Consider a subsonic flow with an upstream Mach number of \(M_{\infty}\). This flow moves over a wavy wall with a contour given by \(y_{w}=h \cos (2 \pi x / l)\), where \(y_{w}\) is the ordinate of the wall, \(h\) is the amplitude, and \(l\) is the wavelength. Assume that \(h\) is small. Using the small perturbation theory of this chapter, derive an equation for the velocity potential and the surface pressure coefficient.