## 2 Materials and Methods
### Research Methods  
The experimental user experience research of Binaural Odyssey used a within-subjects design and was conducted between 23 June-13 August 2021 in a research laboratory of Tampere University with 14 Finnish adult research participants, who used Binaural Odyssey in a controlled setting. However, due to technical problems during the experiment, one research participant was excluded from this study, resulting in 13 research participants. These participants were acquired from both personal and professional contacts of the research group, which resulted in a diverse but relatively academic user group. The study was conducted in the Finnish language. In order to get necessary data for evaluating the therapeutic potential, we gathered both subjective and objective data about the user experience of the participants. The subjective data consisted of interviews and digital questionnaires, and Heart Rate Variability (HRV) data was collected during the study to get objective insight into the functioning of the autonomous nervous system (ANS) of the participants. Previous studies have found higher HRV to be associated with higher emotional well-being [16; 17]. In this study, we focused on SDNN, RMSSD, and Baevsky's stress index (SI) mean values. SDNN and RMSSD values relate to the activity of the parasympathetic nervous system [18], giving insights into the relaxation level of the subject, while Stress Index (SI) is a geometric measure of HRV that reflects cardiovascular system stress [19]. High SI values indicate reduced HRV and an activated sympathetic nervous system, whereas a reduced SI value would indicate the desired effect for a therapeutic relaxation application. During the experiment, the participants were first briefed about the research and their rights as a research participant, before being asked to wear a Polar H10 heart rate monitor chest strap. An Android smartphone with the Elite HRV application was used to collect HRV data from the Polar H10 during the experiment. Figure 1: Two screenshots from the Binaural Odyssey application. The research situation was divided into three phases: Pre-intervention, VR-intervention, and Post-intervention. Research participants were in a sitting posture in each phase, which had a minimum length of five minutes to ensure proper HRV data samples for analysis. During pre-intervention, the participants were interviewed about their background information, with a focus on prior experience regarding virtual reality and their relaxation skills. Participants also filled in several digital questionnaires--a perceived stress scale PSS-10 to measure their initial stress level and a variation of SUXES [20] with a Likert scale (1-7) to evaluate their expectations regarding _Q1 Quick Adaption to VR_, _Q2 Immersivity of VR_, _Q3 Focus to task_ (relaxation), _Q4 Sensory conformity_, _Q5 Visual experience_, and _Q6 Auditory experience_ aspects of the experience with Binaural Odyssey. Finally, participants filled out a Current State Of Mind (CSOM) [21] questionnaire to evaluate current subjective feelings regarding their levels of _Anxiety_, _Tension_, _Relaxation_, and _Resources_ with a Likert scale (0-10). This phase also served as a baseline HRV measurement. In VR intervention, the participants used Binaural Odyssey with an HTC Vive head-mounted display and Audio-Technica ATH-M40 X headphones for a duration of five minutes. The participants were sitting on a rotating chair and encouraged to fully experience the 360-degree nature of the virtual reality experience. The researcher ensured a smooth setup for the experience and observed the situation during the intervention. The HRV measurement from this phase was later compared to both the baseline measurement and post-intervention measurement. In Figure 2, a research participant can be seen with the research setup in the VR intervention phase.