sound wave

Meaning: A sound wave is a mechanical, longitudinal wave: particles of the medium oscillate parallel to the direction of travel, making alternating compressions (high pressure) and rarefactions (low pressure). It needs a medium and cannot cross a vacuum. The source sets the frequency; the medium sets the speed. Pitch tracks frequency, and loudness tracks amplitude and intensity.

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

On the MCAT: A wave crossing a boundary, such as air to water, keeps its frequency while v and λ change together. Speed is generally greater in solids than in liquids, and greater in liquids than in gases. Intensity level questions use β = 10 log(I/I₀) with I₀ = 10⁻¹² W/m², where +10 dB means a tenfold intensity increase. Doppler shifts, pipe resonance and beats also appear.

Common confusion: Sound is not faster just because a medium is denser. Speed depends on stiffness relative to density, and solids are generally faster because they are much stiffer. Also, sound in fluids is longitudinal, unlike light, which is transverse and needs no medium.