Limitations in the Measurement of the Sound Absorption Coefficient on Materials for Highway Noise Barriers
Abstract
Some absorbing materials used to improve the performance of highway noise barriers are rigid and their flow resistance is high. They absorb sound energy in a complex porous and resonant way and their absorption coefficient is strongly dependent upon their mounting. In our study, three rigid materials measured with the standing wave tube exhibited resonant peaks at frequencies depending on their thickness. One of the materials, measured in a reverberant chamber lying on the floor, showed the same resonant peak. However, when measured in the same chamber in a free standing position, the absorption curve was typical of a porous material. The results of the study confirm that "hard," rigid, acoustical materials should not be measured with the standing wave tube but rather in a free standing position in a reverberant room.
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