In this paper we propose a technique based on the phase evolution of the Short Time Fourier Transform (STFT) for increasing the spectral resolution in the time-frequency analysis of a musical signal. It is well known that the phase evolution of the STFT coefficients brings important information on the spectral components of the analysed signal. This property has already been exploited in different ways to improve the accuracy in the estimation of the frequency of a single component. In this paper we propose a different approach, where all the coefficients of the STFT are used jointly to build a measure of how likely all the frequency components are, in terms of their phase coherence evaluated in consecutive analysis window. In more detail, we construct a phase coherence function which is then integrated with the usual amplitude spectrum to obtain a refined description of the spectral components of an audio signal.