Several applications in spatial audio signal processing benefit from the knowledge of the diffuseness of the sound field. In this paper, several experiments are performed to determine the response of a tetrahedral microphone array under a spherically isotropic sound field. The data were gathered with numerical simulations and real recordings using a spherical loudspeaker array. The signal analysis, performed in the microphone signal and spherical harmonic domains, reveals the characteristic coherence curves of spherical isotropic noise as a function of the frequency.