The guided wave theory has been introduced to determine the usable frequency range for reliably using the well-known one-dimensional wave theory to interpret impulse response testing results in pile foundations. Length-to-diameter ratio is usually considered a key factor that affects the analysis results. In this research, 37 testing cases with length-to-diameter ratios widely varying from 4 to 36 were collected to determine the effect. Regardless of the diameter size or pile type, the testing cases with a length-to-diameter ratio (slenderness) below 15 had a consistent scattering along or near the theoretical usable frequency-diameter curves due to a better propagation ability. For the slender cases, the experimental points scatter far below the theoretical usable frequency-diameter curves due to more energy leakage during propagation.