  The dispersion properties of low frequency dust acoustic waves in the strong
coupling regime are investigated experimentally in an argon plasma embedded
with a mixture of kaolin and $MnO_2$ dust particles. The neutral pressure is
varied over a wide range to change the collisional properties of the dusty
plasma. In the low collisional regime the turnover of the dispersion curve at
higher wave numbers and the resultant region of $\partial\omega/\partial k < 0$
are identified as signatures of dust-dust correlations. In the high collisional
regime dust neutral collisions produce a similar effect and prevent an
unambiguous identification of strong coupling effects.
