Mouth-Clicks used by Blind Expert Human Echolocators – Signal Description and Model Based Signal Synthesis

Echolocation is the ability to use sound-echoes to infer spatial information about the environment. Some blind people have developed extraordinary proficiency in echolocation using mouth-clicks. The first step of human biosonar is the transmission (mouth click) and subsequent reception of the resultant sound through the ear. Existing head-related transfer function (HRTF) data bases provide descriptions of reception of the resultant sound. For the current report, we collected a large database of click emissions with three blind people expertly trained in echolocation, which allowed us to perform unprecedented analyses. Specifically, the current report provides the first ever description of the spatial distribution (i.e. beam pattern) of human expert echolocation transmissions, as well as spectro-temporal descriptions at a level of detail not available before. Our data show that transmission levels are fairly constant within a 60° cone emanating from the mouth, but levels drop gradually at further angles, more than for speech. In terms of spectro-temporal features, our data show that emissions are consistently very brief (~3ms duration) with peak frequencies 2-4kHz, but with energy also at 10kHz. This differs from previous reports of durations 3-15ms and peak frequencies 2-8kHz, which were based on less detailed measurements. Based on our measurements we propose to model transmissions as sum of monotones modulated by a decaying exponential, with angular attenuation by a modified cardioid. We provide model parameters for each echolocator. These results are a step towards developing computational models of human biosonar. For example, in bats, spatial and spectro-temporal features of emissions have been used to derive and test model based hypotheses about behaviour. The data we present here suggest similar research opportunities within the context of human echolocation. Relatedly, the data are a basis to develop synthetic models of human echolocation that could be virtual (i.e. simulated) or real (i.e. loudspeaker, microphones), and which will help understanding the link between physical principles and human behaviour

Frequency dependent directivity diagrams.

<p>Top row: Azimuth frequency-dependent directivity diagrams for EE mouth-clicks at 40cm. Lines indicate average of measured data; shaded regions denote 1 SD around the average; different colours denote different frequency bands. Middle row: Azimuth frequency-dependent directivity diagrams for EE mouth-clicks at 100cm. Line and colour coding as in top row. Bottom row: Elevation frequency-dependent directivity diagrams for EE mouth-clicks. Line and colour coding as in top row.</p

Synthetic click parameters for EE1, EE2, and EE3.

<p>Synthetic click parameters for EE1, EE2, and EE3.</p

Illustrations of click waveforms.

<p>Illustrations of waveforms of three clicks for each of the three echolocators.</p

Click synthesis.

<p>Synthetic clicks plotted for EE1 (left), EE2 (middle), and EE3 (right) in the frequency-time-domain (top), frequency-domain (middle), and time-domain (bottom).</p

Directivity diagrams.

<p>Top row: Azimuth directivity diagrams for EE mouth-clicks at 40cm. Markers indicate average of measured data; shaded regions denote 1 SD around the average; red line is fit of a modified cardioid. Middle row: Azimuth directivity diagrams for EE mouth-clicks at 100cm. Symbol and colour coding as in top row. Bottom row: Elevation directivity diagrams for EE mouth-clicks. Markers indicate average of measured data; shaded regions denote 1 SD around the average.</p

Frequency content of clicks for all echolocators.

<p>Top: Averaged spectrograms; Middle: Averaged power spectral density (PSD) plots; shaded regions denote 1 SD around the average; Bottom: Waterfall plots showing a set of PSD estimates for a subset of clicks and the average PSD estimate (black line at back).</p