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Question
The horn of a car emits sound with a dominant frequency of 2400 Hz. What will be the apparent dominant frequency heard by a person standing on the road in front of the car if the car is approaching at 18.0 km h−1? Speed of sound in air = 340 m s−1.
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Solution
Given:
Frequency of sound emitted by horn \[f_0\]= 2400 Hz
Speed of sound in air v = 340 ms−1
Velocity of car \[v_s\] = 18 kmh−1 =\[18 \times \frac{5}{18} \text { m/s }\] = 5 m/s
Apparent frequency of sound \[\left( f \right)\] is given by : \[f = \left( \frac{v}{v - v_s} \right) \times f_0\]
On substituting the values, we get :
\[f = \left( \frac{340}{340 - 5} \right) \times 2400\]
\[ = 2436 \text { Hz }\]
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