The sensor at entrance sampling the frequency of noise signal
and pass it to controller. According to formula (8) and (9), Corresponding
path difference can be calculated by controller and can
be achieved by adjusting closed position of solenoid valve and
bypass tube telescopic length. The sound wave from entrance into
main tube partially go into bypass tube at position I, partially move
forward along the main tube, then flow together at position II. The
two opposite phase waves offset due to fully destructive interference,
so as to eliminate noise.
As to this semi-active muffler device, the path difference of
main tube and bypass tube can be changed rapidly by adjusting
bypass tube solenoid valve. The length of telescopic bypass tube
can be adjusted precisely by step motor and screw. Path difference
can be changed rapidly and accurately corresponding to the changing
of noise frequency by connecting branch bypass tube and telescopic
bypass tube.