Evaluation of Viscoelastic Coatings as Low Frequency Acoustic Absorbers.

摘要

This study examines the effectiveness of elastomer coatings for specular, acoustic backscatter reduction in low frequency, under-water applications. Four representative elastomers were selected: Butyl B252, Neoprene W, Polybutadiene, and thiokol RD. Their coating reflection loss for CW and impulse incident acoustic plane waves was examined using techniques and computer codes developed based on a geometric acoustics approach. The performance of these elastomer coatings was examined in the frequency range from 10 Hz to 10 kHz, the pressure range 0 to 500 psig, and the temperature range 0 degrees to 30 degrees C; ranges commonly encountered in oceanic applications. The results indicate that at low frequencies (order 100 Hz), the available elastomers, applied as homogeneous coatings up to 20 cm thick, are not sufficient to obtain reflection reduction greater than 6 dB for the specified conditions. Greater specular backscatter reduction may be achieved by using layered, inhomogeneous coatings but a more relevant approach for these low frequencies may involve the examination of structural resonant response to the incident acoustic field since the body size and acoustic wavelength are the same order of magnitude (Gaunard, 1977). The structural resonant mechanism of backscatter is probably dominant at low frequencies. (Author)