STRAUSS: Tour of the Solar System Sonifications

Here we give two examples of how we used our new sonification code STRAUSS to turn data into sound for the show Audio Universe Tour of the Solar System. This information is summarised in Harrison et al., to appear in Communicating Astronomy to the Public Journal. The resulting sonification of the data described below can be heard on our page on this website 'Science Behind the Show'

Sonification of Earth's Rotation

Figure demonstrating the data used to create the sonification of the Earth's rotation, as described in the caption.

Fig. 1: Diagram demonstrating our data sonification of the Earth’s rotation. Panel a) shows water covering fraction (left axis) versus longitude (top axis) over two Earth rotations with a world map projection as a grey underlay. These values are used to calculate the low-pass filter cutoff frequency for the sonification (right axis). Panel b) shows the waveform of the sonification as a function of time. Panels c) and d) demonstrate the effect of the filter on the waveform by zooming into 20 milli-second windows around longitudes where the water covering fraction is approximately highest and lowest, respectively (indicated by vertical lines of corresponding colour in Panel a).

We wanted to sonify sunlight bouncing off the spinning Earth through changing timbre as the Sun passes over water (a “brighter” sound) or land (a “darker” sound). For this, we used data of the covering fraction of water as a function of longitude. The data are from the GEBCO 2021 bathymetry, which assigns water or land to each cell of a 15x15 arcsecond grid across the Earth (Fig. 1). To create the sonification, we started with a sustained musical chord, using notes G flat 3, D flat 4, E 4 and B 4. Each note was created from a set of three sawtooth oscillators combined at frequencies on, 2% above and 2% below the target pitch. These choices provide a harmonically rich sound, which is then manipulated by filtering out frequencies (i.e. subtractive synthesis) based on the water covering fraction data. 

The longitude and water covering fraction (Fig. 1a, upper and left axes) were mapped directly to the time in the sequence and the filtering cut-off frequency of the chord, respectively (bottom and right axes). The cut-off frequency, above which frequencies are attenuated, was calculated from the water covering fraction using a logarithmic scale. A low-pass Butterworth filter (Butterworth, 1930) with a 24dB roll-off was used. The conversion of water fraction to frequency cut-off can be seen by comparing the left and right y-axes of Fig. 1a. We note the more jagged, harmonically rich waveform representing a longitude over the pacific ocean (Fig. 1c) relative to the smoother waveform representing a longitude over Europe and Africa (Fig. 1d). The filtering mainly changes the timbre of the sound but a secondary effect on volume is achieved in that the land-dominated regions sound the quietest (see waveform in Fig. 1b). 

Sonification of Stars Appearing in the Night Sky

Figure demonstrating how we turned star colour, star position and star magnitude into sound, as described in the caption

Fig. 2: Diagram demonstrating our sonification of the ‘stars appearing’. Panel a) shows the mapping of V-band magnitude (top axis) and B-V colour (left axis) to triggering time in the audio sequence (bottom axis) and musical note (right axis), respectively. The aligned Panel b) shows the waveform produced for a stereo setup, and the triggering times of the 10 brightest stars (dotted lines). The right panel shows the stellar sky chart, with point size and colour indicating brightness and B-V colour, respectively. In our sonification the observer faces south, with the left and right audio channels corresponding to the east and west cardinal directions, respectively.

The audience ‘listen’ to stars that appear around them at the European Southern Observatory’s Very Large Telescope (VLT). The data we used are presented Fig. 2, which are the magnitudes, colours and coordinates of stars as viewed from the VLT on the 13th September 2019. We only considered stars with V-band magnitudes <6, to roughly correspond to the detection limit of the human eye. 

Each star is represented by a single note on a glockenspiel from one of five pitches: D flat 3, G flat 3, A flat 3, E flat 4 or F 4, with the choice of note based on the star’s colour (specifically, the difference in the star’s B and V magnitude). The reddest stars are assigned the lowest notes and the bluest stars the highest notes. During the sequence each star is heard in an order based on its magnitude, with the brightest stars sounding first and the faintest sounding last. This represents how brighter stars appear first to the human eye after sunset.

The stars’ positions were used to determine in which speaker(s) they should be heard. For example, stars directly in front of the ‘observer’ sound in the front speakers for the surround sound version, or equally in the left and right ear for the stereo version.