An open pipe end inverts a sound pulse’s sign via a reflected rarefaction

Evidence

If a pipe terminates in an open end, it is much the same as a sudden very large increase in pipe diameter. We expect a sharp drop in impedance; the discontinuity will reflect sound amplitude back with the opposite sign. The air at the end of the pipe feels less pushback, overshoots, and pulls on the air behind it, initiating a rarefraction that propagates backwards. — Eric J. Heller, Why You Hear What You Hear- An…, 112

Categorization

• XREF: Connects to stopped/organ-pipe acoustics and why open vs closed organ pipes differ by an octave. — connects to: Open and stopped organ pipes correspond to open-open and open-closed acoustic tubes. — David Howard, Jamie Angus, coustics and Psychoacoustics, 1181