How is acoustic impedance defined?

• A. The product of the density of the material and the velocity of sound in the material
• B. Used to calculate the angle of reflection
• C. Used to determine resonance values
• D. Found by Snell's law

Answer: (A)

Acoustic impedance is a property that describes how much a medium resists the passage of an ultrasound wave. It is defined as the product of the medium’s density and the speed of sound in that medium: Z = ρ c. This quantity determines how the wave behaves at interfaces between materials. When a wave encounters a boundary with a different impedance, part of it is reflected and part is transmitted; the larger the impedance difference, the stronger the reflection. That’s why interfaces with very different Z values (like water and steel) produce strong echoes, which is fundamental to how ultrasonic testing detects flaws. Snell’s law governs the refraction angle based on wave speeds, not impedance, and resonance values relate to standing wave conditions rather than impedance at boundaries.