Overview of passive radiator speakers

Like a closed box, the box of the passive radiator speaker is completely sealed, with only one chamber. However, there is no other similarity with a closed box, because passive radiator speakers also have one or more suspended diaphragms that vibrate with the movement of the woofer's own diaphragm. These diaphragms are called "passive radiators" or "passive basins" because they radiate sound waves without a drive system. In fact, you can turn the woofer into a passive radiator by removing the magnetic circuit components.

Unlike open boxes, passive radiators have compliance and offset limitations. However, if the compliance and offset constraints are large enough, then the function of the passive radiator will be very similar to the air duct of an open box. Note: In general, a passive radiator should be able to push 2 times the volume of air that the woofer can discharge. If the diameter of the passive radiator is the same as that of the woofer, this means that it will require an Xmax greater than 2.5 times.

The principle of passive radiator speakers is similar to that of open boxes, which means that they have many of the same advantages, such as extended low-frequency response and fast cut-off speed, as shown in the first figure below. A distinctive feature of passive radiator acoustics is that there is a gap or depression in the response at the resonance frequency of the passive radiator itself. This produces an initial cut-off rate that is steeper than that of the open box. Unfortunately, the transient response near the resonance of the passive radiator is reduced. Note: The resonance frequency of the passive radiator is usually set at least one octave below the cut-off frequency of the cabinet.

Passive radiator speakers have a unique phase shift. It starts to increase at low frequencies, very similar to an open box. However, the phase shift suddenly decreases near the resonance of the passive radiator, resulting in a decrease in the overall phase shift, and the rise is assumed to be similar to a closed box (see the second image above). In theory, a total phase shift of 450° is possible, but it is rare to see a phase shift greater than 270°. This means that the sound waves produced by speakers usually lag no more than three-quarters of a wavelength. However, the sharp change in phase when the passive radiator resonates reveals the aforementioned weakening of the transient response.
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