Electronic noise attenuation system

An electronic noise attenuation 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 with said propagated sound wave to produce interference between the two sound waves at a given position in said propagation passage, said system comprising:

• first mechano-electric transducer means disposed at a position closer to said source of noise from said given position in said propagation passage to sense said propagated sound wave from said source of noise and convert it into an electric signal;
• electro-mechanical transducer means 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 interposed between the position of said electro-mechanical transducer means and said given position or disposed at said given position to sense said propagated sound waves from said electro-mechanical transducer means and from said source of noise and convert them into electric signals;
• a first digital filter arranged to be given first filter coefficients representing a transfer function between said electro-mechanical transducer means and said first mechano-electric transducer means and to perform the digital operation processing on drive signals to be applied to said electro-mechanical transducer means in accordance with said first filter coefficients;
• operation means for finding a difference between the output signal of said first mechano-electric transducer means and the output signal of said first digital filter;
• an adaptive digital filter arranged to be given time varying filter coefficients so that an amount of noise attenuation can be the maximum, to perform the digital operation processing on output signals of said operation means in accordance with said time varying filter coefficients, and to create said drive signal to be applied to said electro-mechanical transducer means;
• a second digital filter arranged to be given second filter coefficients representing a transfer function between said electro-mechanical transducer means and said second mechano-electric transducer means, and to perform the digital operation processing on output signals of said operation means in accordance with said second filter coefficients;
• and
• control means for inputting therein output signals of said second mechano-electric transducer means and output signals of said second digital filter, for calculating said time varying filter coefficients sequentially in accordance with a VS-31 LMS algorithm, and for updating said time varying filter coefficients of said adaptive digital filter by the thus calculated time varying filter coefficients.