Active silencer

An active silencer provided at a pipe which has one end reaching a noise source and another end reaching the atmosphere, said active silencer comprising:

• means for generating a sound into said pipe so that a noise passing through said pipe from said noise source is reduced;
• means for collecting the sound passing through said pipe from said sound generating means, said sound collecting means being provided at a position further away from said noise source than said sound generating means;
• means connected to said sound collecting means for generating a lead phase output signal by leading a phase of an output signal of said sound collecting means by a phase substantially corresponding to a time determined by a distance between said sound generating means and sound collecting means, comprising:
• means for comparing a phase of said output signal of said sound collecting means with that of a delay signal produced by delaying said lead phase output signal,
• means for oscillating said lead phase output signal according to an output signal of said phase comparing means, a frequency of said lead phase output signal being proportional to said output signal of said phase comparing means, and
• means for delaying said lead phase output signal, wherein said phase comparing means, said oscillating means, and said delay means constitute a phase locked loop;
• and
• means responsive to said lead phase output signal for outputting a drive signal which is an inverted signal of said lead phase output signal and for driving said sound generating means by said drive signal.