Acoustic Metamaterials for Noise Cancellation
A growth in the global population leads to an overall increase in the development of buildings and public or private transportation. The construction of these objects and the use of vehicles contribute to urban noise levels and noise pollution. Acoustic metamaterials have been used to control and manipulate sound waves to reduce noise and improve health and quality of life. Currently, engineers are working towards using acoustic metamaterials—also known as sonic or phononic crystals—to allow for low-frequency sound waves or noise cancellation.
The structure of acoustic metamaterials, made from artificially engineered periodic structures, cause unique acoustic wave properties. These acoustic wave properties are used to control acoustic wave propagation. By adjusting the size and shape of the acoustic metamaterial, it’s possible to control the noise attenuation over a tunable frequency range, and to reach the target frequency range, it’s necessary to select the correct type of acoustic metamaterial. Some examples of acoustic metamaterials include acoustic meta-absorbers, sonic crystals, and meta-diffusers. Researchers use hybrid metasurface absorbers, combining the traditional perforated place of acoustic metamaterials with subwavelength-sized metastructures. When the acoustic impedance of a hybrid metasurface matches the impedance of a sound wave, it can absorb the sound energy.
A potential strategy for noise cancellation that has gained population over the years includes the use of acoustic metamaterials with subwavelength sizes that absorb sound. Metamaterials of this type—specifically deep-subwavelength sizes—include the membrane, Helmholtz, and the Fabry-Perot acoustic material. The membrane acoustic material is composed of a disk and an elastic membrane, which allows for resonance in the audible frequency range. The Helmholtz acoustic material has a sound-absorbing structure and structural parameters, which are used to enhance the absorption of sound energy and more precisely control the sound absorption frequency. Similarly, the Fabry-Perot acoustic material is used for acoustic absorption. A limitation of acoustic metamaterials with deep-subwavelength size is that they can only absorb sound at specific frequencies. The metamaterials are incapable of reducing or absorbing broadband noise. However, a strategy used to broaden the absorption bandwidth was to combine the resonant response units. For example, a popular method is to use multiple Helmholtz acoustic metamaterials for broadband sound absorption.
Membrane-type acoustic metamaterials were used for low-frequency noise attenuation and showed potential to be used for indoor noise and aircraft cabin noise mitigation. The limitations of membrane types are the variation of membrane properties in the environment, external equipment for membrane holding, air leakage from poor adhesion between the membrane and metastructures, and manufacturing challenges.
Sonic crystals, non-homogenous composite structures composed of wave scatterers distributed in a 3D space, have been used for outdoor noise mitigation. On the deep-subwavelength scale, sonic crystals can absorb sound waves on the mid-gap frequency. The band gap structure obstructs acoustic waves while the composite structure allows waves to pass through. The band gap is influenced by the density ratio—density of the scatterer’s material and host medium—and filling factor—volume fraction occupied by the scatterers and lattice designs. The band gap can also be influenced by the distance between scatterers in the lattices. Lattices can be constructed in many shapes, such as a square, rectangle, triangle, sphere, cylinder, and honeycomb pattern.
Acoustic meta-diffusers, based on the “Schroeder diffuser”, are used for noise shielding and sound reflection in a diffuse field. Meta-diffusers are generally used to control the acoustic energy in large indoor settings to prevent the loss of sound energy and are composed of wells of varying depths. The thickness of the wells depends on the sound frequency, making the design of the diffuser rather bulky. The size of acoustic meta-diffusers limits the number of installations in real settings.
Phononic crystals have a periodic elastic medium structure with a phononic forbidden band and are used for vibration isolation and noise reduction. This effect is achieved by changing the internal structure and defects within the phononic crystal and adjusting the propagation path of the elastic wave. Elastic waves can be reflected and scattered at the interfaces of the waves caused by that can cause bad gaps. The frequency range covered by phononic crystals is those that are nearly-inaudible. Phononic crystals are reliant on their lattice size, as it allows the crystal to have the same magnitude as the wavelength in the direction of the sound wave propagation. The resolution of phononic crystals is limited by its anisotropy state and lattice constant.
Currently, various models of acoustic metamaterials could aid with noise reduction or cancellation. Each type is suitable for a specific area or wavelength of sound and each has its limitations for when it can be used. However, with the current rate of technological advancement, engineers can design an efficient way to completely block noise. In the future, it is possible to greatly reduce noise pollution and create an environment with lower noise levels, which can aid with the health of the community and contribute to a healthier planet.
References
Duan, Haiqin, et al. "Acoustic Metamaterials for Low-Frequency Noise Reduction Based on Parallel Connection of Multiple Spiral Chambers." Materials, vol. 15, no. 11, 29 May 2022, p. 3882. National Library of Medicine, https://doi.org/10.3390/ma15113882. Accessed 26 Jan. 2025.
Kumar, Sanjay, and Heow Lee. "The Present and Future Role of Acoustic Metamaterials for Architectural and Urban Noise Mitigations." Acoustics, vol. 1, no. 3, 1 Aug. 2019, pp. 590-607, https://doi.org/10.3390/acoustics1030035. Accessed 26 Jan. 2025.
Liu, Junyi, et al. "A Review of Acoustic Metamaterials and Phononic Crystals." Crystals, vol. 10, no. 4, 15 Apr. 2020, p. 305. Proquest, https://doi.org/10.3390/cryst10040305. Accessed 26 Jan. 2025.