Passive radiator systems are very similar in operation to ported systems.
However, instead of a port, the passive radiator system uses a passive radiator (also known as a "drone cone") to extend the system's low frequency response.

The response of a passive radiator system is similar to that of a ported system using the same driver. However, the cutoff (-3dB) frequency is slightly higher, and the cutoff slope is deeper, mostly due to the presence of a "notch" in the frequency response corresponding to the passive radiator's resonance frequency. This notch is normally located far outside of the passband of the system, and therefore usually of little audible significance.
The larger the passive radiator, the lower the passive radiator's resonance frequency (for the same target Fb), and the further the notch is out of the passband.

To design a passive radiator alignment, start with a simple ported alignment using that driver that provides the desired box size and frequency response. Then, use the diameter of your chosen passive radiator as the "port diameter".
The result is the required mass of the passive radiator.

If it is too small, use a larger passive radiator and repeat the calculations.

Now, we need to select an appropriately-sized passive radiator. ALWAYS use a passive radiator that is larger in diameter than the active driver, as the displacement of the passive radiator usually has to be 1.5 to 2 times that of the driver.

If it's not possible to use one large passive radiator, then you can use two or more smaller ones, and tune them by working out the effective diameter from the combined surface area of the radiators.

Note that the effective diameter of the radiator is approximately equivalent to the diameter of the passive radiator's face plus 1/3 of the surround. If unsure, use the quoted Sd for that radiator, then use the following equation to determine the effective radius:

 Sd = radiator area [cm2] R = radius [cm] PI = 3.1415926 R = (Sd / PI)^0.5

Sd = cm2 in2

Diameter = cm in

cm    inch

## Calculate weight of passive radiator

### Input

Passive Radiator Diameter : cm inch
Tuning Frequency (fb) : Hz
Enclosure Volume (Vb) : liter cubic inch