sentences:  Since the audio amplifier only changes the pitch of the blades, it takes much less power, per dB of generated acoustic sound level, to drive a rotary woofer than to power a conventional subwoofer, which uses a moving electromagnet (voice coil) placed within the field of a stationary permanent magnet to drive a cone which then displaces air.Rotary woofers excel at producing sounds below 20 Hz, below the normal hearing range; when installed in the wall of a sealed room, they can produce audio frequencies down to zero Hz, a static pressure differential, by simply compressing the air in the sealed room.In the early 1970s, researchers noted that while humans could detect frequencies below 20 Hz, the ear was much less sensitive to these frequencies.