When a source of sound passes us, whether it be a car horn or a train whistle, the pitch we hear goes from high to low. Explain why. Use any relation to support your explanation.

Explanation

When a source of sound e.g. car harn or a train whistle passes us, the observed pitch varies from high to low. This is due to Doppler's effect. Initially, the source e.g. the car horn or a train whistle is approaching us, i.e., the source is approaching the observer at rest, then the observed pitch `(v^(')` appears to be more that than the true pitch `(v)` because
`v^(')=(upsilon)/(upsilon-upsilon_(s))v`
When the source of sound has crossed us, (i.e., the source is receding away from the observer), the observed pitch appears to be less thatn the true pitch, because
`v^(')=(upsilon)/(upsilon+upsilon_(s))v`