Acoustic attenuation

Abstract: Ocean ambient noise results from both anthropogenic and natural sources. Different noise sources are dominant in each of 3 frequency bands: low (10 to 500 Hz), medium (500 Hz to 25 kHz) and high (>25 kHz). The low-frequency band is dominated by anthropogenic sources: pri- marily, commercial shipping and, secondarily, seismic exploration. Shipping and seismic sources con- tribute to ambient noise across ocean basins, since low-frequency sound experiences little attenua- tion, allowing for long-range propagation. Over the past few decades the shipping contribution to ambient noise has increased by as much as 12 dB, coincident with a significant increase in the num- ber and size of vessels comprising the world's commercial shipping fleet. During this time, oil explo- ration and construction activities along continental margins have moved into deeper water, and the long-range propagation of seismic signals has increased. Medium frequency sound cannot propagate over long ranges, owing to greater attenuation, and only local or regional (10s of km distant) sound sources contribute to the ambient noise field. Ambient noise in the mid-frequency band is primarily due to sea-surface agitation: breaking waves, spray, bubble formation and collapse, and rainfall. Var- ious sonars (e.g. military and mapping), as well as small vessels, contribute anthropogenic noise at mid-frequencies. At high frequencies, acoustic attenuation becomes extreme so that all noise sources are confined to an area close to the receiver. Thermal noise, the result of Brownian motion of water molecules near the hydrophone, is the dominant noise source above about 60 kHz.

Abstract: We present the experimental realization and theoretical understanding of a membrane-type acoustic metamaterial with very simple construct, capable of breaking the mass density law of sound attenuation in the 100--1000 Hz regime by a significant margin ($\ensuremath{\sim}200$ times). Owing to the membrane's weak elastic moduli, there can be low-frequency oscillation patterns even in a small elastic film with fixed boundaries defined by a rigid grid. The vibrational eigenfrequencies can be tuned by placing a small mass at the center of the membrane sample. Near-total reflection is achieved at a frequency between two eigenmodes where the in-plane average of normal displacement is zero. By using finite element simulations, negative dynamic mass is explicitly demonstrated at frequencies around the total reflection frequency. Excellent agreement between theory and experiment is obtained.

Abstract: We present the experimental realization and theoretical explanation of a membrane-type acoustic metamaterial of very simple structure,capable of breaking the mass density law of sound attenuation in the 100—1000Hz regime by a significant margin(~200 times).Due to the membrane's weak elastic moduli,low frequency oscillation patterns can be found even in a small elastic film with fixed boundaries defined by a rigid grid.The vibrational eigenfrequencies can be tuned by placing a small mass at the center of the membrane sample.Near-total reflection is achieved at a frequency in between two eigenmodes where the in-plane average of the normal displacement is zero.By using finite element simulations,a negative dynamic mass is explicitly demonstrated at frequencies around the total reflection frequency.Excellent agreement between theory and experiment is obtained.We also show that the present mechanism can explain the phenomenon of total microwave transmission through subwavelength slits in metallic fractals,at frequencies intermediate between two local resonances.