Electronic noise attenuation system

An electronic noise attentuation system for achieving attenuation of a sound wave propagated from a source of noise in a propagation passage of a sound wave by generating another sound wave 180.degree. out of phase and having the same sound pressure as said propagated sound wave to produce interference between said two sound waves at a given position in said propagation passage, said system comprising:

• first mechano-electric transducer means comprising a first microphone having a conversion characteristic and disposed at a position closer to said noise source than to said given position in said propagation passage to sense said propagated sound wave from said noise source and convert it into an output electric signal;
• electro-mechanical transducer means comprising a speaker having a conversion characteristic and interposed between the position of said first mechano-electric transducer means and said given position in said propagation passage to generate a sound wave for cancelling said propagated sound wave from said noise source at said given position;
• second mechano-electric transducer means comprising a second microphone having a conversion characteristic which matches said conversion characteristic of said first microphone and interposed between the position of said electro-mechanical transducer means and said given position to sense said generated sound wave from said electro-mechanical transducer means as well as said propagated sound wave from said noise source and convert them into an output electric signal;
• operation means to obtain a difference between the output signals of said first and second mechano-electric transducer means and to generate an output signal representative of said difference;
• drive signal generating means to receive the output signal of said operation means and generate on the basis of a given transfer function a drive signal to be given to said electro-mechanical transducer means so that the amount of sound cancellation of said electronic noise attenuation system can be maximized;
• control means to determine a transfer function to be given to said drive signal generating means, set up in said drive signal generating means a control parameter for specifying said transfer function, and correct said control parameter according to changes of the propagation characteristics of said propagation passage as well as to changes of characteristics of the transducer means of said electronic noise attenuation system;
• wherein said first and second mechano-electric transducer means are located in said sound wave propagation passage with said electro-mechanical transducer means therebetween at positions in which a transfer function Hr indicating a propagation characteristic of the sound wave propagated from said electro-mechanical transducer means toward said first mechano-electric transducer means with the conversion characteristics of said electro-mechanical transducer means and said first mechano-electric transducer means added thereto is equivalent to a transfer function Ht indicating a propagation characteristic of the sound wave propagated from said electro-mechanical transducer means with the conversion characteristics of said electro-mechanical transducer means and said second mechano-electric transducer means added thereto.