Acoustic waves at an interface between media with different impedances Z = ρc (density × sound speed) partially reflect and transmit. The reflection coefficient r = (Z₂−Z₁)/(Z₂+Z₁) and transmission t = 2Z₂/(Z₂+Z₁) determine wave amplitude. A quarter-wavelength matching layer with Z_m = √(Z₁Z₂) achieves perfect transmission. Watch standing wave patterns in a multilayer stack.