Energy conversion in sonochemistry is known to be an important factor for understanding the mechanism of chemical reactions. However, ultrasound characteristics are dependent on the physical properties of the liquid medium. This work describes specific heat capacities (cp) of five hydrophobic bis(trifluoromethylsulfonyl)imide based ionic liquids ([C8mpyrr][NTf2], [C4mpyrr][NTf2], [C8epip][NTf2], [C8mim][NTf2], [C8mpy][NTf2]) and, for the first time, the acoustic power when they are submitted to ultrasound at two selected electric powers. The behavior of ultrasound in ionic liquids is then compared to the one in water. Despite the difference in cp values, the measured acoustic powers are very similar for both media. Indeed, ionic liquids heat up faster than water, and this can lead to interesting effects when using ionic liquids as solvents in organic chemistry.