sound

Meaning: Sound is a longitudinal wave. Particles of the medium oscillate parallel to the direction the wave travels, creating alternating regions of compression (higher pressure) and rarefaction (lower pressure). Because it is mechanical, it needs a medium. The source sets the frequency, which we hear as pitch. Amplitude relates to intensity and loudness. The medium sets the speed, through its stiffness and density. In air at room temperature, the speed is roughly 340 m/s.

Equation: v = fλ = √(B/ρ), where v is wave speed (m/s), f is frequency (Hz), λ is wavelength (m), B is the bulk modulus, a measure of stiffness (Pa), and ρ is density (kg/m³).

On the MCAT: Expect questions that rank the speed of sound in solids, liquids and gases. Expect passages in which sound crosses a boundary: the frequency stays the same, while speed and wavelength change together (v = fλ). Sound also underlies the Doppler effect, beats, standing waves in pipes and strings, and intensity in decibels, where each 10 dB step is a tenfold change in intensity.

Common confusion: Students often assume that a denser medium means slower sound, or that louder sound travels faster. Speed depends on the ratio B/ρ. Solids are usually much stiffer, so sound is usually faster in them even though they are denser. Amplitude changes loudness, not speed.