  Visual to auditory sensory substitution devices convert visual information
into sound and can provide valuable assistance for blind people. Recent
iterations of these devices rely on depth sensors. Rules for converting depth
into sound (i.e. the sonifications) are often designed arbitrarily, with no
strong evidence for choosing one over another. The purpose of this work is to
compare and understand the effectiveness of five depth sonifications in order
to assist the design process of future visual to auditory systems for blind
people which rely on depth sensors. The frequency, amplitude and reverberation
of the sound as well as the repetition rate of short high-pitched sounds and
the signal-to-noise ratio of a mixture between pure sound and noise are
studied. We conducted positioning experiments with twenty-eight sighted
blindfolded participants. Stage 1 incorporates learning phases followed by
depth estimation tasks. Stage 2 adds the additional challenge of azimuth
estimation to the first stage's protocol. Stage 3 tests learning retention by
incorporating a 10-minute break before re-testing depth estimation. The best
depth estimates in stage 1 were obtained with the sound frequency and the
repetition rate of beeps. In stage 2, the beep repetition rate yielded the best
depth estimation and no significant difference was observed for the azimuth
estimation. Results of stage 3 showed that the beep repetition rate was the
easiest sonification to memorize. Based on statistical analysis of the results,
we discuss the effectiveness of each sonification and compare with other
studies that encode depth into sounds. Finally we provide recommendations for
the design of depth encoding.
