Sound proofing and acoustic materials are used to attenuate, deaden, or control sound and noise levels from machinery and other sources for environmental amelioration and regulatory compliance. Sound proofing and acoustic materials can use either noise reduction or noise absorption. Noise reduction reduces the energy of sound waves as they pass through. Noise absorption suppresses echoes, reverberation, resonance and reflection.  Important specifications for noise reduction and noise absorption products include noise attenuation and noise reduction coefficient. Noise attenuation is the reduction in sound pressure level (SPL) that an acoustic product provides. It is measured in decibels (dB). Noise reduction coefficient (NRC) is the average of an acoustic material’s absorption coefficients at a specified set of frequencies, typically 256 Hz, 512Hz, 1024 Hz and 2048 Hz. Absorption coefficients range between 0 and 1 and are often evaluated at many frequencies in the audible range in order to create a performance curve for the material throughout the audio spectrum.