Abstract: A reflecting plate type silencer pipe including a reflecting plate is provided for reducing a dominant frequency noise passing through a pipeline. Sound waves propagated in the pipeline as a one-dimensional plane wave are divided into two, that is, sound waves advancing straight along the pipeline and sound waves caused to pass through a bypass route by the reflecting plate thereby being delayed in phase. Both of the sound waves are joined again so as to synchronously advance in the same direction and in parallel to each other with the sound waves being the same in amplitude and opposite in phase to each other, and wave interference is effected between the sound waves, whereby wave propagation energy of the sound wave of the frequency to be silenced passing through the pipeline is selectively dissipated.

Abstract: A reflecting plate type silencer pipe including a reflecting plate is provided for reducing a dominant frequency noise passing through a pipeline. Sound waves propagated in the pipeline as a one-dimensional plane wave are divided into two, that is, sound waves advancing straight along the pipeline and sound waves caused to pass through a bypass route by the reflecting plate thereby being delayed in phase. Both of the sound waves are joined again so as to synchronously advance in the same direction and in parallel to each other with the sound waves being the same in amplitude and opposite in phase to each other, and wave interference is effected between the sound waves, whereby wave propagation energy of the sound wave of the frequency to be silenced passing through the pipeline is selectively dissipated.

Abstract: A howling protective apparatus comprising a phase fluctuation control means, a wave peak comparison means and a timer circuit, wherein said phase fluctuation circuit is controlled by an output from said timer circuit; a howling protective apparatus comprising a gain control means, a wave peak comparison means and a changing means for varying a coefficient of integration; a howling protective apparatus comprising a phase fluctuation circuit, a variable gain circuit and a wave peak comparison circuit, wherein said phase fluctuation circuit and the variable gain circuit are controlled by an output from said wave peak comparison circuit.

Abstract: An automatic volume adjusting apparatus comprises a volume adjusting device connected between a sound signal source and a loudspeaker; a first rectifier circuit for generating a first signal having a D.C. level corresponding to the level of a sound signal supplied from the colume adjusting device to the loudspeaker; a microphone and a second rectifier circuit for generating a second signal having a D.C. level corresponding to the level of a composite sound constituted by the sound generated by the loudspeaker and the ambient noise; a subtraction circuit for generating a third signal having a level corresponding to the level difference between the first and second signals, i.e. the level of the ambient noise; a gate circuit for sampling the third signal at predetermined intervals; and an integrator circuit for holding the output of the gate circuit for a predetermined period of time. The output of the integrator circuit controls the volume adjusting device.