Speech can produce jet-like transport relevant to asymptomatic spreading\n of virus

Abstract

Many scientific reports document that asymptomatic and presymptomatic\nindividuals contribute to the spread of COVID-19, probably during conversations\nin social interactions. Droplet emission occurs during speech, yet few studies\ndocument the flow to provide the transport mechanism. This lack of\nunderstanding prevents informed public health guidance for risk reduction and\nmitigation strategies, e.g. the "six-foot rule". Here we analyze flows during\nbreathing and speaking, including phonetic features, using order-of-magnitudes\nestimates, numerical simulations, and laboratory experiments. We document the\nspatio-temporal structure of the expelled air flow. Phonetic characteristics of\nplosive sounds like 'P' lead to enhanced directed transport, including jet-like\nflows that entrain the surrounding air. We highlight three distinct temporal\nscaling laws for the transport distance of exhaled material including (i)\ntransport over a short distance ($<$ 0.5 m) in a fraction of a second, with\nlarge angular variations due to the complexity of speech, (ii) a longer\ndistance, approximately 1 m, where directed transport is driven by individual\nvortical puffs corresponding to plosive sounds, and (iii) a distance out to\nabout 2 m, or even further, where sequential plosives in a sentence,\ncorresponding effectively to a train of puffs, create conical, jet-like flows.\nThe latter dictates the long-time transport in a conversation. We believe that\nthis work will inform thinking about the role of ventilation, aerosol transport\nin disease transmission for humans and other animals, and yield a better\nunderstanding of linguistic aerodynamics, i.e., aerophonetics.\n