3. (a) Describe what happens to a wave when it encounters an obstacle/ barrier. When is this effect most pronounced? (b) Explain in your own words why a sound wave is sometimes referred to as a pressure wave. (c) Calculate the wavelength and amplitude of the wave shown below: (d) A man standing 300 m in front of a cliff fires a cannon. After how many seconds later will he hear its echo? Explain your reasoning. Speed of sound in air is 330 m/s