Study Finds What Sounds Propel Virus Droplets Longer Distances

COVID positive? Do not recite ‘Peter Piper picked a peck of pickled peppers’

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A street trader sells face masks from a stall following the outbreak of the coronavirus disease (COVID-19) in Manchester, Britain, October 5, 2020
Street trader selliing face masks from a stall, Manchester, Oct. 5, 2020Credit: Phil Noble/Reuters

Say it, don’t spray it! Good luck with that. When we breathe, we emit particle-laden air flows, let alone when we speak, sing and exercise. Perfectly asymptomatic carriers of the SARS-CoV-2 coronavirus can shed viruses and infect others just by being in proximity. Now a new study has demonstrated that the distance emitted droplets travel depends not only on air circulation and distance but on the sounds we’re producing.

Depending on environmental conditions, the much-vaunted 2-meter distance isn’t always enough for maskless safety, especially if one’s protagonist is loudly issuing sounds including plosive consonants such as B and P, report Manouk Abkarian of the University of Montpellier with Simon Mendez of the French National Center for Scientific Research, and Nan Xue, Fan Yang and Howard Stone from the Princeton Department of Mechanical and Aerospace Engineering. Their findings were published in the Proceedings of the National Academies of Science

Obviously the research applies to people who know they have COVID-19, who are supposed to be isolated anyway. But this is a crucial warning to the newly recovered and to unwitting asymptomatic carriers who may be merrily cavorting maskless.

In fact, the trajectories of disease-bearing droplets emitted by coughing, sneezing and so on have been under study for decades, but technologies have improved and the pandemic gives the research a whole new urgency, Abkarian points out. When the coronavirus lockdowns began, he found himself at Princeton, far from his home in France; and thus began the fruitful collaboration on speech-generated airflow, resulting in the disgrace of the letters B and P.

Projection of CO2 emitted by a member of the MET Orchestra while singing, recorded using an infrared cameraCredit: M. Abkarian, P. Bourrianne and H.A. Stone (with help from the FLIR company and E. Bowman)

With the help of a laser sheet and Halloween fog machine (click here if you want more information on the technique), Abkarian and the team deduced that producing plosives such as P and B changes our mouth structure in a way that creates intense vortical structures in the air. You can think of these vortices as traveling puffs of you-laden air.

P- or B-words (pulse, petit peu) in his normal voice sent his droplets about a meter, he explains to Haaretz. But be not sanguine, mask-free meanderer. As you talk, your mouth emits not one but a sequence of such puffs. You are generating a turbulent, conical jet of you-air that can easily convey your droplets over more than two meters in 30 seconds of conversation.

Abkarian stresses that the brand-new science of aerophonetics is in its infancy. Our dispersal of potentially coronavirus-laden droplet goodness depends on the environment: the movement and direction of the air, distance, how loud we’re talking, shrieking or singing, if we’re panting and so on. When we talk, we are creating waves of pressure, but how they disseminate depends on a host of parameters, including how long You and the Other are in proximity, exchanging ideas and, willy nilly, fluids; how far apart you are; wind, air circulation and direction, et cetera.

Anything you say produces a jet from your mouth, but "we will" for instance sends droplets only about 30 centimeters, compared with meters in the case of words with plosives such as “beep.”

Production of saliva filaments on the lipsCredit: M. Abkarian and H.A. Stone

If two meters in a closed environment isn’t far enough to safely recite “Peter Piper picked a peck of pickled peppers” without masks in the room, can it be said the team recommends a 3-meter distance? Four? No... The research is early and the parameters are myriad. The farther you are, the longer droplets or aerosol will take to travel, but a lovely long chat in a closed room will expose you, two meters or not. The heavier the viral load of the emissions, the worse the situation. Science cannot at this stage tell you how far each droplet – which can range from, say 1 micron to 100 microns in size – will travel if spoken, coughed or heaven forfend, sneezed. Ventilation is good.

The only hard and fast rule is to WEAR A MASK, Abkarian sums up. “When I put the mask on and repeat the same sentence, even screaming Peter Piper – I don’t see anything leaving my mask more than 15 to 20 centimeters,” he says.

Hopefully as their work progresses we will get more information on our inadvertent infection of the surroundings. The Metropolitan Opera Orchestra of New York very much hopes so, we infer from a second study published October 2 in the journal of Physical Review Fluids. By serendipity Abkarian and the team learned that the furloughed musicians were desperately trying to figure out how to return to work, he says.

Armed with his trusty laser sheet, fog machine and a saxophone, he began the investigation into the conundrum of where to safely place musicians, singers and the others. But Abkarian had to return to France, and set up experiments featuring singers lured into the lab, singing before a borrowed (hideously expensive) infrared camera that can “see” the chanteuses’ CO2 emissions. “The first song the mezzo soprano sang was an Armenian lullaby,” says the scientist, himself of Armenian origin. Not that he heard it first hand – he was in France at that point. They’re working on analyzing the data, and not a moment too soon for the orchestras of the world. You can’t play a trombone or sing “Dies Irae” wearing a mask.