DRR (Direct-to-Reverberant Ratio) and Reverberation time (T60) are said to be two of the most important characteristics of sound in Reverberation. DRR is said to be ratio of the energies of the direct signal and its reverberations.
The question that I'm trying to figure out is if it is possible to calculate DRR from either the Room Impulse Response or from a reverberated signal itself.
The direct to reverberant ratio is simply (as the name implies) the energy of the direct sound divided by the energy of the reverberation. Given a room impulse response $h[n]$ this can simply be calculated as
$$DRR = 10 \cdot \log_{10} \frac{\sum_{n=n_0}^{n_0+M-1} h^2[n]}{\sum_{n=n_0+M}^{N} h^2[n]} $$
where $n_0$ is the onset of the impulse response, $N$ is the usable length of the impulse response and $M$ a suitable time window that depends a bit on the specific application and the frequency range of interest but is typically in the low millisecond range.
In practice this requires often a little bit of "fudging" due to the quality of the RIR measurements. Real RIRs typically often have pre-ringing, are high passed somewhere and contain a fair bit of noise.
