Question

Acoustic impedance of a medium is defined as the product of:

• A. Density and wavelength
• B. Pressure and velocity
• C. Density and velocity of sound
• D. Frequency and wavelength

Hint:

Acoustic impedance differences at boundaries dictate ultrasound reflections. If the difference in \( Z = \rho c \) between two touching tissues is enormous (like tissue-to-air or tissue-to-bone), almost 100% of the ultrasound beam reflects backward, creating an acoustic shadow behind it!

Answer 1

The Correct Option is C

Concept: Acoustic impedance (denoted by \( Z \)) is a fundamental structural property of a medium that quantifies the total resistance encountered by an ultrasound wave as it propagates through that medium. It determines how acoustic energy behaves at tissue boundaries.

Step 1: Examining the mathematical formula.
The characteristic acoustic impedance \( Z \) of a homogeneous material or tissue is defined mathematically as: \[ Z = \rho \cdot c \] Where:
• \( \rho \) (rho) represents the mass density of the medium (typically in units of \( \text{kg/m}^3 \)).
• \( c \) represents the propagation velocity of sound waves through that specific medium (in units of \( \text{m/s} \)).

Step 2: Checking units and physical interpretation.
Multiplying these two quantities yields the SI unit of acoustic impedance, which is the Rayl (\( \text{kg} \cdot \text{m}^{-2} \cdot \text{s}^{-1} \)).
• Media that are highly dense or have tightly bound elastic lattices (like cortical bone) show very high velocity of sound and high density, leading to massive acoustic impedance.
• Media that are light or loosely bound (like air or gas) have very low density and lower velocity, leading to extremely small impedance. Thus, the correct option describing this product is density and velocity of sound, matching Option (C).