Silent but deadly? Physics says seldom, memory says mostly

Abstract

The belief that a silent passage of flatus smells worse than an audible one appears in many languages but is untested. We follow a passage from sound through dispersion to a bystander's memory with a lip-reed model of the anal valve, dispersion models and observer models from the literature. Most releases, 85%, never set the valve oscillating. We prove that under linear transport a slower release of the same gas cannot give a higher peak, in any room or flow. In computational fluid dynamics of seated and standing people, the warm air rising along the body takes a slow release and lets a fast one pass, so momentum decides which nose a release reaches. With passages as emitted and settings weighted by how often people share them, a bystander finds the silent passage worse in 0.5% of pairs and the audible one in 6.9%; only in a closed lift are the two about equal. A silent passage is noticed only if smelled, and this selection alone can explain the belief: among remembered pairs the silent passage seems worse in 88%. It predicts that releasers hold the belief less firmly than bystanders and that anosmic people never remember a silent passage.