Patent ID: 8041059
Filing Date: 2011-10-18
Classification: H04R

Abstract:
1. An ultrasonic speaker comprising: an electrostatic transducer driven by a signal in an ultrasonic frequency band, a modulator that supplies the electrostatic transducer with a modulated signal which is obtained by modulating a carrier wave signal in the ultrasonic frequency band with a sound signal in an audible frequency band, and the electrostatic transducer includes: a class-D power amplifier that amplifies an input signal; and a low pass filter that has a plurality of pairs of inductors and capacitors, is connected to an output side of the class-D power amplifier, and serves to eliminate switching carrier components included in an output of the class-D power amplifier, a load capacitor of the electrostatic transducer serving as a driving load is disposed at a capacitor, which is closest to the output side of the low pass filter, of circuit elements forming the low pass filter, a coupling capacitor and an output transformer are interposed between the load capacitor of the electrostatic transducer and an inductor closest to the output side of the low pass filter, and a damping resistor is connected in series to a primary coil of the output transformer; a first-surface-side fixed electrode formed with a plurality of holes; a second-surface-side fixed electrode formed with a plurality of holes, the second-surface-side fixed electrode and the first-surface-side fixed electrode forming a pair; and a vibrating film that is interposed between the pair of fixed electrodes and has a conductive layer to which a DC bias voltage is applied, wherein a center tap is provided in a secondary coil of the output transformer, one end of the secondary coil of the output transformer is connected to the first-surface-side fixed electrode of the electrostatic transducer and the other end thereof is connected to the second-surface-side fixed electrode, and the DC bias voltage is applied to the conductive layer of the vibrating film by using the center tap of the secondary coil of the output transformer as a reference.