  The importance of the information in the direct sound to human perception of
spatial sound sources is an ongoing research topic. The classification between
direct sound and diffuse or reverberant sound forms the basis of numerous
studies in the field of spatial audio. In particular, parametric spatial audio
representation methods use this classification and employ signal processing in
order to enhance the audio quality at reproduction. However, current literature
does not provide information concerning the impact of ideal direct sound
representation on externalization, in the context of Ambisonics. This paper
aims to assess the importance of the spatial information in the direct sound in
the externalization of a sound field when using binaural reproduction. This is
done in the spherical harmonics (SH) domain, where an ideal direct sound
representation within an otherwise Ambisonics signal is simulated, and its
perceived externalization is evaluated in a formal listening test. This
investigation leads to the conclusion that externalization of a first order
Ambisonics signal may be significantly improved by enhancing the direct sound
component, up to a level similar to a third order Ambisonics signal.
