Which of the following is not a mechanical wave?

Which of the following statements about wave is not correct? i. A wavefront is a line which contains all particles whose vibration are in phase. ii. The direction of propagation of a wave is the link drawn parallel to the wavefront. iii. A wavefront is a circle which is common to particles that are in the same state of disturbance

What type of motion does the skin of a talking drum perform when its being struck with the drum stick?

The sound from a bell in an enclosed jar gradually faints away while the jar is being evacuated. Which of the following explains this observation?