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One way to boost the bass response of a loudspeaker is by incorporating a passive radiator. Think of a passive radiator as a speaker without its electromagnetic parts (magnet assembly and voice coil).
How Are They Different?
Passive radiators (also known as passives or drone cones) are different from speakers because they tune the volume of an enclosure to a particular frequency, turning it into a Helmholtz resonator–much the same as a port tube or vent–that works in conjunction with speaker driver(s).
A passive radiator moves back and forth to create sound thanks to the fluctuations in air pressure within an enclosure based on its resonant frequency. The resonant frequency of a passive radiator is determined by its mass and compliance (or springiness). A passive radiator that has a tight suspension and low mass will tend to have a higher resonant frequency, whereas a passive radiator with a loose suspension and more mass will have a lower resonant frequency.
Different materials are considered when designing passive radiators to achieve a particular tuning frequency while ensuring the suspension remains linear (operating within its design characteristics without adding additional distortion).
Air pressure inside the enclosure compresses and then springs back, naturally, like a spring. Passive radiators work with the enclosure’s fluctuating internal air pressure (caused by a speaker driver) and excite a resonance peak, to create sound output.
When Are They Used?
Passive radiators can be useful when you want to reduce the weight and size of a ported speaker enclosure. Since there isn’t air moving in and out of a port, there is also the side-benefit of little to no “port noise” or “chuffing” as long as the passive radiator is of the correct size. Additionally, a passive radiator can help to reduce the travel of the speaker used with it, at around the resonant frequency, and can help improve power handling.
You’ll find passive radiators in a lot of home stereos, subwoofer cabinets, car audio speaker systems, Bluetooth speakers, and some studio monitors. However, passives have lots of uses where richer sounds at lower frequencies are needed but the enclosure size cannot accommodate an appropriate-sized port tube.
Resource: Creating Compact OEM Speaker Systems
Design Considerations for Passive Radiators
Passive radiators must be tuned along with the enclosure and driver so that a speaker produces the desired sound. A general understanding is that the larger and heavier a passive radiator is, the deeper its bass performance will be. Adding mass (weight) to a passive radiator will lower the tuning frequency.
The size of the passive radiator should correspond to the size of its enclosure and the speaker. A good rule of thumb is to have a passive radiator that has twice the surface area of the speaker driver.
Passive radiator speakers are a great and generally inexpensive option if you’re looking for a sound system that is light, portable, requires less power, and tends to be more durable. If you have questions about whether a passive radiator is right for your sound system needs, please contact us today and we’d be happy to talk.
