This paper is concerned with studying the problem of scattering of a spheri
cal wave by a thin hard barrier on a hard plane. This problem is relevant t
o investigating the effect on the early part of the sound field of a large
room, when a simple, thin and hard strip-like element is set horizontally o
n a side wall. This scattering element may be a sound reinforcing reflector
or an idealisation of a side balcony. Three different calculation models b
ased on ray acoustic concepts and on solutions to the problem of diffractio
n by a half plane are dealt with and compared to each other. For the case o
f the half plane, one of the models, the Biot-Tolstoy theory of diffraction
, is a treatment in the time domain whereas the two other ones are approxim
ate and give solutions in the frequency domain (one of them is the Geometri
cal Theory of Diffraction). The expression of the diffracted field in the t
ime domain approach is exact but quite complicated so that, it has not been
possible to give its Fourier transform in an exact form. In an earlier pub
lication on this subject, the problem is overcome by making a simple analyt
ical approximation of the early part the diffracted field, and then, adding
to its exact Fourier transform the Digital Fourier Transform of the remain
ing part of the diffracted field. In this paper, an improvement is given to
the expression of this early part of the diffracted field and it is shown
that for both the time and the frequency domains, this new form is accurate
enough for most engineering purposes. Moreover, the frequency from of this
latter has a very simple expression. As one is interested in covering as l
arge a low frequency range as possible, multiple diffraction is also implem
ented in each model to take into account the finite width of the barrier on
the plane. Some experimental results are presented also supporting the the
oretical predictions quite favourably. (C) 1999 Elsevier Science Ltd. All r
ights reserved.