n-Undecane (C11H24) density and speed-of-sound measurements in liquid phase have been reported along five isotherms at temperatures between (278 and 318) K. The density results have been obtained at pressure up to 60 MPa by means of a vibrating tube DMA 512 while speed-of-sound measurements for pressures up to 100 MPa. The speed-of-sound values have been measured by means of a double-path pulse-echo technique at three different operating frequencies. It has been chosen to repeat the measurements at different frequency values because of the observed discrepancies in the fly-time determination. A particular emphasis has been done to analyze these discrepancies, not directly explicable by traditional diffraction corrections. Indeed we have deduced that the source of differences is ascribable to a combination of sample speed-of-sound, of the signal frequency and to the geometrical configuration of the experiment, that affect the determination of the fly-time of the ultrasonic burst. (C) 2012 Elsevier Ltd. All rights reserved.