In 1997, Dr. Sansalone in her book "Impact-echo: nondestructive evaluation of concrete and masonry" says that this factor is 0.96 due several studies, but they not mention the reason for it.
It's a form factor, representing the volume in which the wave propagates and is reflected, and is used if impact-echo is applied to slabs (i.e. an element where H
It's a form factor, representing the volume in which the wave propagates and is reflected, and is used if impact-echo is applied to slabs (i.e. an element where H