The Absorption Characteristics of Empty Water Bottles

Abstract

This work examines experimentally the absorption characteristics of empty plastic water bottles. It focuses on the three common sizes of bottles widely available in the Saudi market (1500, 600 and 330 ml). Measurements were performed according to ISO 354 and ISO 9613-1 standards in the reverberation chamber. Three systematic groups of measurements were performed, each of which examined only one of the three categories. Each group included four tests with a predetermined density (number of bottles per square meter) and alignment. The impact of combining the three sizes of the bottles in one configuration was tested as well. Then, the impact of filling the best absorptive bottle with sand and the impact of the bottles’ position and alignment were investigated. Results demonstrated that water bottles are not a true Helmholtz resonator, as there is more than one resonance frequency for each category. Nevertheless, their fundamental frequencies can be predicted with a certain degree of accuracy, by applying the equation for true Helmholtz resonators. Results showed that the absorption is directly proportionate to the density and the bottle volume. Using either medium or small water bottles alone does not yield a remarkable absorption, unlike large or mixed bottles, which give the best results. Sand has almost no effect on the absorption of sound. Finally, results demonstrate that a remarkably higher level of absorption is achieved in the mid- and high-frequency ranges by placing the 1500-ml bottles onto the floor of the room at random. Although the values of the different measures of absorption are not too high, results of the combined volumes seem promising. Further investigation to improve this absorption is required.