Hard boundaries reflect sound because pressure energy cannot cross a zero-velocity point.

Evidence

This means that for bounded waves, the impedance depends on the surface area within the bounding structure and so will change as the area changes. As we shall see later, changes in impedance can cause reflections. This effect is important in the design and function of many musical instruments, as discussed in Chapter 4. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 174 (#expands, via linked claim)

Air pressure acts in all directions at the same time and therefore for sound it can be considered to be a scalar quantity without direction; we can therefore talk about pressure at a point and not as a force acting in a partic­ ular direction. Velocity on the other hand must have direction; things move from one position to another. It is the velocity component which gives a sound wave its direction. — David Howard, Jamie Angus, coustics and Psychoacoustics, 146 (#expands, via linked claim)

Categorization

• XREF: connects to: Sound wave molecules have zero velocity at compression peaks due to inertia; Sound reflecting off a hard boundary has zero particle velocity — Toole, Floyd E, Sound reproduction- the acoustics and…, 1024; Pressure is scalar while velocity gives a sound wave its direction — David Howard, Jamie Angus, coustics and Psychoacoustics, 146 (+1 more)

Broader Connections

• Bounded sound waves change impedance with tube area, causing reflections #expands (0.78) — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 174
• Pressure is scalar while velocity gives a sound wave its direction #expands (0.75) — David Howard, Jamie Angus, coustics and Psychoacoustics, 146