Abstract: Conception of normal modes of vibration is generalized to finite-amplitude sound fields in enclosures,taking into acount of the nonlinear effects.This includes the solution the wave equation in the from of nonlinear partial differential equations,for the natural vibrations in the closed space.In doing sos the standing wave nature of the solution is emphasized.The first order solution is taken as the basic solution of the problem,and all the higher order solutions but the modification or correction at various points on basic waveform due to the nonlinearity and do not change the standing wave nature of the solution.In this way,the finite-amplitude normal function is derived for the first time,containing the basic solution,the same normal function as in linear acoustics,and its harmonics.And besides,time-independent terms exist in the complete solution,representing the radiation pressure in the finite-amplitude sound field