Definition
Pressure disturbances that travel through an elastic medium such as air, water, or a solid, alternately compressing and rarefying the medium as they pass. The energy moves through the medium, but the medium itself only oscillates back and forth around its rest position. In air at sea level, sound waves travel at roughly 661 knots (about 761 mph), and their frequency determines pitch while their amplitude determines loudness.
Plain English
Tiny pressure ripples that move through the air (or another material) and reach your ears as sound. The air doesn't actually travel anywhere with the sound -- it just wiggles in place as the ripple passes through.
Context Anchor
Seen in discussions of aircraft noise, hearing protection, propeller and engine sound, and the speed of sound.
Derivation
From Old English 'sund' (the sense of hearing) and 'wave' (a moving ripple). Calling sound a 'wave' captures the key idea: it spreads outward like ripples on a pond, with peaks and troughs of pressure rather than peaks and troughs of water.
Why Pilots Care
The speed of sound sets a hard physical boundary in flight. As an aircraft (or even just a propeller tip) approaches it, airflow behaves very differently -- shock waves form, drag rises sharply, and control responses change. Understanding sound waves is the foundation for understanding Mach number, compressibility effects, and supersonic flight.
Analogy
A sound wave is like a ripple moving across a pond, except it moves through air as pressure changes instead of through water as visible ripples.
Grounding Statement
Picture pushing a slinky in short bursts -- the coils bunch up and spread out as the pulse travels along. Air does the same thing when sound moves through it.
Intuition Check
Sound waves are not pieces of air flying from the source to your ear. They are pressure changes moving through the air.
Example Sentence 1
As a propeller tip approaches the speed of sound waves in the surrounding air, efficiency drops sharply and noise increases.
Example Sentence 2
Cabin insulation reduces the sound waves reaching passengers from the engines and slipstream.