  We propose a natural way to generalize relative transfer functions (RTFs) to
more than one source. We first prove that such a generalization is not possible
using a single multichannel spectro-temporal observation, regardless of the
number of microphones. We then introduce a new transform for multichannel
multi-frame spectrograms, i.e., containing several channels and time frames in
each time-frequency bin. This transform allows a natural generalization which
satisfies the three key properties of RTFs, namely, they can be directly
estimated from observed signals, they capture spatial properties of the sources
and they do not depend on emitted signals. Through simulated experiments, we
show how this new method can localize multiple simultaneously active sound
sources using short spectro-temporal windows, without relying on source
separation.

Sound