  Mid-air ultrasound technology offers new design opportunities for contactless
tactile patterns (i.e., Tactons) in user applications. Yet, few guidelines
exist for making ultrasound Tactons easy to distinguish for users. In this
paper, we investigated the distinguishability of temporal parameters of
ultrasound Tactons in five studies (n=72 participants). Study 1 established the
discrimination thresholds for amplitude-modulated (AM) frequencies. In Studies
2-5, we investigated distinguishable ultrasound Tactons by creating four Tacton
sets based on mechanical vibrations in the literature and collected similarity
ratings for the ultrasound Tactons. We identified a subset of temporal
parameters, such as rhythm and low envelope frequency, that could create
distinguishable ultrasound Tactons. Also, a strong correlation (mean Spearman's
$\rho$=0.75) existed between similarity ratings for ultrasound Tactons and
similarities of mechanical Tactons from the literature, suggesting vibrotactile
designers can transfer their knowledge to ultrasound design. We present design
guidelines and future directions for creating distinguishable mid-air
ultrasound Tactons.
