Open-tube end reflection arises from air-parcel overshoot into lower impedance beyond the opening.

Evidence

The push and pushback cellular impedance picture (see section 1.1) explains why there is an impedance mismatch at the end of an open tube: when an air cell gets pushed inside the tube, the reflection of the pressure disturbance off the nearby walls of the tube stiffens the pushback (impedance) of the air inside, provided there is enough time for the disturbance to reach the walls and back in one period. For sinusoidal forcing, this corresponds to a tube diameter of a wavelength (or less) of sound at the frequency of the sinusoid. Just outside the end of the tube, the impedance due to reflections off the walls drops very rapidly. This picture explains why short wavelengths reflect less and transmit more than long wavelengths do. The negative pressure reflection at the open end of the tube results from the “overshoot” of parcels of air near the opening as they encounter lower impedance. A rarefaction is created behind the overshoot, which then propagates back down the tube. — Eric J. Heller, Why You Hear What You Hear- An…, 1432

Categorization

• DEFINE: Explains the physical mechanism behind the impedance mismatch at an open tube end.
• DEEPTHINKING: A genuinely physical, mechanistically satisfying account of end reflection—the overshoot/rarefaction picture is the kind of explanation that sticks.