While for the test sample WOG2/4 due to the shift of critical frequency to lower frequency range of 12 mm glass pane (higher
bending stiffness) and composition of two glass panes of different
thicknesses (4 and 12 mm) result in smoothed sound insulation
curve because when thinner glass coincidence dip occur the more
rapid increment of thicker glass compensate it and result in higher
value of sound reduction index