Sound
Sound gets its own section because it isn’t a metaphor for pressure — it’s one of the clearest cases where pressure is the literal, physical mechanism, not a comparison borrowed to make something else easier to picture (see Why Pressure for the fuller argument). This section covers the real, standard mathematics of sound as a pressure wave: wave speed, frequency, intensity, decibels, resonance, and the Doppler effect. See Practical Applications: DC to AC Across the Spectrum for where audible and ultrasonic frequencies sit relative to the broader electromagnetic and power-engineering spectrum.