The influence of the vibration frequency on the friction reduction of an ultrasonic haptic surface has been reported in the literature. The models predict that increasing the frequency of the vibration leads to higher friction reduction at constant vibration amplitude, but this has not been reported experimentally. In this paper, we study the friction reduction on a prototype which can vibrate at low (66kHz) and high (225kHz) frequency. By estimating the Point of Subjective Equivalence between a standard at low frequency with a sample at high frequency, we have found that high frequencies can indeed reduce the friction, with the advantage of much smaller vibration amplitude. Moreover, we show that the invariant between the two frequency conditions is not the vibration amplitude. Our conclusion is that the invariant could be the acceleration instead.