Exploring Common DIY Terminology - Passive Radiator

Passive Radiator

A passive radiator, often shortened to simply 'PR', is a component used in loudspeaker and subwoofer design to enhance bass performance. It is a type of driver without a voice coil and magnet structure but has a diaphragm and suspension that moves in response to changes in air pressure inside the enclosure. A passive radiator is located within the same volume as the driver it is supplementing, but could be placed on any side of the cabinet. When the active driver moves and generates sound pressure inside the enclosure, the passive radiator reacts to the air pressure changes, causing it to vibrate. The diaphragm's movement results in an additional contribution to the overall bass output of the loudspeaker. Much liked a ported (vented/bass reflex) enclosure, the passive radiator is 'tuned' to the box and driver. Adjustments for tuning are often included with the PR in the form of washers with known mass; adding mass to the PR tunes it lower.

Since the passive radiator has a suspension, it carries associated Thiele/Small parameters. Manufacturers will sometimes include the T/S parameters for various mass combinations, which help a designer determine the correct tuning for a given application. Several freeware programs are available to DIYers for this purpose.

As a system using passive radiators will function very similarly to a properly tuned vented enclosure, one might ask what are the advantages? The usual answer is cabinet volume - often a passive radiator system can be physically smaller than a similar vented system, plus a designer does not need to find additional space in the cabinet for the vent (which could be very long, depending on the diameter and tuning of the port). Ported enclosures also come with some potential downsides, including port noise (chuffing) and port resonances. However, the primary downside is cost; a system may require more than one passive radiator to ensure excursion is not a concern and that adds to the cost of the project. As an example, using a pair of 12" passive radiators with a 12" subwoofer may cost around $200-$250 (at the time of this writing, at least) for the PRs alone. A vented enclosure with a similar tuning may cost less than $10 in PVC for the port.

Cabinets utilizing passive radiators are also commonly perceived as being complex to design, however there are several freeware programs available to DIYers for this purpose, and while the process is a bit more involved than getting a sealed or vented cabinet right, it is worlds easier to design a PR box than a bandpass enclosure!



Main Page