Diamond is a covalent material with no fundamental infrared active vibrational modes. Intrinsic diamond is completely transparent below the band gap except for two-, three-, and higher-order multiphonon absorption bands located in the mid-IR. Multiphonon absorption band models have been successfully developed for ionic materials; this paper reports the first application of this model to a purely covalent material. To obtain a fit to the experimental data, the phonon density-of-states function required considerable modification relative to the other ionic materials studied. The Gaussian density-of-states parameters were significantly modified relative to typical values for ionic materials. However, the two-phonon red wing is poorly modeled by this density-of-states function. Diamond absorptance near 10 micrometers is of particular interest because many infrared sensors operate at this atmospheric window. Evidence indicates that the absorption in this region is caused by two-phonon acoustic-acoustic interactions. In most materials, pure acoustic multiphonon absorption is not measurable because it is obscured by strong one-phonon optical mode absorption. Diamond has very high acoustic frequencies owing to its strong bonds, and the lack of fundamental absorption unmasks the pure acoustic contribution. This acoustic contribution is modeled by applying a Debye acoustic phonon density-of-states distribution function. Good agreement with experimental data is obtained.