The mass attenuation coefficient or mass narrow beam attenuation coefficient of the volume of a material characterizes how easily it can be penetrated by a beam of light, sound, particles, or other energy or matter. In addition to visible light, mass attenuation coefficients can be defined for other electromagnetic radiation (such as X-rays), sound, or any other beam that attenuates. The SI unit of mass attenuation coefficient is the square metre per kilogram . Other common units include cm²/g (the most common unit for X-ray mass attenuation coefficients) and mL·g^-1·cm^-1 (sometimes used in solution chemistry). "Mass extinction coefficient" is an old term for this quantity.