  Estimation of the direction-of-arrival (DOA) of sound sources is an important
step in sound field analysis. Rigid spherical microphone arrays allow the
calculation of a compact spherical harmonic representation of the sound field.
A basic method for analysing sound fields recorded using such arrays is steered
response power (SRP) maps wherein the source DOA can be estimated as the
steering direction that maximises the output power of a maximally-directive
beam. This approach is computationally costly since it requires steering the
beam in all possible directions. This paper presents an extension to SRP called
steered response power density (SRPD) and an associated, signal-adaptive search
method called hierarchical grid refinement (HiGRID) for reducing the number of
steering directions needed for DOA estimation. The proposed method can localise
coherent as well as incoherent sources while jointly providing the number of
prominent sources in the scene. It is shown to be robust to reverberation and
additive white noise. An evaluation of the proposed method using simulations
and real recordings under highly reverberant conditions as well as a comparison
with state- of-the-art methods are presented.
