of motions of the source and the listener. In all such cases, the expression for the apparent frequency is given in table . . Acoustics Both source and listener move They move one behind the other Listener follows the source a) Apparent frequency depends on the velocities of the source and the listener.
b) v S and v L become opposite to that in case- . n' = v + v L v + v s n Source at rest Listener moves towards the source a) Distance between source and listener decreases. b) Apparent frequency is more than actual frequency. c) v S = in case- .
n' = v + v L v n Source at rest Listener moves away from the source a) Distance between source and listener increases. b) Apparent frequency is less than actual frequency. c) v S = in case- . n' = v − v L v n Listener at rest Source moves towards the listener a) Distance between source and listener decreases.
b) Apparent frequency is more than actual frequency. c) v L = in case- . n' = v v − v s n Listener at rest Source moves away from the listener a) Distance between source and listener increases. b) Apparent frequency is less than actual frequency.
c) v L = in case- . n' = v v + v s n Suppose the medium (say wind) is moving with a velocity W in the direction of the propagation of sound. For this case, the velocity of sound, ‘v’ should be replaced with (v + W). If the medium moves in a direction opposite to the propagation of sound, then ‘v’ should be replaced with (v – W).
Solved problems . A source producing a sound of frequency Hz is approaching a stationary listener with a speed equal to ( / ) of the speed of sound. What will be the frequency heard by the listener? Solution: When the source is moving towards the stationary listener, the expression for apparent frequency is