How Passive Radiators Work
A passive radiator speaker is a simple device that increases the low frequency response (Bass) of a speaker system*. When used properly, a passive radiator can give a speaker system the comparable performance characteristics of a much larger system; that's the point, in a nutshell.
A lot of air needs to be moved in order to produce audible bass frequencies. Traditionally, when it comes to bass production, a larger woofer meant louder, clearer bass. Passive radiator technology has broken this tradition.
What is a Passive Radiator?
A passive radiator looks like a normal driver (speaker) from the front, but on the backside, it appears to have had all of its "guts" removed. See the picture on the right. A passive radiator is a speaker without the magnet, and electronic structure attached to it; it is just the cone, suspension, and frame. Really though, a normal speaker can be used as a passive radiator, the "guts" would just be superfluous.
Basically, a passive radiator is a reactionary device as the name suggests. When a driver (such as a subwoofer) is mounted to a sealed speaker box (enclosure), the physical forward/back movement of the speaker affects the internal air pressure of the enclosure. When a passive radiator is mounted to the same speaker box, the internal air pressure fluctuations (caused by the movement of the driving speaker) causes the passive radiator to begin moving forward/back as if it was also a driven speaker. When the passive radiator moves, it creates sound frequencies just as a normal driver does. The diagram below shows how a passive radiator functions.
Advantages of Passive Radiators
Passive radiators allow woofers to produce deep tones that otherwise may not have been possible. Depending on the tuning when incorporated, a passive radiator will start to move as the drive woofer begins to produce lower frequencies. By engaging at this point, the passive radiator absorbs energy from the drive woofer thus reducing the drive woofer's motion (we call this dampening). Since the drive woofer is now moving less, there is less risk of damage occurring due to over excursion (speaker moves so far it breaks). There are two main advantages here:
1. The motion of the drive woofer is dampened thus flattening the frequency response; this makes for smooth sound reproduction.
2. Passive radiator systems can have the same sonic output as larger speaker cabinets, but in a much smaller size*.
In a nutshell, when a passive radiator is tuned properly it can greatly enhance the sonic capabilities of a speaker in regards to low frequency production.
Passive Radiator vs. Bass Reflex Tube
Passive radiators are not super advanced alien technology; in fact they are very simple. A passive radiator performs the same duties that a "bass-reflex" port tube/hole does, but a passive radiator has many advantages over ported designs.
The physical positioning of a passive radiator is not as crucial as that of a tuned port hole. This fact is important as it means there is less risk of error during the engineering process.
As stated above, passive radiators also enable the use of smaller enclosures. This is because in order for a port tube to provide the same air mass as the speaker cone, the port tube may be physically large, which would require a larger speaker cabinet. (Remember, the port tube must be inside the cabinet.)
Speaker boxes featuring passive radiators are much easier to adjust/tune than ported cabinets.
Tuning a bass reflex port tube is a very complicated process which involves much mathematics to achieve. Tuning a passive radiator is very simple because all that is needed is an exact duplicate of the powered (driven) speaker. This simplicity greatly reduces the margin of possible error, thus enabling quicker, more accurate tuning.
We hope you found this article enlightening. There are many more details and aspects to passive radiator technology that this article did not cover. Our intention is to provide you with a simple, broken-down explanations of what a passive radiator does and what some of its benefits are. We encourage you to research this topic more if you are seeking more technical information to better understand a passive radiator's function; knowledge is power.
*Please see "All About Speakers", for more details on how speakers work.