Ultrasound propagation velocity depends on which properties?

• A. Amplitude and frequency
• B. Impedance and attenuation
• C. Density and compressibility
• D. Dynamic range and compression

Answer: (C)

Ultrasound speed through a medium is set by how massive the medium is per unit volume and how resistant it is to being compressed. The fundamental relation is v = sqrt(K/ρ), where K is the bulk modulus (stiffness) and ρ is density; equivalently v = 1/√(ρβ) with β being compressibility. This means the wave travels faster in a stiffer (less compressible) and less dense medium, and slower in a denser or more compressible one. The pair density and compressibility captures these intrinsic factors that determine speed. Other terms like amplitude, frequency, impedance, attenuation, dynamic range, or compression describe how the wave behaves at interfaces, how much energy is lost, or how signals are processed, not the propagation velocity.