Is a passive radiator a free lunch?

[oldenough]

I was giving my Kg-4's a listen today and i got to thinkin' (something i try to avoid). What is the rationale behind the idea of a passive radiator. At first glance it would seem to be the elusive free lunch, but after more thought that can't be, otherwise it would be more widely used than it is. So could someone enlighten me to the reasoning and pro's/cons behind this idea. I apologise right up front for my ignorance.

[Mark Hardy]

A passive radiator is a special case of a vent (or port). Its major merit is that it cannot "chuff". They are (to this day, AFAIK) usually tuned empirically, by adding mass to the PR diaphragm.

I have owned, and enjoyed, Polk Monitor 7As for almost 30 years. Polk made good use of PRs in their early designs, starting with their first consumer loudspeaker, the Model 9.\

EDIT: Here's a pretty good overview of the PR from good ol' Huw Powell of Human Speakers (keeper of the EPI/Epicure/Genesis flame):

http://www.humanspeakers.com/whatis/PR.htm

'n here's a Polk 7A, with a 10" PR and 6" "midbass":

'n HERE's an original Polk Model 10 (10" non-coated PR and two 6" midbass drivers):

[Daddy Dee]

[]The PR also keeps mice out of the cab.

[oldenough]

Thanks for the link Mark, i have a better understanding of how a PR works, but it still seems to my untechnical mind that better results would be achieved with an active woofer. What am i missing here ? Is it that you dont have to design a 3 way system, ie cross-over etc, or do these things still come into play. Again i apologise for my lack of understanding.

[oldenough]

The PR also keeps mice out of the cab.

Okay! Now all we need is JacksonBart to come along with a smart reply and i'll have a complete understanding[]

[Mark Hardy]

...but it still seems to my untechnical mind that better results would be achieved with an active woofer.

Indeed it does, comparing apples to apples.

http://www.diysubwoofers.org/prd/

And of course, all things being equal, the rate of LF rolloff of an acoustic suspension speaker (6 dB/octave, IIRC) in a sealed box is much shallower than either "ported" design shown in the graph above.

I think Polk, for example, went with a PR in their early speakers

1) To enable use of a small woofer and a relatively small box and still get decent LF performance. The 6" midbass was a good sounding, good performing midrange driver; a 12" woofer likely wouldn't be.

2) To save the expense of an onboard "subwoofer" and crossover. A separate woofer in a small sealed box could've actually provided better LF performance, but at considerably enhanced cost.

3) It was fashionable at the time

4) It does keep small mammals out of the speaker box.

I think reason 4) is the best one, looking back over 30-plus years!

EDIT: More on PRs (good ol' Google):

http://www.subwoofer-builder.com/passive.htm

The advantages of a passive radiator design are:

No port noise

Low tuning can be achieved in a small box. This normally requires long ports with troublesome "pipe mode" resonance and compression

Disadvantages:

Cost

Faster rolloff may require a high-pass filter that an equivalent ported system might just scrape by without.

[Daddy Dee]

Well, I'm a really non tech type here, so my understanding is always lay and simplistic.

In picking up on a few conversations among folks who really do understand this stuff, I have come to understand that there is more to a PR than I thought at first. They allow for more fine tuning than ports do and that Klipsch added PR's to some speakers, like Chorus II, indicates that there is merit to the idea. They are more costly than ports which also suggests that Klipsch thinks so to add cost to the design.

In addition to the size of the PR, they are also variable in the amount of mass and stiffness, which would be similar to variations in port diameter, determining how much and how easily air moves out of the cab.

[jacksonbart]

Its like an olive loaf on pumpernickle

[STL]

Sometimes PRs are used instead of ports because a properly sized port (for the venting freq. desired) won't fit in the enclosure -- whether it's due to the length of the port or its diameter. I have replaced the PR with a port (keeping the same venting frequency) on both a KG4.2 as well as a pair of KG3.2s -- in both cases the change was seemingly transparent.

[oldenough]

Thanks everyone, interesting, so it seems it's just another way to skin the same cat.

Bart, as usual thanks for the invaluable input[]

[oldbuckster]

Isn't it just a way to enhance lower bass response by using the speakers own cabinet mass along with the passive radiator,instead of a noisy port, or adding a sub woofer. Not many people used sub woofers back in the 70's, yes some did, but most didn't ................. Sub woofers never really took off until Home Theatres became popular.......I remember taking a pair of Acoustic Suspension speakers, and drilling ports in them ................. That was dumb, sounded awful, I'll let others design speakers, ..............

[DrWho]

Faster rolloff may require a high-pass filter that an equivalent ported system might just scrape by without.

That

picture from diysubwoofers.org is extremely misleading and I can only

imagine how many people are referrencing it (like the above quote).

The

largest advantage to a passive radiator is that it offers an extra

degree of freedom over a standard port. Namely, with a passive radiator

you can choose the compliance of the suspension, the surface area of

the cone, and the mass of the vibrating area. With a port, the

"compliance of the suspension" is directly related to the mass of the

air which is determined by the volume and surface area of the pipe.

Basically, you don't get to arbitrarily choose the compliance of air.

Ultimately, ports are terribly non-linear devices where the tuning

frequency is constantly shifting around and there is all kinds of power

compression too. Passive radiators solve all those problems, but also

allow the engineer to choose the Q of the tuning point by balancing the

ratio of suspension compliance to diaphragm diamater. In other words,

there is absolutely no reason why a passive radiator couldn't be tuned

to have a lower Q than a classical port (which is to say that there is

no reason that it couldn't roll off slower).

The diysubwoofers.org picture is misleading because they aren't taking

advantage of the Q control that a passive gives you. They design their

passive radiator suspension to approximate the compliance of air in a

port the same diameter, which of course will yield a sharper Q.

However, it is possible for one to deviate from that approach and

achieve a slower rolloff. The classic tradeoff here is gain versus

bandwidth. As you make the Q wider, the gain becomes less - which

requires a different tuning alignment on the active driver.

To ask "why wouldn't I use an active driver?" is the same thing as "why

wouldn't I use a port?" - in which case, the answer mostly comes down

to physics and money. Adding another driver just increases the gain of

the system at every frequency (assuming you also double the internal

volume of the cabinet). The only way to make the system go lower by

moving to two active drivers would require a significant redesign of

the active drivers - which ultimately is going to sacrifice the "high"

frequency behavior of those drivers (which may or may not be an issue

depending on the system the driver will be used in). Assuming that the

high frequency behavior was engineered to be just good enough, then

trying to increase the low frequency extension by adding another driver

isn't an option. However, with a port/passive radiator one can use that

same driver and extend the low frequency response - and do so while

decreasing the distortion of the system (because at the tuning point,

the active driver's excursion is at a minimum). The cabinet needs to be

about 1.5x larger which is better than 2x larger. Or if one doesn't

want to make the cabinet bigger, one could change the active driver to

not dig as low and raise the tuning frequency to yield the same

bandwidth as the original, but with increased efficiency.

Ultimately, it's all comes down to choosing the least compromised

solution given a fixed price point and a fixed cabinet size (since

money and aesthetics seem to be the main driving force of the industry).