The effect of diffraction in increasing the velocity of propagation over the plane wave value is determined experimentally by phase comparison of reflected longitudinal waves in fused silica blocks. It is shown that the parameters of transducer diameter, path length traveled, and frequency can usually be adjusted to keep the fractional excess velocity under 0.02% for measurements in the 20–100‐Mc range. This consideration is of particular importance in connection with precision measurement of elastic moduli.