The current study focuses on 1-propyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide, [C3mim][NTf3], and 1-pentyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide, [C 5mim][NTf2]. The objective is to study the influence of pressure as well as that of the cation's alkyl chain length on several properties of this type of ionic liquids. Speed of propagation of ultrasound waves and densities in pure ionic liquids as a function of temperature and pressure have been determined. Other thermodynamic properties such as compressibilities and expansivities have been obtained. Speed of sound measurements have been carried out in broad ranges of temperature (298 < T/K < 338) and pressure (0.1 < p/MPa < 200). A novel high-pressure ultrasonic cell was designed and built to allow for precise speed of sound measurements in liquids. The uncertainty of the speed of sound measurements is 0.05 %. A detailed description of the cell is presented. Density measurements have been performed over a broad range of temperature (298 < T/K < 333) and pressure (0.1 < p/MPa < 60) using a vibrating-tube densimeter. The density overall uncertainty is 0.02 %.