Experimental measurements of the phase speed are performed in a medium of air bubbles in castor oil. Castor oil has been chosen as the host liquid due to both its density and sound speed being closely matched to that of water. The viscosity of castor oil is about 1000 times greater than water which allows bubbles to rise relatively slowly in a cylindrical container. The phase speed will be measured in an apparatus similar to the standing wave tube setup reported by E. Silberman [J. Acoust. Soc. Am. 29, 925--933 (1957)]. We are interested in comparing our results with the phase speed theory that was reported by K. W. Commander and A. Prosperetti [J. Acoust. Soc. Am. 85, 732--746 (1989)]. The volume void fraction (beta) (of air volume to total mixture volume) is estimated by electronic measurements which involve measuring the capacitance of a cylindrical coaxial capacitor in air, in oil, and in bubbly oil.