The heart rate variability data are sonified with a software sound synthesis (SWSS) program. Such programs, the basis of computer music [1], have existed since the 1950s. Music is represented digitally by converting the air pressure changes of musical events into a discrete series of numbers or samples.  The samples are audified by being passed into a digital to audio converter (DAC), which converts the sample values into voltage values that are used to vibrate the cone of a loudspeaker, thus producing the desired sound. For example, an audio compact disc (CD) contains a set of discrete samples. The CD player contains a DAC that feeds the numbers to an amplifier, which in turn sends energy proportional to the discrete sample values to a loudspeaker. Software sound synthesis programs enable a composer to create a set of samples so that a composition may be realized and stored digitally.

We sonify HRV data with the SWSS program SuperCollider, a specialized programming language designed for real-time audio applications. SuperCollider displays exceptional versatility, with capabilities including real time signal processing, algorithmic composition and inter-machine remote control.

SuperCollider is well suited to our sonification model because of its computational efficiency, its array and list processing capability, its methodology for generating (spawning)  musical events according to a programmer's instructions, and its interactive potential through the use of custom-designed graphical user interfaces (GUIs).

A series of signal processing operations are saved as separate files, loaded into SuperCollider and stored as array variables. The arrays are iterated simultaneously, with each successive value employed as the source of a musical event. The model employs the following mappings:

• Each interbeat interval is mapped to a pitch, sounded by an oscillator that produces short sine wavae sounds ("grains").  Higher heart rates correspond to higher pitches.  To produce a harmonious blend of sounds, the same pitch mapping formula is used as the basis for all sonification tracks.
• Successive intervals differing by more than 50 ms are given an additional timbral annotation, a "tinkling" sound produced by phase modulation synthesis.
• The current interval is considered to be the center of a sliding window of 300 values, a number corresponding to roughly five minutes of activity, a time window used for some heart rate variability analyses [11].  The window's standard deviation is sonified by a percussive, tapping sound. The standard deviation value is mapped to the speed of the tapping.
• Two smaller sliding windows sonify running means.  The window sizes are smaller than that used for the standard deviation window in order to hear changes on shorter time scales. The first is a window of fifteen values, sounded by a glassy hum.  The second is a window of 5 values, with rounded values so that changes occur with a coarser degree of precision, sounded by a clarinet-like timbre.

To run the sonification patch:

Download SuperCollider here

Then download the file HRV_sonification.zip (zip archive, 240 KB).