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.

回声定位（echolocation）是指利用声波回声推断环境空间信息的能力。部分盲人通过口腔咔嗒声掌握了超凡的回声定位技能。人类生物声呐的第一步为发射信号（口腔咔嗒声），并通过耳朵接收由此产生的声波。现有头部相关传递函数（head-related transfer function, HRTF）数据库已对该接收声波的相关特性进行了描述。针对本研究报告，我们收集了由三位经过专业回声定位训练的盲人产生的大量咔嗒声发射数据集，这使得我们得以开展前所未有的精细分析。具体而言，本报告首次描述了人类专业回声定位发射信号的空间分布（即波束方向图，beam pattern），同时还提供了此前从未达到的精细时频特征描述。我们的数据分析结果显示，在以口腔为中心的60°锥角范围内，发射强度基本保持恒定，但随着观测角度增大，强度会逐渐衰减，且该衰减幅度大于语音信号。在时频特征方面，数据表明发射信号始终极短（时长约3毫秒），峰值频率处于2-4kHz频段，同时在10kHz频段也存在能量分布。这与此前基于较粗糙测量得到的3-15ms时长、2-8kHz峰值频率的研究结果存在差异。基于本次测量结果，我们提出将发射信号建模为衰减指数调制的单音信号之和，并结合改进的心形指向性（modified cardioid）实现角度衰减。我们为每位回声定位者提供了对应的模型参数。这些研究结果为构建人类生物声呐的计算模型迈出了关键一步。例如在蝙蝠研究中，发射信号的空间与时频特征已被用于推导并验证基于行为的模型假设。本次发布的数据同样为人类回声定位领域的类似研究提供了可行方向。相关地，本数据集可作为构建人工人类回声定位合成模型的基础，这类模型既可以是虚拟（即仿真）形式，也可以是实体（即通过扬声器、麦克风搭建）形式，将有助于厘清物理原理与人类行为之间的关联。