In this paper, a new type of metric that defines the similarity between musical audio signals is proposed. Based on the spectral flatness criterion, those metrics achieve low computational cost and low sensitivity to acoustical degradations. Validation is performed by studying the ability of the proposed metric to determine whether two audio signals have been played by the same musical instrument. For this task, proposed metrics are shown to overcome metrics based on the comparison of standard spectral features especially when the request and the records of the database are of different acoustical properties.