The results of a study on the properties that influence the performance of acoustical materials at high sound levels is reported. Experimental data for nonlinear acoustic resistance of simple structures such as screens and perforated plates agree well with theoretical calculations. Similar data obtained for more complex structures such as fibrous or cellular materials indicate that in general the performance of an acoustical material at high intensities is determined by the conventional parameters such as its dc flow resistanceR0, applicable at low intensities, plus a resistive term that depends on the particle velocityv. The dynamic flow resistance can in general be described asR = R0(1 + v/vcr), wherevcris the velocity at which the resistance becomes twice the low velocity value. Other properties of several acoustical materials, including dynamic, mechanical, and thermal performance at high intensities, are reported. (Supported by the U. S. Navy Bureau of Ships.)