The product of the sound velocity and the density of a material is known as

• A. Acoustic impedance of the material
• B. Refraction value of the material
• C. Elastic constant of the material
• D. Poisson's ratio of the material

Answer: (A)

In ultrasonics, the quantity you get by multiplying the sound velocity by the density is the acoustic impedance, usually written as Z = ρc. This property tells you how much a material resists the passage of an acoustic wave. It has units of kg/(m^2·s), often called rayls.

Acoustic impedance is what controls reflections and transmissions at interfaces: a large mismatch in Z between two media causes most of the wave to reflect back, while a smaller mismatch allows more transmission. The other options don’t describe this product: refraction value isn’t a standard material property, the elastic (stiffness) constant relates to how a material resists deformation, and Poisson’s ratio describes lateral strain under axial loading.