Mid-air haptic feedback technology produces tactile sensations that are felt without the need for physical interactions, wearables or controllers. When designing mid-air haptic stimuli, it is important that they are sufficiently different in terms of their perceived sensation.This paper presents the results of two user studies on mid-air haptic feedback technology, with a focus on the sensations of haptic strength and haptic roughness. More specifically, we used the acoustic pressure intensity and the rotation frequency of the mid-air haptic stimulus as proxies to the two sensations of interest and investigated their Just Noticeable Difference (JND) and Weber fractions. Our results indicate statistical significance in the JND for frequency, with a finer resolution compared to intensity. Moreover, correlations are observed in terms of participants' sensitivity to small changes across the different stimuli presented. We conclude that frequency and intensity are mid-air haptic dimensions of depth 5 and 3, respectively, that we can use for the design of distinct stimuli that convey perceptually different tactile information to the user
