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Sound morphing is a transformation that gradually blurs the distinction between the source and target sounds. For musical instrument sounds, the morph must operate across timbre dimensions to create the auditory illusion of hybrid musical instruments. The ultimate goal of sound morphing is to perform perceptually linear transitions, which requires an appropriate model to represent the sounds being morphed and an interpolation function to obtain intermediate sounds. Typically, morphing techniques directly interpolate the parameters of the sound model without considering the perceptual impact or evaluating the results. Perceptual evaluations are cumbersome and not always conclusive. In this work, we seek parameters of a sound model that favor linear variation of perceptually motivated temporal and spectral features used to guide the morph towards more perceptually linear results. The requirement of linear variation of feature values gives rise to objective evaluation criteria for sound morphing. We investigate several spectral envelope morphing techniques to determine which spectral representation renders the most linear transformation in the spectral shape feature domain. We found that interpolation of line spectral frequencies gives the most linear spectral envelope morphs. Analogously, we study temporal envelope morphing techniques and we concluded that interpolation of cepstral coefficients results in the most linear temporal envelope morph.