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 engine of train A blows a long whistle. Assume that the sound of the whistle is composed of components varying in frequency from $\mathrm{f}_1=800 \mathrm{~Hz}$ to $\mathrm{f}_2=1120 \mathrm{~Hz}$, as shown in the figure. The spread in the frequency (highest frequency - lowest frequency) is thus 320 Hz . The speed of sound in still air is $340 \mathrm{~m} / \mathrm{s}$.
The spread of frequency as observed by the passengers in train B is