the flap pair is actuated to perform a common mode movement and a differential mode movement simultaneously;
wherein the film structure performs the common mode movement, so as to form an amplitude-modulated ultrasonic air pressure variation with an ultrasonic carrier frequency;
wherein the film structure performs the differential mode movement, so as to form an opening at a rate synchronous with the ultrasonic carrier frequency;
wherein the air-pulse generating device produces a plurality of air pulses according to the amplitude-modulated ultrasonic air pressure variation.