Passive radiators replace the port in a vented enclosure with a tuned, undriven cone. The result: the low-frequency extension of a ported box without the larger internal volume or the port noise. Audiomobile's M-CAR Series uses Modular-Coupled-Acoustic-Reflex technology to deliver up to 2 inches of peak-to-peak excursion in a low-profile cast-alloy frame, designed to pair with Audiomobile GT and E-Series subwoofers in compact-enclosure builds. Available in 8, 10, and 12 inch sizes. Series breakdown and integration notes below the lineup.
Why Buy From Audio Intensity
CNC-Matched Enclosures
Spec-built to your driver's Thiele-Small parameters
A passive radiator is a tuned, undriven cone mounted in a sealed enclosure to extend the system's low-frequency response. It works by acoustic coupling. The active subwoofer's cone movement pressurizes the enclosure, and that pressure drives the passive radiator's cone in opposition. The radiator's mass and compliance are tuned to a specific frequency, where it contributes additional output below the active driver's natural roll-off.
The result is similar to a ported enclosure: extended bass response below the system's resonance frequency. The differences are practical. A passive radiator doesn't have port noise. It needs less internal volume than a properly tuned port. And it can be tuned to a specific target frequency by adjusting the radiator's added mass.
When a Passive Radiator Makes Sense
Compact Enclosures with Limited Volume
The most common application. A buyer wants the low-end extension of a ported box but doesn't have the internal volume to build one properly. A sealed enclosure with a matched passive radiator delivers similar bass extension in a smaller footprint. For under-seat truck builds, sport coupes, motorcycles, and stealth installs where every cubic inch matters, this is the right tool.
Port-Noise-Sensitive Builds
SQ builds where chuffing or port turbulence would compromise the listening experience. A passive radiator has no port to make noise. The cone moves cleanly, the bass extends, and there's no airflow artifact at high excursion levels.
Vehicles with Vibration or Mounting Constraints
Some installs (motorcycles, off-road vehicles) introduce vibration that can affect ported tuning. Passive radiators are mechanically tuned, not pneumatically, which makes them more predictable in vibration-sensitive applications.
The M-CAR Series Lineup
Three sizes available, each tuned for a specific subwoofer pairing.
M-CAR 200 (8 inch, $149.95): Pairs with Audiomobile 8 inch and 10 inch subwoofers in compact sealed enclosures. The smallest profile in the lineup. Suited to under-seat builds and tight custom installs where a vented box won't fit.
M-CAR 250 (10 inch, $199.95): The most versatile size. Pairs with Audiomobile 10 inch and 12 inch subwoofers in mid-sized sealed enclosures. The right pick for most behind-seat truck builds and sealed sub installs that need extended bass without going ported.
M-CAR 300 (12 inch, $229.95): The high-output size. Pairs with Audiomobile 12 inch and 15 inch subwoofers in larger sealed builds. For installs where you have the volume for a full-size sealed enclosure but want the extended response of a ported box.
How to Choose the Right Size
Match the Radiator to the Subwoofer
The passive radiator should be sized to roughly match the active subwoofer's cone area. An 8 inch sub pairs with the M-CAR 200 (8 inch). A 10 inch sub pairs with the M-CAR 250 (10 inch). A 12 inch sub pairs with the M-CAR 300 (12 inch). Going larger on the radiator extends the tuning frequency lower; going smaller raises it. Most builds work best with matched sizes.
Calculate Internal Volume
Passive radiator enclosures need less volume than ported equivalents but more than purely sealed. As a starting point, use the manufacturer's recommended sealed volume for the active driver, then add 15 to 25 percent. The exact target depends on the driver's Thiele-Small parameters and the radiator's tuning. Use the EBP Calculator with your specific driver to dial it in, or contact our tech team for a sealed-with-radiator design.
Tune the Radiator
The M-CAR series ships with adjustable mass weights that let you tune the radiator's resonance frequency. Adding mass lowers the tuning frequency (extends the bass deeper). Removing mass raises it. Start with the manufacturer's recommended mass for your active driver and tune by ear or measurement. Get this wrong and the system either lacks low-end punch or sounds boomy at one specific frequency.
Pairing With Audiomobile Subwoofers
The M-CAR series is engineered specifically to work with Audiomobile's GT and E-Series subwoofers. The Thiele-Small parameters and acoustic loading are calculated to mate with Audiomobile's driver characteristics. While M-CAR radiators can technically work with subwoofers from other manufacturers, the published tuning recommendations assume an Audiomobile driver on the active side.
Browse the Audiomobile subwoofer collection to find the matching driver, or browse all subwoofers if you're not committed to Audiomobile yet. Send us your enclosure dimensions and target tuning if you want a custom build recommendation.
Frequently Asked M-CAR Passive Radiator Questions
What is a passive radiator and how is it different from a port?
A passive radiator is a tuned, undriven cone that contributes low-frequency output to a sealed enclosure. It does the same job as a port (extends bass below the system's natural roll-off) but uses mechanical mass and compliance instead of pneumatic tuning. Passive radiators don't have port noise, need less internal volume than ported equivalents, and can be tuned more precisely by adjusting the radiator's mass.
Do I need an amplifier for an M-CAR passive radiator?
No. Passive radiators are not driven. They contribute to the system's bass response acoustically, driven by the air pressure inside the enclosure. The amplifier powers the active subwoofer; the passive radiator works automatically as a result of the enclosure pressurization.
Can I use an M-CAR passive radiator with non-Audiomobile subwoofers?
Technically yes, but the tuning recommendations assume an Audiomobile GT or E-Series driver on the active side. The M-CAR's mass, compliance, and excursion are designed around Audiomobile's driver characteristics. Pairing with another manufacturer's driver requires modeling the system in enclosure software with both drivers' Thiele-Small parameters to confirm the tuning works.
What size enclosure does an M-CAR build need?
Less volume than a ported enclosure but more than a purely sealed one. As a starting point, use the active subwoofer's recommended sealed volume and add 15 to 25 percent. The exact target depends on driver and radiator parameters. Use the EBP Calculator with the active driver's specs, or contact our tech team for a custom enclosure design.
How do I tune an M-CAR passive radiator?
The M-CAR series ships with adjustable mass weights. Adding mass lowers the tuning frequency (deeper bass extension). Removing mass raises it. Start with the manufacturer's recommended mass for your active driver, then tune by ear or measurement. Most builds end up close to the manufacturer's starting recommendation.
Is a passive radiator better than a ported enclosure?
Different tools for different jobs. Ported enclosures need more internal volume but can hit higher SPL at the tuning frequency. Passive radiator enclosures are more compact, have no port noise, and offer more precise tuning control. For SQ-focused builds and compact installs, passive radiators usually win. For maximum SPL with available enclosure volume, ports usually win.
