# A Wearable Sonification System to Improve Movement Awareness: A Feasibility Study
## 3 Materials and Methods
### Developing the Sonification System  
The auditory feedback used in this study combined continuous and discrete tones to indicate the participant's balance state. The discrete tone was a bell sound triggered by the participant rotating outside the reference region, as illustrated at the top of Figure 3. The reference region was based on a 5% rotational deviation on either side of the reference datum collected in the calibration process. The continuous tones were shifted via an increase or decrease in the center frequency of a bandpass filter over a selection of tuned oscillators. The auditory effect was to brighten the tones as the rotation increased away from the reference region or, conversely, dull these tones as the rotation decreased in the region through the sagittal plane of movement, as shown in the lower half of Figure 3.  
Figure 3: The CAF system takes sagittal rotation data to generate continuous and discrete tones.  
Cueing movement with auditory feedback that changes outside a reference region follows the approach used by [42; 44]. In those studies, an auditory sinusoidal tone was increased in frequency when the participant accelerated in the anterior direction and decreased when the participant accelerated in the posterior direction whilst trying to balance. We sought to implement our sonified signal based on this relationship, as it has been demonstrated as a promising approach. In this study, we further improve it by combining continuous tones that were brightened as the rotation increased or conversely dulled as the rotation decreased, with a discrete bell tone that signaled the movement outside of the reference region in the sagittal plane. Figure 4 shows the proposed flow structure for retrieving data from the sensor and providing sonification to the user.  
The auditory feedback system was designed to guide users in real-time to maintain balance while performing a one-leg balance task. The IMU sensor, attached to the participant, started to read sagittal plane rotation data. This data was used to establish a baseline (datum) from the average of 50 sensor samples. From this datum, a minimum or maximum threshold was set at 5% on either side to detect significant deviations in balance.  
Information collected by the sensor was sent to MATLAB, which processed and converted them into readable acceleration and angular data. The processed data were then graphed in MATLAB for real-time observation and sent concurrently to Max. In Max, the data underwent further processing to become sonified signals, triggered based on one of two responses: (1) If the rotation data crossed the predefined thresholds, a discrete bell tone was generated to alert the user of a loss of balance. (2) For less critical deviations, the data were scaled and mapped to modify the cut-off frequency of a continuous tone. This continuous tone changes in pitch (or brightness) to provide subtle feedback on the user's position relative to balance. These auditory cues are transmitted to the user through headphones, providing a sonic representation of their balance in real time. The movement sonification system files can be found in the Supplementary Materials for reference.  
Whilst not seeking to time movement events as in other research, we used discrete tones to indicate a threshold was exceeded as an unmistakable sound moment in a corrective manner. This discrete sound moment can contrast and define movement regions with the continuously changing tone.  
We agreed with [PERSON] et al. [47] on the need for a dynamic movement unfolding using CAF. While [PERSON] et al. [32] applied this concept to walking gait to denote the leg's swing phase, we have adapted it to balance training as the CoM movement could benefit from real-time CAF to correct this movement back into a reference region and enhance kinesthetic awareness.  
Figure 4: Diagram of the algorithm and auditory feedback system.