It Looks Like a Driver, But Isn't

• A passive radiator is a device seated in a hole in the front of the cabinet of a large home audio loudspeaker. Its function is to augment the lowest range of the "woofer," or bass driver. From the front panel with the screen removed, the passive radiator resembles a larger version of the woofer. However, the passive radiator actually has no electrical connection to the speaker. It is "tuned" to vibrate in response to the bass energy coming from the woofer, but at a range several steps lower than the lowest pitches that the woofer can reproduce. The passive radiator lends to the loudspeaker the aural perception of bass notes about half an octave lower than it would otherwise be able to reproduce.
  
  

How the Passive Radiator Works

• The passive radiator, which possesses a cone like that of a loudspeaker, typically fills the largest aperture in the cabinet and is able to move inward and outward even more than the woofer. This is referred to as the cone's "excursion." Neither magnets nor wires are attached to the passive radiator. It sits apparently motionless until pitches within its purposefully very narrow range are called for. Some newer passive radiators are tunable by the user to center on one of several frequencies. Passive radiators are used to deepen the response of many models of subwoofers, and are starting to be used to boost bass response in car audio as well.
  
  

Advantages and Disadvantages of the Passive Radiator

• One obvious advantage of the passive radiator is that it can allow for a smaller woofer, with more moderate power requirements, to be used. A smaller woofer can also deliver greater clarity in the mid-low register, leaving the range of the "rumble" sounds to the passive radiator.
  
  One disadvantage in operation is that the radiator is exactly "out of phase" with the woofer, meaning that its cone moves out when the woofer cone moves in, and vice versa, lending marginally audible effects in certain registers. This is inevitable since the action of pushing out or pulling in by the woofer exerts a powerful effect on the air inside the speaker enclosure. A good analogy would be to think of how the air in a small room reacts when an airtight door is quickly opened or closed: air travels through another window or door elsewhere in the room in the opposite direction. As a result, passive radiators are most effective in adding extra bottom to an already large woofer, so that any audible effects of being out of phase occur at pitches near the bottom of the range of human hearing rather than in the mid-low range.