Speech air flow with and without face masks

It took a while due to the absolutely shocking amount of work required for the “Gait change in tongue movement” article, but Natalia Kabaliuk, Luke Longworth, Peiman Pishyar‑Dehkordi, Mark Jermy and I were able to get our article on “Speech air flow with and without face masks” accepted to Scientific Reports (Nature Publishing Group). The article is now out (though a pre-review version had been available since we submitted this article to Sci Rep). You can also watch my YouTube video describing many of the results.

Here is an example of a low-stiffness air-flow from a porous mask, which allows leaks from the tops, bottoms, and sides, and forward flow prevention, as taken from Figure 5 of the article.

Figure 5. Audio and Schlieren of speech through a porous face mask (Frame 621, 1st block, CORI Supermask). Image from 88 ms after the release burst for the [kh ] in “loch”. Note that the k’s puff is smoother and less well defined than the one in Fig. 2, but still has eddies that change air-density across the span of the puff. The red-dashed line in the audio waveform indicates the timing of the schlieren frame.

And here is an example of typical higher-stiffness flow from a less porous mask from Figure 8.

Figure 8. Audio and Schlieren of speech with a tightly fitting surgical mask (Frame 334, 1st block, Henry Schlein surgical mask [level 2]). Air slowly flows out above the eyes, floating out and upward continuously. The red-dashed line in the audio waveform indicates the timing of the schlieren frame.

Masks can be made to fit tighter, as in well-designed KN95/N95 masks and masks with metal strips at the nose to prevent upward-escaping air flow. However, for all the masks we studied, the tradeoff was not entirely avoided. And with that, here is our abstract:

Face masks slow exhaled air flow and sequester exhaled particles. There are many types of face masks on the market today, each having widely varying fits, filtering, and air redirection characteristics. While particle filtration and flow resistance from masks has been well studied, their effects on speech air flow has not. We built a schlieren system and recorded speech air flow with 14 different face masks, comparing it to mask-less speech. All of the face masks reduced air flow from speech, but some allowed air flow features to reach further than 40 cm from a speaker’s lips and nose within a few seconds, and all the face masks allowed some air to escape above the nose. Evidence from available literature shows that distancing and ventilation in higher-risk indoor environment provide more benefit than wearing a face mask. Our own research shows all the masks we tested provide some additional benefit of restricting air flow from a speaker. However, well-fitted mask specifically designed for the purpose of preventing the spread of disease reduce air flow the most. Future research will study the effects of face masks on speech communication in order to facilitate cost/benefit
analysis of mask usage in various environments.

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