# A Review of Recent Literature on Audio-Based Pseudo-Haptics
## 2 The Review
### Audio-Based Pseudo-Haptics  
[PERSON] et al. [54] conducted a qualitative study to explore cross-modal illusions, particularly inducing haptic feedback using auditory cues. A within-subject study was conducted to explore participants' experience when presented with different auditory combinations while interacting with a block in VR. The study consisted of two sessions: (i) a usability test and (ii) a focus group session. As part of the usability test, 23 participants were tasked with exploring nine virtual rooms, each featuring distinct auditory stimuli, and engaged with tactile blocks (suspended in the air) by rubbing them. A Leap Motion Controller (optical hand tracker) allowed the participants to interact with virtual objects without physically touching the blocks. The purpose of the usability test was to select \"information-rich\" participants to participate in the focus group session collected through a questionnaire, observation, and an interview. In the virtual environment, the participants interacted with these blocks. The blocks responded to touch by returning to their original position. Three auditory stimuli were used during these interactions: (i) pure texture: audio recordings of real-world textures being rubbed, providing a direct tactile experience; (ii) atomal abstract: pitch-bent continuous notes played using sine, triangle, sawtooth, or square waveforms, varying based on hand movement velocity; and (iii) total abstract: synthesized notes across all waveforms played across different octaves, simulating minute textural interactions for a pseudo-haptic experience. These auditory strategies aimed to enhance the immersive nature of the virtual reality (VR) simulation and create intuitive mappings between audio and tactile sensations. Participant behaviors were video-recorded, and at the end of the session, participants were asked to complete a questionnaire and participate in a semistructured interview. During the focus group sessions, based on their usability test scores, 10 of the 23 participants engaged in a moderately structured discussion guided by pre-defined questions. These questions aimed at testing the participants' experience and the sensations they felt while in the virtual environment. The results indicated that believable, real-world-like auditory and visual feedback, along with a consistent and immersive VR environment, were necessary to induce a pseudo-haptic sensation. [PERSON] et al. [42] presented a novel method to vary the sensation of heaviness by modifying the auditory feedback presented to users in response to user input. Their method manipulated the response delay, frequency, and loudness of a sound. A total of 131 participants were asked to rate the heaviness of the two squares displayed on their screen. By clicking on a button associated with each of the two squares, one of the sound conditions was played. The sound conditions included two frequencies, (i) 200 Hz and (ii) 400 Hz, accompanied by one of five delay conditions: (i) 0 ms, (ii) 100 ms, (iii) 200 ms, (iv) 300 ms, and (v) 500 ms. The output sound consisting of one frequency and one delay condition was randomly assigned to each of the two buttons. A control condition without any sound was also included. Eleven sound conditions (two frequencies \(\times\) five delays + one no sound condition) were considered, each repeated four times for a total of 44 trials. After the sound condition was played from the two squares, the participants were asked to compare the two squares and rate which one they perceived to be heavier using a 5-point Likert scale (1 being the square on the left was heavier and 5 being the square on the right is heavier). Results showed significant differences between the two frequencies, with the lower frequencies being perceived as heavier. Moreover, there was significance between the delay times, where the participants perceived a stronger heaviness sensation with increased delay. However, no significant difference was found between frequency and delay interaction.