Fariha Mir

Date of Award

Fall 2021

Document Type

Open Access Dissertation


Mechanical Engineering

First Advisor

Sourav Banerjee


In our surroundings abundant energy, we either feel through component vibration or hear noises from acoustic sources. Harvesting these unused and untapped green energy in the form of ambient vibration, and acoustic sounds is an emerging field of research in recent years. Utilization of the energy within a wide band of the frequency spectrum originated from the vibrational sources alone stands as one of the most promising ways to power small electronic devices, smartphones, local structural health monitoring sensors, home, and workshop appliances. These abundant sources of green energy are available in almost all the engineering industries, workshops, manufacturing facilities, construction zones, and in our daily operations. Particularly aerospace, mechanical, and civil sectors have plenty of such scenarios where the energy used is lost through vibration and acoustic noises. Continuously running machinery in a workshop, ambient vibration in a manufacturing facility, vibrating wings of an aircraft, high dB aircraft noise near airports, noise in metallurgical plants, power plants, vehicle noise near a roadside facility, etc. are few examples of the ambient source of energy that can be harvested which are otherwise wasted. If a suitable mechanism is devised, the vibration and acoustic noise sources can be equipped to trap and reclaim the energy to create local power sources.

Researchers proposed many such methods in the past two decades. However, only recently researchers including us proposed that carefully engineered metamaterials can also be used for energy harvesting. Metamaterials are man-made materials that behave uniquely and possess exclusively desired properties that are not found in natural materials. Usually, it is the combination of two or more materials and can be engineered to perform specific tasks that are not possible with traditional materials. These were initially discovered in photonics while working with electromagnetic radiation. An electromagnetic counterpart of wave propagation in mechanics, i.e. phononics with acoustic waves were found to be affected by the metamaterials. These acoustic metamaterials when carefully designed are also capable of affecting the wave propagation characteristics through fluids such as air. Many acoustic metamaterials have gone beyond its definition but still, characterize the waveguiding properties. They are classified under the passive modalities of acoustics to affect the sound and vibration mitigation. Incorporation of smart materials while constructing acoustic metamaterial, can enhance the multifunctionality of these materials in both passive and active ways. A prospective application field for such acoustic metamaterials is energy harvesting from low-frequency vibrations. Conventionally, passive acoustic metamaterials are visualized as noise barrier materials to filter roadside and industrial noises. This application can get extended to the aerospace application where mitigation of engine noise inside the cabin is challenging. Irrespective of their target applications, acoustic metamaterials integrated with smart materials can scavenge the very green energy that they are designed to absorb and mitigate.

First, in this research work, a recently proposed method of creating Acoustoelastic Metamaterial (AEMM) is used to investigate further if that can be used to harvest energy from the industrial noise barriers. It is known that noise barriers are designed to minimize noise outside the boundary like the noise barriers seen beside the highways. Construction materials like concrete, steel, vinyl, wood, or earth mounds are used in the industrial sound barriers that can reduce the sound pressure level (dB) on the other side of the barriers. In this work, a novel metamaterial wall (MetaWall) is proposed to redefine the industrial sound isolation wall using the integrated AEMM units. In this part, wave isolation and energy harvesting capabilities of the acoustic metamaterial is fused to propose MetaWall unit bricks, which are made of rubber-metal-concrete composite, as an industrial building material.

Secondly, it is proposed that such acousto-elastic metamaterial (AEMM) models can also be used in the aerospace industry to power the online NDE/SHM sensors, e.g. piezoelectric wafer active sensors which are widely used. Hence, further in this part, a rigorous study is made to find the actual power required by the online NDE / SHM sensors such that a similar amount of power can be harvested by the AEMM model and stored in a battery for scheduled scans. The ultimate goal of this second study is to minimize the size of the proposed AEMM model to make it suitable for aerospace applications on-board. With changes in the materials of the cell constituents, it is shown that the power outputs from a similar model can be significantly altered and further optimized. A parametric study is also performed to show the variation of the output power. Finally, based on the learning a plate-type metamaterial is proposed to harvest a required optimum amount of energy from the ambient vibration with dominant frequency as low as 100Hz.

In the third section, a spiral-shaped acoustic metamaterial is proposed which has dual functionality of noise filtering and energy harvesting over a wider range of frequencies. A work in progress presented with a proposed timeline to complete the dissertation. This acoustic metamaterial has a comparatively high reflection coefficient closer to the anti-resonance frequencies, resulting in high sound transmission loss. The filtered noise is trapped inside the cell in the form of strain energy. The spiral design is also sensitive to the vibration due to trampoline shaped in highly flexible polymeric piezoelectric material attachments inside the cell. This also makes it capable of harvesting energy using vibration. This is a promising acoustoelastic metamaterial with multifunctionality properties for future applications.

Hence, it is claimed that if metamaterials are employed to reduce or suppress the noise and make use of the trapped energy which is any way wasted could be harvested to power the local electronic devices. The new solution could make a transformative impact on the 21st century’s green energy solutions. Calculated placement of smart materials in the cell-matrix can help to extract the strain energy in the form of power. The acoustic metamaterial cell designs presented in this research have the capability of isolating noise and reducing diffraction by trapping sound in a wide range of frequencies and at the same time recover the trapped abundant energy in the form of electrical potential using piezoelectric materials.