In other words, acoustic stiffness is the measure of how difficult it is to move a material's surface when subjected to an acoustic wave. The higher the acoustic stiffness, the more difficult it is to move the surface, and the less efficient the acoustic wave will be at transmitting energy through the material.
Acoustic stiffness is an important parameter in the design of acoustic materials, such as soundproofing materials and acoustic absorbers. Materials with high acoustic stiffness are often used to block or absorb sound waves, while materials with low acoustic stiffness are used to transmit sound waves with minimal loss of energy.
For example, lead is a material with high acoustic stiffness, and is therefore often used as a soundproofing material in construction. On the other hand, fiberglass is a material with low acoustic stiffness, and is therefore often used as an acoustic absorber in sound studios and other noise-sensitive environments.
The acoustic stiffness of a material can be affected by a number of factors, including its density, porosity, and elasticity. In general, materials with high density and low porosity tend to have high acoustic stiffness, while materials with low density and high porosity tend to have low acoustic stiffness.