When to choose a passive radiator over a vent

From my experience so far, there are three scenarios where it would be benefitial to design around a passive radiator:

  1. When shrinking a subwoofer enclosure (and therefore the port diameter), chuffing may occur when the air velocity out of the port approaches 5% the speed of sound.
  2. When designing small enclosures where space is at a premium, enclosure volume is taken up by the comparatively voluminous port.
  3. It's neat.

Like when deciding whether or not to select a sealed or vented enclosure, passive radiators will typically only introduce beneficial effects when the specified driver has a Qts between 0.2 and 0.5. A driver with a Qts above 0.4 would most likely do best in a sealed enclosure.

How to choose and tune a passive radiator

The process begins with choosing a driver, then designing and tuning a vented enclosure. Next, follow the step-by-step procedure below:

  1. Choose a passive radiator with an Sd between 1.5 and 2.0 times as large as the Sd of the driver. Alternatively, you can also select a passive radiator with a Vd twice the Vd of the driver.
    Vd = Sd * Xmax
    In addition, it is commonly seen in many designs that implement passive radiators to use a duplicate of the woofer and just leave the terminal disconnected.
  2. In WinISD, replace the port diameter with the Sd of the passive radiator. Note the new port length is very long.
  3. Calculate the volume of air this "port" would take up.
    V = pi * (Sd/2)^2 * length (m^3)
  4. Calculate the mass of this volume of air.
    M = V * 1.21 (kg)
    If this mass is lower than the Mms of the passive radiator, a different one will have to be chosen with either a smaller Mms or a larger Sd, or make the enclosure volume smaller.
    If this mass is higher than the Mms of the passive radiator, congrats, you're almost done! All you have to do is add the difference between the calculated mass of air and the Mms to the passive radiator itself.

Design Example: Dayton Audio DC200-8