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Novel Acoustic Materials for Passive Hearing Protection

A material which rejects a high amplitude acoustic signal but transfers a lower intensity sound without attenuation is introduced. More specifically, the material is designed to transmit a sound having intensity below 65 dBA and to reject any acoustic signal exceeding 90 dBA which satisfies military standards for hearing protection. A wide range of situations would benefit from such capability: The first envisioned application targets protection of military personnel from hearing loss while keeping the high level of situation awareness. Obvious immediate commercial application of the developed material includes hearing protection in the law enforcement, hunting, sport shooting and other similar activities. Proposed approach also has a potential for setting hazardous noise threshold level to 85 dBA which is required for civilian applications. According to National Institute for Occupational Safety and Health more than 30 million workers in the USA are exposed to high intensity noise levels and suffer from hearing loss. DoD alone pays more than $3 billion per year in directly related to hearing loses legal and rehabilitation compensations. Moreover, passive hearing protection gears which reject noise without affecting the sound transmission at moderate sound levels would form $1.3 billion USA market for the next five years. The market includes noise isolation in motorcycle and snowmobile helmets, earplugs for highway and airline workers, policemen, and power tool operators. In addition, such meta-material layer could serve as a passive protection of sensitive acoustic equipment from occasional high-intensity signals in sonars, medical ultrasound detectors, acoustic intelligence and security devices, and hearing aids.

This project develops a meta-material exhibiting the required bi-stable acoustic response. More specifically, the material is composed by meta-structural elements transmitting acoustic energy in two different states which are switchable by the sound intensity. A meta-element transmits low intensity sound as a longitudinal elastic wave; however, due to the element design, uniaxial elastic deformations caused by the elastic wave become unstable above some critical level. When the sound intensity increases, the system ability to transmit acoustic energy is limited and the energy exceeding certain pre-defined level is rejected in the form of reflected waves. To satisfy the project requirement the meta-material is designed to ensure threshold intensity of the acoustic signal near 90 dBA level. Our analysis shows that bi-stable meta-elements prone to mechanical instability and switchable by the acoustic pressure would perform best by having linear sizes much smaller than the wave length. Therefore the desired acoustic performance can be achieved within few millimeters layer of meta-elements. This is a critical advantage for designing wearable protective devices. Finally, the meta-material can be mass fabricated using low cost manufacturing approaches.

Different types of meta-elements were considered and their response to excitations at various amplitudes and frequencies are modeled and numerically simulated. Acoustic pressure and frequency of meta-element vibrations are linked to displacements of their boundaries, both for low and high amplitudes of vibrations. Obtained dynamic relationships are used to match material response and its ability to transmit acoustic waves. This analysis yields the acoustic energy flux through the meta-layer. Transmission and reflection coefficients between the incident acoustic wave and the transmitted and the reflected by material layer waves are obtained. Main result of the project is summarized in the transfer function for acoustic energy at different wave amplitudes and frequencies. Relationships between geometrical parameters and acoustic performance of the meta-elements are established for different engineering materials. All features of the desired meta-material are in the reach for current mass-production technologies. Several meta-structures having similar acoustic performance but required different manufacturing approaches are presented. Acoustically all considered structures perform similarly but would exhibit different structural properties of the final layer. The developed software and analytical tools can be applied to future analysis of other perspective structures which would further reduce the manufacturing costs and improve performance of the resulting material. The project is ready for the next stage which would be manufacturing a prototype material and conducting performance testing of the acoustic meta-layer.

 

 

Highlights of the project:
 

A material which rejects a high amplitude acoustic signal but transfers a lower intensity sound without attenuation is introduced. More specifically

Acoustic transmission is controlled by the sound intensity

 

 

Sound wave below some critical value, Acr, is transmitted without attenuation

 

 

Acoustic energy exceeding the critical value, Acr,is reflected

Transfer Function for Acoustic Energy

Sound intensity below 65dB is transferred without attenuation; anything above the transition level (somewhere between 65 to 90 dB depending on frequency) will be reflected back