  In this paper we report multi-frequency measurements of pulse broadening
times (tau_d) for nine medium dispersion measure (DM $\approx 150-400$ pc
cm$^{-3}$) pulsars observed over a wide frequency range. The low frequency data
at 243, 325 and 610 MHz are new observations done with the Giant Metrewave
Radio Telescope (GMRT). The frequency dependence of tau_d for all but one (PSR
B1933+16) of our sources is consistent with the Kolmogorov spectrum of electron
density fluctuations in a turbulent medium. PSR B1933+16, however, shows a very
flat spectrum as previously observed for high DM pulsars. Our observations
combined with earlier published results enable us to study the spectral index
of tau_d over the whole observed DM range. While the spectral properties are
generally consistent with a Kolmogorov spectrum, pulsars seen along
line-of-sights towards the inner Galaxy or complex regions often show
deviations from this expected behaviour.
