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.
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.