A "thin" half-value layer (or a quick drop of -3 dB) results from a high frequency sound wave and a medium with a high rate of attenuation, such as bone. HVL is measured in units of length.
A thin half-value layer (or a rapid decrease of -3 dB) occurs when a high-frequency sound wave encounters a medium that attenuates sound waves rapidly, like bone. The thickness of this layer is measured in units of length.