Extended Data Figure : Mean particle number concentration as a function of the distance from the participant's mouth to the apex of the funnel. For this experiment, a participant sang the "Happy Birthday" song for 20 s at 80-90 dB five separate times at each distance. The reported value is the mean and standard deviation for each distance. When the participant vocalised 8-10 cm from the funnel apex, the measured number concentrations did not vary significantly (factor of ~1.5), whereas beyond 12 cm the measured number concentrations decreased by an order of magnitude or greater. For all participants in this study, the distance between the subject mouth and the funnel apex was 8-10 cm.