Which statement is false about the acoustic variables?

• A. Pressure is an acoustic variable.
• B. Distance is an acoustic variable.
• C. Density is an acoustic variable.
• D. Mass is an acoustic variable.

Answer: (D)

In a sound wave, the quantities that actually carry the wave’s information are those that change in the medium: pressure, density, and the motion of particles (often described by particle velocity or displacement). Among the statements, pressure is indeed an acoustic variable, and density is also an acoustic variable because both vary as the wave propagates through the medium. Mass, on the other hand, is not treated as an acoustic variable. The wave causes changes in density, not changes in the total mass within a region; mass is conserved for a given amount of fluid, and what varies is how that mass is distributed (density) and how it moves (velocity/ displacement). Labeling mass as an acoustic variable would be incorrect. The idea that distance equals an acoustic variable isn’t part of the standard set, since distance refers to displacement, which is related but not typically listed as a primary acoustic variable; the essential point is that mass does not vary as an acoustic descriptor, unlike pressure and density.