A siren mounted on the roof of firehouse emits sound at a frequency of 815 Hz. A steady wind is blowing with a speed of 14.4 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 m f he or she is approaching from an upwind (c) Firefighters are approaching the siren from various directions at 19.5 m/s. What frequency does a firefighter hear position, so that he or she is moving in the direction in which the wind is blowing? Hz (d) What frequency does a firefighter hear if he or she is approaching from a downwind position and moving against the wind?

Question

Physics

Corinne Hodges

14 March, 11:41

A siren mounted on the roof of firehouse emits sound at a frequency of 815 Hz. A steady wind is blowing with a speed of 14.4 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 m f he or she is approaching from an upwind (c) Firefighters are approaching the siren from various directions at 19.5 m/s. What frequency does a firefighter hear position, so that he or she is moving in the direction in which the wind is blowing? Hz (d) What frequency does a firefighter hear if he or she is approaching from a downwind position and moving against the wind?

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Answers (1)

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Tatiana Palmer · Answer 1

Wind increases the velocity of sound in case of downwind and decreases in case of upwind.

a) In this case velocity of sound = 343 - 14.4 = 328.6

wavelength = velocity / frequency = 328.6 / 815 = 40.32 cm

b) In this case velocity of sound = 343+14.4 = 357.4

Wavelength = 357.4 / 815 = 43.85 cm

c) In this case apparent frequency = 815 x (328.6 + 19.5) / 328.6

= 863.36 (sound is moving against the wind)

d) Apparent frequency = 815 x (357 + 19.5) / 357 (sound is moving in the direction of wind)

= 859.51