Experimental measurements have been carried out on natural vibration frequencies of steel and aluminum cylinders of diverse radii and lengths in symmetric longitudinal modes. A new method of noncontact detection has been used, based on speckle heterodyne interferometry. As opposed to other authors’ experiments, these experiments do not only include slender specimens but include diameter-to-length ratios in the interval from 0.0156 to 0.6254. A study was performed using the results of the experiment. The relationship between the frequency and some variables of interest were analyzed and the dispersion of the velocity was determined. A systematic dependence of the wave velocity on the diameter was observed, even for small diameter-to-wavelength ratios. The results obtained were compared with the theoretical ones for infinite rods and limits were established for those the commonly used simplified theory may not be applied to. These results can be used to determine the Young’s modulus from the frequencies of the longitudinal modes.