to use multiple large woofers in a large box (ala Brian Cheney's subs) using a PR allows you to really fine tune the low frequency cut off by dialing in the exact amount of mass loaded to the cone of the PR. The roll off rate below the low frequency tuning point of the enclosure is slightly less steep using a PR instead of a port, but not quite as shallow as the roll off rate for a sealed box or transmission line.

[BobM]

The placement of the port makes a difference in the answer here. I personally don't like front facing ports, because sometimes I hear the chuffing. But rear ported speakers need space behind them.

[toocool4]

Have a look at this vid it may help. https://www.youtube.com/watch?v=g4M3rb8QMSU