Two trucks heading in opposite directions each with speed $0.1 u$, approach each other. The speed of the sound is $u$. The driver of first truck sounds his horn of frequency 495 Hz . Let $v_1$ and $v_2$ are the frequencies heard by the driver of second truck, when the trucks approach each other and when the trucks have passed each other. The magnitude of $v_1-v_2$ is

An organ pipe with both ends open has a length $L=25$ cm . An extra hole is created at position $L / 2$. The lowest frequency of sound produced is (assume, speed of sound $=340 \mathrm{~m} / \mathrm{s}$ )

The transverse displacement $y(x, t)$ of a wave on a string is given by $y(x, t)=e^{-\left(a x^2+b t^2+2 \sqrt{a b x} t\right)}$. This represents a

A bus moving with an uniform speed of $72 \mathrm{~km} / \mathrm{h}$ towards a building blows a horn of frequency 1.7 kHz . If speed of sound in air is $340 \mathrm{~m} / \mathrm{s}$, what will be the frequency of echo heard by bus driver?