half-value layer

The quarter-value layer is equal to the sum of the first and second half-value layers.

A "thin" half-value layer (or a quick drop of -3dB) results from a high frequency sound wave and a medium with a high rate of attenuation, such as bone.

A material's half-value layer (HVL), or half-value thickness, is the thickness of the material at which the intensity of radiation entering it is reduced by one half.

HVL can also be expressed in terms of air kerma rate (AKR), rather than intensity: the half-value layer is "the thickness of specified material which attenuates the beam of radiation to an extent such that the AKR is reduced to one-half of its original value.