Two trains $A and B are moving with speeds 20 \mathrm{~m} / \mathrm{s} and 30 \mathrm{~m} / \mathrm{s} respectively in the same direction on the same straight track, with B ahead of A. The engines are at the front ends. The engines of train A blows a long whistle.
Assume that the sound of the whistle is composed of components varying in frequency from f_{1}=800 \mathrm{~Hz} to f_{2}=1120 \mathrm{~Hz}, as shown in the figure. The spread in the frequency (highest frequency - lowest frequency) is thus 320 \mathrm{~Hz}. The speed of sound in still air is 340 \mathrm{~m} / \mathrm{s}$.