A criterion is proposed to obtain an optimal wavelet packet (WP) tree based on the critical band structure of the human auditory system for time-frequency decomposition of speech and audio signals. The criterion minimizes a perceptual cost function based on Zwicker's model of the critical band structure and allocates an optimal number of terminating nodes at different decomposition depths of the WP tree. The criterion is used to obtain the optimal WP tree and the corresponding critical band ordered wavelet packet basis for some typical sampling frequencies