A high transmission broadband gradient index lens using elastic shell acoustic metamaterial elements

Area of Science:

• Acoustics
• Materials Science
• Metamaterials

Background:

• Acoustic metamaterials offer unique wave manipulation properties.
• Cylindrical elastic shells present a novel building block for acoustic devices.
• Transformation acoustics principles guide the design of wave-focusing structures.

Purpose of the Study:

• To demonstrate the efficacy of cylindrical elastic shells in acoustic metamaterial devices.
• To design and test a cylindrical-to-plane wave lens using elastic shell metamaterials.
• To validate simulation predictions with experimental underwater measurements.

Main Methods:

• Utilizing transformation acoustics to define target effective properties (constant density).
• Employing a piecewise approximation of effective compressibility with an array of elastic shell elements.
• Fabricating a neutrally buoyant device with 48 elements of nine commercial shell types (aluminum, brass, copper, polymers).
• Conducting underwater measurements to characterize the device's acoustic response.

Main Results:

• Simulations predicted a broadband performance for the cylindrical-to-plane wave lens.
• Experimental findings confirmed a broadband quadropolar response from 20 to 40 kHz.
• Positive gain in the radiation pattern was observed in the four plane wave directions.

Conclusions:

• Cylindrical elastic shells are effective elements for acoustic metamaterial devices.
• The demonstrated device successfully functions as a broadband cylindrical-to-plane wave lens.
• The experimental results validate the theoretical design based on transformation acoustics and elastic shell metamaterials.