Originally Posted by Zorba922
J In full sized speakers that are already crossed over to subs, they are gimmicky at best.
Just because you don't understand how they work does not mean they are "bad"--actually, sometimes they are the only reasonable option.
To put in the simplest terms, passive radiators are "mechanical ports". Think of a tuning fork, have you've ever seen a "adjustable tuning fork?" It is a tuning fork with a weight strapped across the forks (tines?) and you adjust it up and down with the weight to change the tuning frequency. Passive radiators do the same thing, they resonate at a specific frequency (like a port does) and it is tuned with weights.
Passive radiators were theorized in the 1950's and first used by JBL in 1964 so they have been around for quite awhile. Think Polk Audio with their SDA series in the 1980's--they all used them to tune the cabinet.
Why would you use an expensive passive radiator instead of a simple port? The main reason is for very low tuning of small enclosures where a port won't fit. They are also useful if you need ported speakers in wet areas, outside areas to prevent critters from setting up house keeping or a way to prevent port overload "chuffing"
Anyone that has ever modeled a port in a small enclosure, say down below 30 Hz or lower realizes quickly that the port must be very, very long to be tuned low. Get one of those CSS SDX10 subwoofers and attempt to tune it to 20Hz in a 2.2 cubic foot box and it becomes obvious the port lenght will be very long which takes up a ton of space, demands the box becomes very large to handle such a long port and when dealing with long ports--it has it's own frequency it resonates. You don't want a port resonating on it's own at 100Hz even crossing over at 80Hz because it destroys the frequency response etc. Make the port smaller in diameter to shorten the port to prevent it resonating and it will overload quickly because the port restricts the air flow and "chuffs" The solution is to use passive radiators, generally one on either side of the box to keep them in balance and provide enough stroke to prevent damage or distortion. Generally speaking, since you need to purchase two passive radiators and they need to be long stroke (or larger diameter passive radiators) sometimes they cost more than the active subwoofer.
I know this personally, I have a 12" subwoofer tuned to 20 Hz with passive radiators--the PRs cost more than the sub. However, the cabinet I used was the perfect size for the 12 incher if I used "external ports" and my wife was not a fan. Picked up the PRs, they really don't remove internal air space like long ports do (bonus!) modeled them with software to get an idea of how much weight I needed, measured and tested--retuned a bit lower with more weight and done. You change the tuning by adding or subtracting weights and some passive radiators allow you to remove the external cap and do that without removing the device from the box. This allows simple tuning and really helps if you have other subs with different tunes to keep them about the same for better performance.
I have a boombox design sitting on my bench that will use a 2.1 channel chipamp board. A Tang Band 6.5 inch neodymium subwoofer in a 0.65 cubic foot box tuned to 29Hz works very well. The 6.5 inch sub has an Xmax of 12mm so around 1 inch of stroke so it requires a rather large port to prevent chuffing... the box size is too small, the driver dispaces too much air and lightness is required as it will be carried around. The only real option is to use two 8 inch passive radiators which gives an extra bonus, no ports for ants, spiders etc. to crawl into the boom box. Another cool oddity, ports completely unload below tuning frequency but since a PR
is a physical thing--it unloads also but not completely because it must be a mass that requires force to move it. I'm not saying you don't require a high pass filter below tuning, you do but it helps.
Granted, audio salesmen really don't know what they are talking about (as usual) so why would you expect them to inform people what PRs do? Sure, they look cool because more cones increases the coolness factor and I'm sure some manufacturers spec them specifically because they look cool (KEF cough, cough) No harm in that, they have to eat but calling them "Auxilliary Bass Radiators" like it is some kind of high tech new thing is the traditional BS found in audio.
Why would they use passive radiators in bluetooth speakers? Simple, kids like watching cones move back and forth. Since passive radiators will stroke 2 to 3 times farther than an active driver--that is really COOL! Look at some car audio videos, the guy with the most cones and the longest stroke wins.
Now you can sell something that is cooler that the other stuff AND it seals up the box which is a great design idea when using small, portable speakers that get exposed to rain, grass, bugs and teenagers.
In summation, PRs have been in use for the past 56 years--they are not a "gimmick" Sure, they are very, very expensive compared to a plastic tube and they must be designed right but for some uses the only option. Some people equate them to stuffing a sock in your pants--I get it but once you understand how it works--makes sense. My first speaker years ago was a 6.5 inch two-way with an 8 inch passive radiator--wonderful sounding speaker although rather large to hold the thing. These days I use passive radiators for subwoofers when ports take up too much space, I need adjustable tuning, they chuff or resonate so a nice option. I would not use them in cars, PA equipment or use them in vertical orientation but sometimes they are the perfect solution. As with everything in audio, it is always best to learn what it does before throwing it under the bus. PRs need love too!