Loud and Shouted Speech Perception at Variable Distances in a Forest

To increase the range of modal speech in natural ambient noise, individuals increase their vocal effort and may pass into the 'shouted speech' register. To date, most studies concerning the influence of distance on spoken communication in outdoor natural environments have focused on the 'productive side' of the human ability to tacitly adjust vocal output to compensate for acoustic losses due to sound propagation. Our study takes a slightly different path as it is based on an adaptive speech production/perception experiment. The setting was an outdoor natural soundscape (a plane forest in altitude). The stimuli were produced live during the interaction: each speaker adapted speech to transmit French disyllabic words in isolation to an interlocutor/listener who was situated at variable distances in the course of the experiment (30m, 60m, 90m). Speech recognition was explored by evaluating the ability of 16 normal-hearing French listeners to recognize these words and their constituent vowels and consonants. Results showed that in such conditions, speech adaptation was rather efficient as word recognition remained around 95% at 30m, 85% at 60m and 75% at 90m. We also observed striking differences in patterns of answers along several lines: different distances, speech registers, vowels and consonants.