# Drone Rider: Foot Vibration Stimuli to Enhance Immersion and Flight Sensation in VR
## 5 Discussion  
In Experiment 1, the vibrations generated by the drone propeller sounds did not result in high ratings for the sense of flight and immersion. Two potential reasons for this outcome are as follows. First, the primary frequencies of the vibrations from Drones 1, 2, and 3 were 115 Hz, 320 Hz, and 580 Hz, respectively, along with their harmonic frequencies. Human cutaneous mechanoreceptors adapt quickly to such high-frequency stimuli [40, 41, 42]. As shown in Figure 4, participants noted experiencing numbness in their soles due to these vibrations. This sensory adaptation likely reduced their sensitivity to the vibratory stimuli, diminishing the effects on their sense of flight and immersion. Second, participants reported that the vibrations generated by Drones 1, 2, and 3 did not align with the sensations they expected from the drone they were virtually riding. As shown in Figure 3B, the Q3 scores (\"How well did the vibrations on your soles match the simulated drone?\") were lower for the vibrations of Drones 1, 2, and 3 compared to those of the chainsaw, motorcycle, and laundry machine. Some participants noted that the vibrations of Drones 1, 2, and 3 felt inconsistent with what they typically associate with a flying vehicle. This mismatch between participants' expectations and the vibratory feedback likely contributed to the lower questionnaire scores. In contrast, the vibration produced by the motorcycle was rated highly in terms of both the sense of flight and immersion. This vibration consisted of relatively low frequencies, with prominent peaks at 70, 90, 110, 125, 140, and 150 Hz, as shown in Figure 2c. Additionally, the vibration was intermittent rather than continuous, which may have reduced the likelihood of participants adapting to the stimuli. However, we did not objectively assess the level of numbness experienced by participants in response to the vibrations during the experiments. Thus, the role of sensory adaptation in influencing the questionnaire responses remains an open question that warrants further investigation in future studies. We had anticipated that superimposing the vibration sources from the non-dominated solutions in Experiment 1 would further enhance the sense of flight and immersion. However, the superimposed vibration sources did not yield higher scores compared to the original vibration of motorcycle. Although the results did not align with our expectations, the identification of the best vibratory stimuli from among the several candidates remains a significant outcome of this study. A previous study suggested the effectiveness of pure sinusoidal vibration at 80 Hz to evoke the sense of flight or hovering [8]. However, in our study, vibrations primarily composed of a single frequency did not receive high ratings from the participants. This is in agreement with a report by [PERSON] et al. [24], where they observed the effectiveness of  
Figure 5: Results of Experiment 2. Mean scores and standard errors of the sense of flight and immersion for each vibration. The motorcycle vibration outperformed all other vibrations. an engine-like vibratory stimulus presented by a seat in enhancing the sense of self-motion. The vibrations of Drones 1, 2, and 3 also included one prominent frequency, resembling the condition of pure tones. These results suggest that the vibrations synthesized by multiple frequency components can be more effective than pure tones. If this is correct, we need a method to synthesize vibrations designed to increase target subjective ratings. Such approaches were attempted in some earlier studies [44; 45; 46; 47]. The limitations of this study are noted. All participants in this experiment had no prior experience with operating drones or flying airplanes. Individuals with such experience might show different experimental results compared to those without. The influence of participants' experience on the experimental results remains unclear.