Problem 3

A stationary source emits sound at

f=500\,\mathrm{Hz}

in still air, where sound speed is

c=340\,\mathrm{m/s}

. An observer moves directly toward the source at constant speed

u=20\,\mathrm{m/s}

; there is no wind and the source is not moving. (a) Find the first requested quantity. (b) Find the second quantity. (c) Determine the final result and check its units. (a) Find the sound wavelength (a) Find the sound wavelength in air. (b) Find the frequency heard by the observer. (c) Explain the Doppler sign and check that the result exceeds

f

problems.proof.stepOf

problems.proof.prevStep problems.proof.nextStep

f_{obs}=f(c+u)/c

problems.proof.analysis

Rearranging the governing relation isolates the first requested result. Keeping the signs explicit also checks that its direction and magnitude are physically sensible.