Theoretical curves for acoustic impedance vs. frequency, obtained by solving the equation for wave motion within the material, are compared with recently measured values of the impedance of various sound absorbing materials. The satisfactory agreement in most cases measured indicates that that for homogeneous materials the effective values of flow resistance, porosity and material density are nearly constant over the frequency range 200 to 6000 c.p.s. This makes it possible to express the whole acoustic behavior of these materials by three constants. The behavior of two nonhomogeneous materials is also discussed. The relation between impedance and the various types of absorption coefficient is discussed, and specific formulas for the coefficients are given in terms of the acoustic impedance. It is indicated that in rectangular chambers, when the sound does not have a random diffusion throughout the room, the decay curve for pressure squared in db, vs. time in seconds, is a broken line, with the initial slope corresponding to a coefficient for normal incidence, and the final slope corresponding to a coefficient for grazing incidence. If enough scattering objects are inserted to make the diffusion of sound complete, so that no standing wave is all grazing or all normal to any surface, then the decay curve is a straight line with slope corresponding to the coefficient defined by Sabine. A formula relating this coefficient to the acoustic impedance is given. The Acoustical Materials Association values of absorption coefficient, used in practice, are compared with the curves of normal and Sabine coefficient, computed from the experimental curves for impedance. It is apparent that above 2000 c.p.s. the A.M.A. values equal the computed Sabine coefficients, indicating sufficient diffusion of sound in the reverberation chambers at these frequencies. Below 500 c.p.s. the A.M.A. values correspond more closely to the normal coefficient, indicating an insufficient diffusion of sound, and that the initial slope of a broken line decay curve is being measured. In these two ranges of frequency, therefore, one can compute the A.M.A. coefficients from the impedance. It is suggested that field coefficients for large auditoriums correspond to the computed Sabine coefficient for lower frequencies than do the A.M.A. coefficients. A simple formula is given for the broken line decay curve in a rectangular room with no diffusion, and is checked by experimental curves. A case of partial diffusion is discussed, where the decay curve is intermediate between the theoretical curves for complete and for no diffusion.