Many car audio enthusiasts start out by taking their parents' home audio speakers and placing them in the rear of their cars to blast out tunes. If you had parents that were hip then maybe you were slightly more sophisticated. Back in the day "alternative enclosures", like passive radiator and transmission line, were cutting edge. Not everyone knows about these enclosures; after all, they are a bit obscure. But it's worth covering.
Passive RadiatorHarry Olson first described passive radiators (PRs) in his patent Loudspeaker and Method of Propagating Sound that was issued back in 1935. Except for an article by Olson in 1954, very little was published about PRs until Nomura and Kitamura in their IEEE paper in October 1973, and Small's JAES paper in October 1974. Since then, passive radiator systems have had a relatively mild impact in the home audio market, while the car audio market failed to embrace it. However, two car audio manufacturers, Boston Acoustics and Earthquake, have recently initiated a movement towards the use of PRs in the automotive industry, taking from their experience in home audio.
So what are passive radiators, what do they do, how do they work, and what are the positive and negatives?
Passive radiators are often deceiving as they commonly look like a conventional subwoofer. However, these certainly are not subwoofers. While they look like and even appear to move like a normal subwoofer from the outside of a speaker system, what is behind them tells a different story. Passive radiators lack a key ingredient that assures they are not a woofer: the motor structure. In other words, they have no voice coil, magnet, top plate, t-yoke, tinsel leads or terminals. PR's are essentially an unpowered driver, therefore they must be mated with a powered woofer in an otherwise sealed enclosure.
Passive radiators systems are referred as a variation of a ported enclosure. While they are mathematically identical, PR's use a diaphragm to take the place of the port.
A PR is essentially made of two parts: a "weighted diaphragm" and a "spring". The weight of the diaphragm is a critical element the design and must be correct for the part to function properly. By changing the weight of the diaphragm, the resonance frequency will change, thus effecting the tuning of the enclosure. The spring is a combination of the stiffness of the suspension materials, and the air trapped within the cabinet. This too can change the tuning of the enclosure, much the same as the enclosure volume of a vented box changes its response.PRs are tuned, by the mass loading, to resonate at a frequency below the active woofer's linear response range. A passive radiator has a useful range about a 1/4 octave above and below its resonance. However, the typical roll off is a fairly steep 18 dB/octave. The combined response of the woofer and passive radiator should produce about a half an octave bass extension at low frequencies that add up to the level produced by the woofer on its own at higher frequencies, if the PR is tuned properly. In other words, a small amount of low frequency bass that the system would normally have difficulty reproducing now exists.
In a passive radiator system, both the cone of the active woofer and PR could move in phase with each other, or any combination of opposite motions, up to 180 degrees out of phase. Keeping both cones exactly in phase would be ideal in order to reinforce the output of the woofer, but as physics would have it, this sort of resonant system is not exactly possible.