Sound-absorbing block not only absorb sound within a specific location, but also are used to diffuse sound, helping make an area quieter and more acoustically palatable. How efficiently concrete block does this is measured by determining its sound absorption coefficient (SAC). If a surface were able to absorb 100% of the sound waves striking it, its sound absorption coefficient would be 1; a surface absorbing 65% of the sound waves striking it has an SAC of .65. The higher the SAC, the more sound absorbed by the surface. Directly related to SAC is noise reduction coefficient (NRC). NRC is a single-figure index used as a measure of noise absorption. It is based on how well a surface absorbs sound over a set range of four frequencies that include high, medium, and low sounds. SLOTTED BLOCK One of the most effective types of sound-absorbing block has slots molded in the face and connected to a cavity. The slot generally is funnel-shaped to allow as much sound as possible to enter through the opening. The cavity behind the slot acts as a resonator. Based on the principle of the Helmholtz resonator, the cavities can be configured to absorb specific frequencies of sound. The cavities in slotted block can be left unfilled, or contain a fibrous filler, metal septum, or a fibrous filler combined with a metal septum.