ध्वनि के आयाम से संबंधित, ध्वनि की तीव्रता के स्तर या सापेक्ष लाउडनेस को मापने के लिए आमतौर पर किस इकाई का उपयोग किया 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)