What Is a Passive Radiator [For Subwoofer or Speaker Box]

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What Is a Passive Radiator?

A passive radiator (see our best picks.) is essentially a “passive driver” without magnets and circuitry. It uses the dispersion created by the active driver (speaker/sub) to enhance the low-frequency reproduction. Passive radiators are mounted on the enclosure box along with active drivers to improve the bass response of the sound system.

Low-frequency response (bass) is one of the most critical factors for many people while choosing/designing their car audio systems. While subwoofers are primarily used for improving the low-frequency response of a stereo, they sometimes can’t prove to be enough. This is when you can add a passive radiator to complement the bass driving performance of speakers and subs.

It’s similar to a regular driver (speaker or sub) in its shape. It has a cone, suspension, and frame. However, it doesn’t have magnetic and other field circuitry like normal drivers. In other words, it is not an electric component and is more of a mechanical piece that works with a reactionary mechanism to improve the bass response of a speaker system.

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How Does a Passive Radiator Work?

A passive radiator is used (mounted)in sealed speaker boxes (enclosures) along with a normal speaker or sub. When the driver (sub or speaker) starts producing sound with its protruding movement, it creates forward/backward air movement within the enclosure.

This change in air pressure also brings about forward/backward movement in the passive radiator. When the radiator moves in sync with the active driver (speaker or sub), the enclosure produces more bass.

Benefits of Using Passive Radiators

Passive radiators are primarily mounted in enclosures as bass boosters. However, there is more to the installation. A passive radiator gets in action when drivers start producing low-end frequencies.

This reverb usually happens at higher excursions that can damage the speaker/sub. Passive radiators absorb those reverbs to dampen the driver’s response. This doesn’t just rule out the over-excursion but also smoothens the sound reproduction.

The other benefit is they can let you have more sonic output from a speaker system than other speaker cabinets/enclosures of the same size.

Passive Radiator Calculator

Usually, passive radiators are larger than active drivers in the enclosure. There are a couple of values that you must know to make sure you are using the passive radiator in the right manner. Here we will share with you some formulas to calculate relevant passive radiator parameters.

  • The radius of passive radiator = Square root of Radiator area/Pi
  • The volume of the box = Equivalent compliance of the driver/ the compliance ratio of the passive radiator
  • Tuning Frequency: Harmonics x Resonate frequency of the driver

Passive Radiator vs. Port – Are Passive Radiators Better than Ports?

Ports in cabinets or ported subwoofers also offer the same function as passive radiators i.e. improve the low-end frequency of the sound system. However, a passive radiator lets a wider range of frequencies pass through because they are not restricted like ports with a certain resonant frequency cutoff.

In short, a passive radiator can create a better bass response than ports if it is correctly aligned and tuned.

Bass Reflex vs. Passive Radiator

A Bass reflex tube is just another name for a port. If we compare bass reflex tubes/ports with passive radiators, we can come to the following conclusions.

  • Bass reflex tubes are easier to tune.
  • Bass reflex tubes sound more like a single note. Therefore, whatever bass they produce sounds more or less the same. In comparison, a passive radiator gives the system a fine detailing for low-end frequencies.

How to Tune a Passive Radiator?

Besides the size of the passive radiator and how it is aligned with the driver and fitted in the enclosure, the resonant frequency of the enclosure also plays an integral role in “tuning” the passive radiator.

For many people, tuning is the most difficult aspect of using a passive radiator. You can make it easy by using WinISD. It is a freeware program that lets you use driver specification to simulate how the driver will act in a particular alignment given a certain enclosure volume and radiator size.

Hey, there mobile audio lovers! My name is Vincent Talbot, founder and chief editor at 99carstereo.com. Ask any mobile audio fanatic, installer, or company rep what makes a good car speaker, sub, or amp, or, better yet, why he or she prefers a certain brand over another, and be prepared to endure a litany of opinions, viewpoints, and passion-fueled perspectives. To be honest, mobile audio shopping can be a daunting task without a guide, so I’ve assembled what I feel are the best products to consider to make things easier for you. More.

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