यदि कोई वस्तु ध्वनि की गति से अधिक तेज (विशेष रूप से ध्वनि की गति से 1 से 5 गुना के बीच) चलती है, तो इस स्थिति को क्या कहा जाता है?

Consider the following statements regarding waves: 1. If the particles of a medium move along the direction of propagation of the wave, the wave is called a longitudinal wave. 2. If the particles of a medium vibrate perpendicular to the direction of propagation of the wave, the wave is called a transverse wave. 3. Transverse waves involve changes in the shape/shear of the medium, whereas longitudinal waves involve changes in the density/volume of the medium. Which of the statements given above are correct?

At what speed should a source of sound move away from a stationary observer so that the observer perceives the apparent frequency to be half of the original frequency? (where v is the speed of sound)