Acoustic impedance at a boundary influences both reflection and transmission of ultrasound. Which term describes this property?

• A. Impedance
• B. Attenuation
• C. Density
• D. Velocity

Answer: (A)

Impedance is the property that describes how much of the ultrasound energy is reflected at a boundary and how much is transmitted. Acoustic impedance is defined as Z = ρc, the product of a medium’s density (ρ) and the speed of sound in that medium (c). When a wave encounters an interface between two media with different impedances, the mismatch determines how the energy splits: a larger mismatch yields more reflection and less transmission, while similar impedances allow more energy to pass through. Attenuation explains energy loss as the wave travels, not the boundary behavior; density and velocity contribute to impedance but do not alone describe the boundary reflection/transmission process.