# Drone Rider: Foot Vibration Stimuli to Enhance Immersion and Flight Sensation in VR
## 3 Experiment 1: First Round of Optimization Procedure  
### Sound and Vibratory Stimuli  
The authors, along with two colleagues, tested vibrations generated from the sounds of various drone propellers. These sounds included intense frequency components above 100 Hz. We found that the perceived magnitudes of these vibrations decreased shortly after exposure began, likely due to sensory adaptation effects [40; 41]. For example, fast-adaptive type-II mechanoreceptors, which are sensitive to vibratory stimuli, undergo significant adaptation after only a few seconds of continuous vibration exposure [42]. Participants  
Figure 1: System configuration of Drone Rider. (**A**) Entire view of the hardware. (**B**) Image seen through the VR headset. (**C**) Vibratory foot plate.  
reported numbness in their feet, which detracted from the quality of the flying experience. Consequently, we decided to include vibrations from rotary machines as well as those of drones in our tests.  
Thirteen types of vibrations were selected for investigation in Experiment 1, following the steps below. First, the authors and two colleagues gathered sound sources from 50 types of rotary machines, including seven types of drones, using recordings from video streaming sites. Second, six colleagues evaluated the vibrations produced by each sound, assessing whether each vibration could evoke a sense of flying using a two-alternative forced-choice method by responding to the question, \"Did you feel like you were flying in the sky?\" For each vibration, participants stood on the vibratory plate and experienced a free virtual flight for 50 s. Of the initial 50 sounds, 20 received more than three votes, and 12 of these received five or six votes. To ensure practical feasibility during the main trials, we selected the 12 sounds with the highest votes (five or six). Additionally, an 80 Hz vibration was included in the set based on a prior study [8], bringing the total number of vibrations tested in Experiment 1 to 13.  
During the evaluation, vibration levels (sound volume) were adjusted to ensure all vibrations were perceived as subjectively equal in intensity. This adjustment was performed by the authors, who set the sound levels individually, and the mean levels were applied. All sound signals recorded from the video-sharing sites were transformed into monaural format at a sampling rate of 44,100 Hz.  
The thirteen selected sounds were as follows: propeller sounds from three types of drones (Drone 1, Drone 2, Drone 3). Drone 1 was a human-ridable drone, while Drones 2 and 3 were mid-sized drones with payloads of several tens of kilograms. Additionally, we included the sound of a helicopter propeller (Helicopter) and two jet aircraft sounds (Planes 1 and 2). Further sounds included a motorcycle, chainsaw, laundry machine, chemical plant, compression machine, gears, and the 80 Hz tone.  
Figure 2 shows the amplitude of vibratory stimuli in both the time and frequency domains for representative vibration patterns: Drone 2, Plane 1, motorcycle, and laundry machine. These vibrations were measured using an accelerometer (2302B, Showa Sokki Corp., Tokyo, Japan) attached to the center of the vibratory surface of the voice coil motor. The measurements were conducted without any load applied to the voice coil motor.