An approximation of the differential equation for compressible duct flow is presented. The equation is linear and of the second order. The duct transfer function and response characteristics are obtained by applying small-perturbation theory to the differential equation. The resulting equations describe duct natural frequency as a function of duct areas and volumes, and damping ratio as a function of the slope of the steady-state mass flow, pressure-recovery curve. The calculated response agrees, to a first approximation, with measuied response as obtained from tests of a fixed-geometry, sugar-scoop inlet model with bypass for matching airflows. Testing was done in the 10 X 10 and 8 X 6 ft. supersonic tunnels at NASA Lewis Flight Propulsion Laboratory. Further agreement was obtained during flight tests of the F8U-3 airplane.