question_answer 19)

Explain why (or how): (a) In a sound wave, a displacement node is a pressure antinode and vice-versa, (b) Bats can ascertain distances, directions, nature and size of obstacles without any eyes, (c) a violin note and sitar note may have the same frequency, yet we can distinguish between the two notes, (d) Solids can support both longitudinal and transverse waves, but only longitudinal waves can propagate in gases, (e) The shape of pulse gets distorted during propagation in a dispersive medium.  

Answer:

(a) Node (N) is a point where the amplitude of oscillation is zero (and pressure is maximum). Antinode (A) is a point where the amplitude of oscillation is maximum (and pressure is minimum) These nodes and antinodes do not coincide with pressure nodes and antinodes. In fact, displacement node N coincides with pressure antinode and displacement antinode A coincides with pressure node, as is clear from the definitions. (b) Bats emit ultrasonic waves of large frequencies. When these waves are reflected from the obstacles in their path, they give them the idea about the distance, direction, size and nature of the obstacle. (c) Though the violin note and sitar note have the same frequency, yet the overtones produced and then-relative strengths are different in the two notes. That is why we can distinguish between the two notes. (d) This is because solids have both, the elasticity of volume and elasticity of shape, whereas gases have only the volume elasticity. (e) A sound pulse is a combination of waves of different wavelengths. As waves of different travel in a dispersive medium with different velocities, therefore, the shape of the pulse gets distorted.