The aim of the paper is to give a physical explanation of the absence of the
feature in the Calzetti extinction curve. We analyze the dust attenuation of a
homogeneous source seen through a distant inhomogeneous distant screen. The
inhomogeneities are described through an idealized isothermal turbulent medium
where the probability distribution function (PDF) of the column density is
log-normal. In addition it is assumed that below a certain critical column
density the carriers of the extinction bump at 2175 Angstroem are being
destroyed by the ambient UV radiation field. Turbulence is found to be a
natural explanation not only of the flatter curvature of the Calzetti
extinction curve but also of the missing bump provided the critical column
density is N_H >= 10^21 cm^-2. The density contrast needed to explain both
characteristics is well consistent with the Mach number of the cold neutral
medium of our own Galaxy which suggests a density contrast sigma_(rho/) 6.Comment: 6 pages, 6 figures accepted for publication in A&A, section
