ABSTRACT
Sound wave is a pressure wave, which is a longitudinal wave. A pressure wave travels in a medium at the speed of sound. The medium can be a solid, liquid, or gas, or a multiphase mixture. Any change in properties of the medium might lead to the change of speed of sound. This chapter focuses on the speed of sound in air. The Mach number, M, is an important parameter to indicate the characteristics of a high-speed compressible fluid flow: airflow is subsonic when M < 1; airflow is sonic if M = 1; and airflow is supersonic when its M > 1. The pressure disturbance caused by the object can only propagate within the Mach cone, so the outside of the Mach cone is called the “silent region”. When a shockwave propagates in the air, and there are sudden changes in air property, in particular, the air pressure. The sudden change of pressure strikes every object along its path including the ears of observers.
