A constriction at a pressure node slows air and lowers resonant frequency

Evidence

Suppose instead that we introduce a constriction at an interior node. At the nodes, pressure is not varying, but the back-and-forth air motion is maximal there. The airflow will be hindered by a constriction, which should slow things down and lower the frequency of the corresponding mode. — Eric J. Heller, Why You Hear What You Hear- An…, 1641

Categorization

• XREF: connects to: Pipe bore constriction shifts mode frequencies depending on standing-wave node position — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1222