Fig. 7 also shows that between 200 Hz and 465 Hz, transmission
loss of the semi-active muffler reduces more than 10 dB compared
with that below 200 Hz. Transmission loss of experiment is overall
lower than simulation, and it is obvious different in some frequency
bands. The main reason is: as frequency increases, wavelength
becomes shorter and shorter and more but more precision
is required by path difference. But in the experiment, device precision
is limited, it is difficult to precisely adjust the best matched
path difference. Therefore, it is unable to meet its best noise reduction
performance and there is a certain error between experiment
and simulation.