Definition
A wave in which the particles of the medium vibrate back and forth in the same direction that the wave is travelling. The motion produces alternating regions of compression (particles squeezed together) and rarefaction (particles spread apart) moving along the direction of travel. Sound waves in air are the most common example.
Plain English
A wave where the stuff carrying the wave moves back and forth along the same line the wave is moving. Think of pushing and pulling a long spring end-to-end — the squeeze travels down the spring in the same direction you pushed.
Context Anchor
Seen in basic aviation physics discussions of sound, vibration, and pressure changes moving through air.
Derivation
From Latin longitudo, meaning 'length.' A longitudinal wave moves the medium along its length — in line with the direction of travel — which is exactly what distinguishes it from a transverse wave, where the medium moves across the direction of travel.
Why Pilots Care
Sound — including engine noise, propeller noise, and radio audio reaching the ear — travels as a longitudinal wave. Understanding this helps make sense of how pressure disturbances move through the atmosphere and why the speed of sound depends on air properties.
Analogy
Imagine a Slinky lying on a table. If you push one end straight toward the other end, you see a squeeze travel down the Slinky in the same direction you pushed. That squeeze moving along the Slinky is a longitudinal wave.
Grounding Statement
When a propeller makes sound, tiny pressure changes move through the air while nearby air moves back and forth along the path of the sound.
Intuition Check
Longitudinal does not mean the wave is necessarily lined up with the airplane’s nose-to-tail axis. Here it means the back-and-forth motion is in the same direction the wave travels.
Example Sentence 1
Sound from the engine reaches the cockpit as a longitudinal wave travelling through the air.
Example Sentence 2
Vibrations along the fuselage can send longitudinal waves through the metal structure.