Temporal Decomposition (TD) is an efficient technique for modeling speech spectral evolution through orthogonalization of the matrix of spectral parameters which reduces the amount of spectral information in TD-based speech coding. We have shown in earlier work that ``event'' functions can be approximated by fixed-width Gaussian functions with a minor degradation in the reconstructed speech, leading to further bit-rate reduction in such systems. In this paper, through perceptually-based spectral distortion measurement, we show the impact of events shape on the speech quality, and propose a new composite function and discuss its effect on the coder performance using different combinations of spectral parameters in event detection and speech synthesis.