An arrangement and an approach of experiment procedure on image visualization of gas flow in cave of firing sound reduction device (FSRD) are considered. Visualization images obtained with the use of Toepler’s device and polarization interferometer with birefringent Wollaston prisms used under “infinitely broad band” regime are given. The analysis of obtained visualization images with the use of a digital processing method is given. Some flow structure features in FSRD input caves and chambers which can have influence on its acoustics are found.