It casts what is known as an acoustic shadow, which blocks passage of the higher frequencies from one side of the head to the other.
Then he remembered that there are natural phe- nomena to which some one has given the name 'acoustic shadows.'
If you stand in an acoustic shadow there is one direction from which you will hear nothing.
An acoustic shadow apparently prevented Grant and Ord from hearing the sounds of the battle starting.
(These voids are also visible in the previous image as dark acoustic shadows.)
He lowered it to a depth of twenty feet to assure that it was out of the acoustic shadow of the boat's hull.
An acoustic shadow can be created by terrain as well that will leave spots that can't easily hear sounds from a distance.
Due to a possible acoustic shadow Ord's forces were never engaged and Rosecrans fought alone.
As one website refers to it, "an acoustic shadow is to sound what a mirage is to light".
Acoustic Shadows - What is an acoustic shadow and how does it work?