Abstract: The 1/2 subharmonics distortion in a direct radiation loudspeaker is investigated when the driven frequency is lower than the lowest nature frequency associated with the axisymmetric mode of the loudspeaker shell.The laser triangulation method based on the PSD(Position Sensitive Detector) is used to measure the vibrations and modes of the loudspeaker shell.With the participant asymmetric mode determined,the method of multiple scales is then used to obtain the first-order approximations to the nonlinear modal equations for the loudspeaker shell.The formula of the threshold voltage of the subharmonics is given.The results show that the subharmonics in the loudspeaker are caused by the asymmetric mode excited parametrically by the axisymmetric vibration.Increasing the loss factor,the Young's modulus of the loudspeaker shell material or the shell thickness can increase the threshold voltage of the subharmonics occurrence.