In Fig19-2, as the piston (活塞) moves back and forth, it alternately compresses and expands certain (使稀薄)) the air next to it conditions X This disturbance travels down the tube as a sound AP wave 0 X p(x,t)=po+△p(x,D) △x ipm (x,D)=P0+△P(x,D) It is convenient to use density and pressure to describe the V X properties of fluids Fig 19-2Fig19-2 In Fig19-2, as the piston (活塞) moves back and forth, it alternately compresses and expands (使稀薄)) the air next to it. This disturbance travels down the tube as a sound wave. x x x x v m u P0  0 m S  m pm x (x,t) (x,t)  = 0 +  P(x,t) P P(x,t) = 0 +  It is convenient to use density and pressure to describe the properties of fluids. Under certain conditions