The quality of unit selection based concatenative speech synthesis mainly depends on how well two successive units can be joined together to minimise the audible discontinuities. The objective measure of discontinuity used when selecting units is known as the `join cost'. The ideal join cost will measure `perceived' discontinuity, based on easily measurable spectral properties of the units being joined, in order to ensure smooth and natural-sounding synthetic speech. In this paper we describe a perceptual experiment conducted to measure the correlation between `subjective' human perception and various `objective' spectrally-based measures proposed in the literature. Also we report new objective distance measures derived from various distance metrics based on these spectral features, which have good correlation with human perception to concatenation discontinuities. Our experiments used a state-of-the art unit-selection text-to-speech system: `rVoice' from Rhetorical Systems Ltd.