What a passive radiator is
A passive radiator (also called a drone cone) is a driver frame and diaphragm with no magnet and no voice coil. Rod Elliott of Elliott Sound Products (ESP) describes it as a variation on a bass-reflex box: the radiator is tuned for a resonant frequency somewhat below the woofer’s, often with addable weights. R. H. Small’s 1974 JAES papers call the passive-radiator loudspeaker system a close relative of the vented-box system, capable of similar low-frequency performance. Accuton (manufacturer) puts the same idea in Helmholtz terms: together with the active driver, the radiator works as a resonator at the tuning frequency, with a response similar to a vented enclosure. The radiator’s moving mass stands in for the mass of air in a vent that would otherwise be too long for the cabinet. If you are still choosing between a sealed box and a port, start with sealed vs ported. A passive radiator is a third option, used instead of a port.
Why choose one over a port
ESP’s main practical point: with a passive radiator there is no possibility of port “chuffing” or the other noises a ported box can make when air velocity in the vent is too high. Accuton adds that there is far less air turbulence, and that a rigid passive radiator does not pass midrange pipe resonances and standing waves the way an open port can. Accuton also notes that you do not spend enclosure volume on a long port tube, and that added mass can set a low tuning frequency even in a small box. Small’s analysis is the theoretical backbone: the system is a close relative of the vented box, with similar low-frequency performance when the radiator is designed correctly. Prefer a suspension with high compliance and low mechanical losses (Small).
What it costs
Baffle area. ESP is plain about it: a passive radiator takes up more space on the baffle than a port. Accuton goes further for a good working setup: the radiator’s effective piston area should be at least twice the active driver’s, because the radiator must move more air at its lower resonance (Accuton: typically 2 to 4 times, depending on the alignment). Excursion. Small’s requirement: the radiator should possess a linear volume displacement limit at least twice that of the system driver. For program-rated systems, Small states that the passive-radiator displacement volume must typically be about twice the rated driver displacement volume. If driver and radiator share the same effective area, their maximum linear excursions must be in that ratio (Small). The notch below cutoff. Unlike a simple vented box, Small shows that a passive-radiator system’s frequency response has a notch (a dip) at the passive radiator’s own resonance. That frequency is normally below the system cutoff. The notch sharpens the corner of the response and steepens the initial cutoff compared with an equivalent vented-box alignment. The region below the notch is deep in the stop band, so it is not a listening-band feature, but it is part of how the alignment behaves.
Tuning with added mass
ESP notes that some radiators have weights you can add or remove to set the radiator’s resonant frequency. Accuton (manufacturer) describes the same lever: higher moving mass lowers the tuning frequency, and vice versa. Accuton’s datasheets include a mass-vs-volume chart for each radiator; Accuton also prints the calculation path if you prefer to work it yourself. Accuton’s formulas, as printed (symbols as Accuton labels them):
- Resulting compliance in the box:
Cmsr = 1 / [ (1/Cms) + 1/((Vb/Vas) × Cms) ] where Cms is the radiator’s compliance, Vas its equivalent volume, and Vb the enclosure volume.
- Tuning frequency:
Fres = sqrt[ 1 / (Cmsr × Mres × 4π²) ] where Mres = Mms + extra mass. Accuton says it measures each radiator’s stated Fs at 3 mm excursion, because the suspension gets stiffer as it moves farther. If you use two radiators on one enclosure, Accuton says to halve Vb for the chart or the calculation. Pick the radiator and target Fb from the maker’s chart or the formulas above, then measure.
How to check it
Treat the finished box like any other tuned enclosure:
- Confirm net volume after driver, bracing, and the radiator’s own volume (how to calculate net box volume).
- Measure the driver’s Thiele-Small parameters rather than trusting the sheet alone (measure Thiele-Small parameters).
- Sweep near-field and impedance with a measurement loop (REW basics). Look for the radiator’s contribution around the intended tuning, and watch that excursion stays inside the radiator’s linear limit at the levels you care about.
If you already modeled a ported alignment and are only swapping the vent for a radiator, re-check tuning after the mass is set. Port-length math does not transfer one-for-one; see how to calculate port length for the vented case you are leaving behind.
Decision table
| Goal | Prefer |
|---|---|
| Simplest box, no tuning parts | Sealed (sealed vs ported) |
| Extra bass extension, you can size a quiet port | Ported (port length) |
| Port-like extension without port noise; small box, low Fb; baffle has room for a large radiator | Passive radiator |
| Baffle too tight for ~2× driver area, or radiator excursion is undersized | Port or sealed instead |