Passive Car Subwoofer SS 15 D2 by Hertz. This speaker has a nominal power (RMS) of 1200 Watts and a diameter of 15 inches. The speaker does not have a box, which helps save space in the cabin or trunk of the car. Passive subwoofers like this one do not have a built-in amplifier, so an external amplifier is required for them to operate, which also determines the quality of sound reproduction. Passive speakers do not experience the phenomenon of overheating of the speaker and air during operation due to the absence of an internal amplifier.