The inertia of air when the intake valve closes forms a pressure wave that travels back to the induction system.

• A. Pressure wave
• B. Sound wave
• C. Vibration
• D. Drag wave

Answer: (A)

When the intake valve closes, the air rushing toward the cylinder has momentum and can’t stop instantly. That inertia creates a disturbance in the pressure inside the intake tract, and this disturbance travels back toward the throttle body and runners as a pressure wave. This wave is a propagating fluctuation in air pressure within the gas column, moving at roughly the speed of sound in air and shaped by the manifold geometry and valve timing. A pressure wave is the precise way to describe this phenomenon, whereas a general sound wave is a broader term for audible pressure disturbances in air, and doesn’t capture the specific gas‑column propagation here. Vibration involves movement of solids, and “drag wave” isn’t a standard term for this effect.