Which is not an acoustic variable?

• A. Density
• B. Pressure
• C. Distance
• D. Intensity

Answer: (D)

Acoustic variables describe the immediate state of the sound field in the medium—pressure, density, and particle velocity. Intensity is the energy flow carried by the wave per unit area and is derived from those state quantities (via pressure and velocity and the medium’s impedance). Because it reflects energy transport rather than a direct, intrinsic state of the wave, intensity is not considered a fundamental acoustic variable. Density and pressure are direct properties of the wave, while distance is a spatial parameter describing propagation. Therefore, intensity stands out as not being a basic acoustic variable.