Passive radiators

[ZO6killinsbf 10+ year member]

Whats the deal with these? I've read that they act like a 2nd subwoofer but without being a 2nd sub and without the extra box space? Is this correct? Hows that work? Also, how can you make "on the fly" box tuning with these? I rarely see these used so it is a subject I am curious about. Thanks

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[Rich B 5,000+ posts]

A speaker enclosure using a passive radiator usually contains an "active driver" (or main driver), and a "passive cone" (or drone or passive radiator). The active driver is a regular driver, and the passive is typically the same or similar, but without a voice coil and magnet assembly. It is only a suspended cone, not attached to a voice coil or electrical circuit. The passive radiator usually has some means to adjust its mass, thereby allowing the speaker designer to change the box tuning.

Internal air pressure produced by movements of the active driver cone moves the passive radiator cone as well.

Passive radiators are used instead of a reflex port for much the same reasons—to tune the small volume and driver for better low frequency performance. Especially in situations in which a port would be inconveniently sized (usually too long for practical box configurations). They are also used to eliminate port turbulence and reduce motion compression caused by high velocity airflow in small ports (especially small diameter ones). Passive radiators are tuned by mass variations (Mmp), changing the way their compliance interacts with motion of the air in the box. The weight of the cone of the passive radiator should be approximately equivalent to the mass of the air that would have filled the port which might have been used for that design. Passive radiators do not behave exactly as do (more or less) equivalent bass-reflex designs in that they cause a notch in system frequency response at the PR's free air resonant frequency; this causes a steeper roll-off below the system's tuned frequency Fb, and poorer transient response. Due to the lack of vent turbulence and vent pipe resonances, many prefer the sound of PRs to reflex ports. PR speakers, however, are more complex to design and likely to be more expensive as compared to standard reflex enclosures.

The frequency response of a passive radiator will be similar to that of a ported cabinet, with two exceptions. The system low frequency roll-off in a passive radiator design will be slightly steeper, and will have a notch (dip) in frequency response due to the Vas (compliance, or stiffness of the speaker cone) of the passive radiator. The goal in designing a passive radiator is to adjust the tuning so that this notch is below audible levels.

http://en.wikipedia.org/wiki/Passive_radiator_(speaker)

 

[truckramrod 10+ year member]

passive radiators function as a port does in a vented enclosure. to tune them you can change the diameter, amount of passive radiators, and the amount of weight attached to the cone to change the tuning.

I believe you do have to use a larger enclosure similiar to a ported enclosure though I could be wrong.

Edit: this is a very general explanation and not at all a guide to using a PR setup in your car.

I used to own some 1980's JVC floorstanding speakers with 12" woofers with PR's and they had some great low end for such low quality drivers and building materials.

 

[perfecxionX 10+ year member]

AE Speakers --- Superb Quality, Unforgettable Performance, Definitely.

 

[mikeyr4444]

I got to work on a pair of KEF reference monitors with passive radiators in them, they add more to the mix then you would expect.

 

[CHEMMINS 10+ year member]

Rule of thumb is 2 passive radiators the same size as the sub used.

 

[ZO6killinsbf 10+ year member]

Awesome info. Thanks guys. So basically cost and complexity are the main reasons they aren't used more often?

 

[Wingman0121 10+ year member]

Rule of thumb is 2 passive radiators the same size as the sub used.

True but he can get away with using just one passive radiator if that radiator has at least twice as much excursion capability than the actual driver, it does put more wear and tear on the PR tho.

 

[Audiohazzard 10+ year member]

Sunfire home subs uses a PR in 1cuft for there 12" EQ series and they absolutely rock!!! I had to get rid of mine due to complainants. Lol

 

[R34P3R 10+ year member]

AE Speakers --- Superb Quality, Unforgettable Performance, Definitely.

Great Read. I consider myself learn't.

 

[T3mpest 10+ year member]

One thing to take into consideration with passives that few guides will tell ya. Even though the box can be small since there is no port takaking up air space, you still have to make sure the box is big enough to allow clearance inside for the passives to not hit the main drivers. If you do a 2 sub setup with the max height you can fit in your trunk then you need 4 passives, so some woudl be on the side. Since 2 sub setups are common i though it'd bring this up. If space is an issue and that's why you used them you do still have to be careful with yoru dimensions on the box and watch which way they fire.

Anyway I do use PR's in my setup. 2 av15h's in 4.7 cubes tuned to 25hz. It really does allow you get the extra outuput of ported without giving up all your space. Plus you can tune low easily this way. I do feel like it gives up just a bit on some of the lows compared to some smaller sealed setups. Not in actual SPL, but more in frequency response. Just a bit easier to get that strong low end balance I and alot of daily listeners want. White Girl style moving air lows. Super lows the car digs tho SNoopdan "love story mix" moves my rear seat and deck an inch or two on that 20hz opening. This is in a cadillac too, not a litttle toyota. Just saying for those who might try to go a normal size up from what they can fit ported since they saving air room, if you still cant' quite give the drivers normal air room, while it will sill play low, it wont' do it with the response you normally expect from such a low tuning.

IDk what' I'd do in my car if i reddid it. 12's with Pr's could play silly loud at tunign and when you tune low that is still pretty musical but fun. My box now has the frequency response of a big sealed box, but just has great overall output since I used 2 15's do it it. A sealed box with 15's wouldn't have the extra oomph around 40hz but still extend that low.

 

[Evyn1]

Rule of thumb is 2 passive radiators the same size as the sub used.

What about an 18” PR X DD 712?