Which two physical properties are used in the impedance equation Z = ρ × c?

• A. Density and propagation speed
• B. Viscosity and density
• C. Pressure and density
• D. Frequency and density

Answer: (A)

Impedance in this context is determined by density and the speed of sound in the medium. The product ρ × c comes from the idea that how much pressure is needed to set the medium in motion depends on how much mass there is to move (density) and how quickly disturbances travel through the material (speed of sound). A denser medium resists motion more, and a medium where sound travels faster carries pressure changes more quickly, both increasing impedance. In SI units, ρ is in kg/m^3 and c in m/s, so Z has the units of pressure per particle velocity, matching the acoustic impedance concept.

Viscosity isn’t part of this simple Z = ρ c relationship, though it can affect energy loss and more complex forms of impedance. Pressure or frequency don’t appear in this basic equation.