Open pipe ends invert compression pulses while closed ends preserve them

Evidence

When a compression (positive amplitude) pulse of sound pressure energy is launched into a pipe, for example at the instant in the air reed cycle illustrated in the lower-right snapshot in Figure 4.14, it travels down the pipe at the velocity of sound as a compression pulse. When the compression pulse reaches the far end of the pipe, it is reflected in one of the two ways described in the “standing waves” section of Chapter 1 (Section 1.5.7), depending on whether the end is open or closed. At a closed end, there is a pressure antinode, and a compression pulse is reflected back down the pipe. At an open end, there is a pressure node, and a compression pulse is reflected back as a rarefaction pulse to maintain atmospheric pressure at the open end of the pipe. Similarly, a rarefaction pulse arriving at a closed end is reflected back as a rarefaction pulse but as a compression pulse when reflected from an open end. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1153