In this work, we present a reference material for measuring the acoustic attenuation coefficient of biological tissues or tissue-mimicking materials by a transmission method. Accurate measurements by the method require a reference material that has the same acoustic velocity and density as the biological tissue. The newly developed reference material was prepared from NaCl and MgSO4 aqueous solutions. The inorganic material is stable through time. To formulate materials with the desired acoustic velocity and density, the ratio of three constituents (i.e., water, NaCl, and MgSO4) can be determined depending on the method of experiments with mixtures by Scheffe.