  This paper proposes a practical approach to estimate the
direct-to-reverberant energy ratio (DRR) using a spherical microphone array
without having knowledge of the source signal. We base our estimation on a
theoretical relationship between the DRR and the coherence estimation function
between coincident pressure and particle velocity. We discuss the proposed
method's ability to estimate the DRR in a wide variety of room sizes,
reverberation times and source receiver distances with appropriate examples.
Test results show that the method can estimate the room DRR for frequencies
between 199 - 2511 Hz, with $\pm$ 3 dB accuracy.
