The equation of a progressive wave can be given by $$y = 15\sin (660\pi t - 0.02\pi x)$$ cm. The frequency of the wave is

A hollow cylinder with both sides open generates a frequency f in air. When the cylinder vertically immersed into water by half its length the frequency will be

Two stretched strings have lengths l and 2l while tensions are T and 4T respectively. If they are made of same material the ratio of their frequency is

When sound is produced in an aeroplane with a velocity of 200 m/s horizontally its echo is heard after $$10\sqrt 5 $$ seconds. If velocity of sound in air is 300 ms^$$-$$1 the elevation of aircraft is