Elucidating the Sound Absorption Characteristics of Foxtail Millet (Setariaitalica) Husk

The current study deals with the analysis of sound absorption characteristics of foxtail millet husk powder. Noise is one the most persistent pollutants which has to be dealt seriously. Foxtail millet is a small seeded cereal cultivated across the world and its husk is less explored for its utilization in polymer composites. The husk is the outer protective covering of the seed, rich in silica and lingo-cellulose content making it suitable for sound insulation. The acoustic characterization is done for treated foxtail millet husk powder and polypropylene composite panels. The physical parameters like fiber mass content, density, and thickness of the composite panel were varied and their influence over sound absorption was mapped. The influence of porosity, airflow resistance, and tortuosity was also studied. The experimental result shows that 30-mm thick foxtail millet husk powder composite panel with 40% fiber mass content, 320 kg/m3 density showed promising sound absorption for sound frequency range above 1000 Hz. We achieved noise reduction coefficient (NRC) value of 0.54. In view to improve the performance of the panel in low-frequency range, we studied the efficiency of incorporating air gap and rigid backing material to the designed panel. We used foxtail millet husk powder panel of density 850 kg/m3 as rigid backing material with varying air gap thickness. Thus the composite of 320 kg/m3 density, 30-mm thick when provided with 35-mm air gap and backing material improved the composite’s performance in sound frequency range 250 Hz to 1000 Hz. The overall sound absorption performance was improved and the NRC value and average sound absorption coefficient (SAC) were increased to 0.7 and 0.63 respectively comparable with the commercial acoustic panels made out of the synthetic fibers. We have calculated the sound absorption coefficient values using Delany and Bezlay model (D&B model) and Johnson–Champoux–Allard model (JCA model) and compared them with the measured sound absorption values.

Metal-Polymer Composite as an Acoustic Attenuating Material for Ultrasonic Transducers

This work reports the acoustic evaluation of a tungsten-epoxy composite which is functioned as a backing material of an ultrasonic transducer. The composite is prepared by mixing tungsten powder in epoxy at a weight ratio of 4:1 using a shaker milling and curing the mixture under a vacuum condition. The sound velocity and acoustic attenuation in the prepared composite is measured by propagating a 200-kHz ultrasonic wave with a through transmission mode. The prepared composite is subsequently assembled with a 1-MHz piezoelectric disk (PZT-8) and an epoxy layer as the active element and the matching layer, respectively, into a fabricated ultrasonic transducer. A square-wave burst pulse is used as an excitation signal for the evaluation of the fabricated transducer. The generated ultrasonic wave shows a higher damping in the presence the backing layer. In addition, the measured sound velocity and acoustic attenuation of prepared composite showed that the ratio of 4:1 is sufficient composition in order to apply as a backing material. In conclusion, control the transducer characteristics is determined by the acoustic properties of tungsten-epoxy composite.