Barrass S (1998) Some Golden Rules for Designing Auditory Displays. In: Boulanger R (ed) Csound Textbook – Perspectives in Software Synthesis, Sound Design, Signal Processing and Programming, MIT Press.

Dirt/Gold and Figure/Ground was a sonification designed in response to Sara Bly’s challenge to classify gold from dirt in soil samples that cannot be represented in less than six dimensions [Bly, 1994]. The application of the theory of Auditory Scene Analysis in the design of this sonification was described in a chapter in the Csound Book [Barrass 1998].

The sonification was realised using a six parameter FM algorithm that produces a rich non-linear space of timbres that range from vibrating chirps and bell like dongs to deep gurgles and noisy drones. The data points in the gold category were mapped to brighter timbres, while the dirt category were mapped to duller deeper timbres.

The Van Noorden gallop was used to perceptually segregate gold from dirt. Ten points from the gold category were alternated with ten points from the dirt category, and played at a rate of 5 tones per second (an intertone onset interval (IOI) of 200ms). The parameters of the FM algorithm were adjusted to maximise the segregation between gold and dirt categories.

Twenty-seven subjects between the ages of 20 and 60 were asked to classify 30 random points from the soil dataset. The subjects could take as long as they liked with as many repetitions as they needed. The results show a 74% success rate that is significantly better than chance (p < 0.01) [Barrass, 1998].

In further development the use of the Van Noorden effect to scale the FM parameter space also pointed to the potential to listen to more than one point at a time. This led to an experiment in which 1000 random points of gold and dirt were played back at a rate of 10 points per second to produce a 100 second long granular synthesis texture.

The sonification gave an impression of the distribution and quantity of gold that stood out as brighter figures against the duller background of the dirt. This was a significant development because it demonstrates a way that stream-based sonifications may allow answers to questions about a dataset as a whole based on figure/ground streams.

The Dirt and Gold case study raises the question of whether the Van Noorden gallop could be used to perceptually scale a general range of timbre spaces. Other questions emerging from this case study include:  How well can listeners judge statistical information about the distribution of two categories from a granular stream-based sonification? What is the relation between playback rate and perception of statistical information?