A siren mounted on the roof of a firehouse emits sound at a frequency of 890 Hz.
ID: 1787099 • Letter: A
Question
A siren mounted on the roof of a firehouse emits sound at a frequency of 890 Hz. A steady wind is blowing with a speed of 13.5 m/s. Take the speed of sound in calm air to be 343 m/s (a) Find the wavelength of the sound upwind of the siren (b) Find the wavelength of the sound downwind of the siren. (c) Firefighters are approaching the siren from an upwind position, so that he or she is moving in the direction in which the wind is blowing? fromv (d) What frequency does a firefighter hear if he or she is approaching from a downward position and moving against the wind?Explanation / Answer
(A) upwind,
v =343 + 13.5 = 356.5 m
wavelength = v / f = 356.5 / 890
= 0.400 m
(B) donwwind,
v = 343 - 13.5 = 329.5 m
wavelength = v / f = 0.370 m
(C) from doppler's effect,
f = f0 ( v + v_r) / (v + v_s)
f = 890 ( 343 + 19) / (343 + 13.5)
f = 903.7 Hz
(d) f = 890 (343 + 19) / (343 - 13.5)
f = 977.8 Hz