Patent ID: 7899197

Claim:
An ultrasonic speaker comprising: an electrostatic transducer which is driven by a boosted driving signal by boosting a modulated signal obtained by modulating a carrier wave with an acoustic signal in an audio frequency band and includes an output transformer T which connects the electrostatic transducer to a secondary side winding thereof in parallel and boosts the modulated signal, and a resistor R and a coupling capacitance C 1 connected in series to a primary side winding of the output transformer T and in which a circuit constant of a primary side circuit of the output transformer T including a serial circuit of the resistor R and the coupling capacitance C 1 and a circuit constant of a secondary side circuit of the output transformer including a self-inductance L 2 and a load capacitance CL of the secondary side winding of the output transformer T are set such that a resonance frequency f 0 of a circuit formed by the self-inductance L 2 of the secondary side winding of the output transformer T and the load capacitance CL(F) of the electrostatic transducer is matched or approximately matched to a carrier wave frequency fc of the electrostatic transducer; an audio frequency signal source which generates a signal wave in an audio frequency band; a carrier wave signal source which generates and outputs a carrier wave in an ultrasonic frequency band; a modulator which modulates the carrier wave with the signal wave in the audio frequency band; and a power amplifier which amplifies the signal modulated by the modulator and applies the amplified signal to the primary side winding of the transformer T through the resistor R and the coupling capacitance C 1 , wherein, when the load capacitance of the electrostatic transducer is CL(F) and a rated carrier wave frequency of the electrostatic transducer is fc(Hz), the self-inductance L 2 of the secondary side winding of the output transformer T is set to L 2 =1/(4π2fc 2 CL).