You're stopped at a lite & a truck travels thru the lite at a constant speed of 35 m/s. The truck's siren normally produces a sound of frequency 1355 Hz. After the truck has passed you & is traveling away from you, what's the frequency you'd hear?

Please note that in your situation the Doppler effect is involved. We can suppose that the sound speed in the air at say 25°Celsius is 341 m/sec. So we have two cases:

case 1)

if the siren is approaching to the observer, then the sound frequency f_m measured by that observer, is given by the subsequent formula:

f_m= f/(1-V_s/v), wher f is the sound frequency emitted by the siren, v is the sound speed and V_s is the siren speed. So substituting your numerical data, we find:

f_m=1355/(1-35/341)=1510 Hz (approximated value)

case 2)

if the siren is going away from the observer, then the measured frequency f_m of the sound received by that observer is given by the subsequent formula: