Mechanical waves, echoes and Doppler shift

Mechanical waves require matter to transmit a disturbance. Sound in air is longitudinal and cannot cross a vacuum. An echo is reflected sound, so its measured delay includes the outward and return trips. For a stationary source and reflector, distance to the reflector is d=2vt​. Keep the speed appropriate to the stated medium and conditions.

Sound frequency is set by the source, while propagation speed depends mainly on the medium and its conditions. Refraction occurs when a speed change alters propagation direction. Relative approach generally raises observed frequency; relative recession lowers it, the Doppler effect. A supersonic source can produce a shock front because it moves faster than its own wavefronts. These classifications concern observed frequency, not a change in the source’s vibration rate.

Watch out: The detected echo has travelled to the reflector and back, so divide the total travel distance by two.