A small speaker delivers 2 W of audio output. At what distance from the speaker will one detect 120 dB intensity sound ? [Given reference intensity of sound as 10^–12 W/m² ]

A progressive wave travelling along the positive x-direction is represented by y(x,t) = Asin(kx – $$\omega $$t + $$\phi $$). Its snapshot at t = 0 is given in the figure.
For this wave, the phase $$\phi $$ is :

A submarine (A) travelling at 18 km/hr is being chased along the line of its velocity by another submarine (B) travelling at 27 km/hr. B sends a sonar signal of 500 Hz to detect A and receives a reflected sound of frequency $$\upsilon $$. The value of $$\upsilon $$ is close to: (Speed of sound in water =1500 ms^–1)

The correct figure that shows, schematically, the wave pattern produced by superposition of two waves of frequencies 9 Hz and 11 Hz, is :