We have modelled, designed and printed a metamaterial with a structure that reduces the effective speed of sound in air by half, opening up the potential for space-saving noise cancellation devices.
Noise pollution can reduce our quality of life, and even our life expectancy. A 2014 report from the European Environment Agency estimates that 10,000 premature deaths and 43,000 hospital admissions for coronary heart disease and stroke can be attributed to noise exposure in Europe each year. (1) The urgent need for efficient noise cancellation devices is clear.
Noise pollution can reduce our quality of life, and even our life expectancy. A 2014 report from the European Environment Agency estimates that 10,000 premature deaths and 43,000 hospital admissions for coronary heart disease and stroke can be attributed to noise exposure in Europe each year. (1) The urgent need for efficient noise cancellation devices is clear.
Destructive Interference
Noise cancellation devices often work on the principle of destructive interference, where sound waves are combined so that they cancel each other out.
Here’s how it works:
Here’s how it works:
One tool that may be employed to reduce noise from engines, fans and other devices is a Quarter Wavelength Resonator (QWR). This is a side branch in a duct that redirects sound waves so that they interfere destructively. The length of the QWR is ¼ of the wavelength of the noise frequency to be attenuated.
The animation below shows how a QWR functions:
The animation below shows how a QWR functions:
Part of the sound wave is diverted into the QWR, reflects off the end wall and then recombines with the undiverted wave. The diverted wave has travelled an additional ½ wavelength, so the waves now re-combine destructively, cancelling the noise.
There are limitations to this approach: the size of the resonator depends on the wavelength of the noise we want to reduce. Low frequency noises have long wavelengths, so the size of the resonator required soon becomes impractically large.
There are limitations to this approach: the size of the resonator depends on the wavelength of the noise we want to reduce. Low frequency noises have long wavelengths, so the size of the resonator required soon becomes impractically large.
Acoustic Metamaterials
We used a mathematical mapping process called Transformation Acoustics to design and 3D print a metamaterial that has the potential to overcome the size limitations of QWRs for reducing low frequency noise.
Metamaterials are special materials with properties that are not found in conventional materials. Unlike conventional materials, the properties of metamaterials are defined by their structures rather than by their chemical make-up. Acoustic metamaterials have geometries that allow the manipulation of sound waves in ways not previously achievable.
Our metamaterial has arrays of elliptical cylinders which stretch the apparent space inside the resonator, with the result that the effective speed of sound is reduced by half. Crucially, it does this while maintaining a close match to the impedance of sound in air across a range of frequencies.
Metamaterials are special materials with properties that are not found in conventional materials. Unlike conventional materials, the properties of metamaterials are defined by their structures rather than by their chemical make-up. Acoustic metamaterials have geometries that allow the manipulation of sound waves in ways not previously achievable.
Our metamaterial has arrays of elliptical cylinders which stretch the apparent space inside the resonator, with the result that the effective speed of sound is reduced by half. Crucially, it does this while maintaining a close match to the impedance of sound in air across a range of frequencies.
Acoustic impedance is a measure of how easily sound travels through a substance. When sound passes between media with very different impedance, some of the sound is reflected and not transmitted. For the destructive interference from the QWR to work effectively, transmission of sound energy into the side branch must be efficient, so a close match to the impedance of air is required.
Experimental results illustrating the sound reduction from three resonators of equal length. Branch (a), dotted red curve, has a metamaterial structure of ellipses centred on the branch centre. Branch (b), solid green curve, has a metamaterial structure of ellipses centred on the branch walls. Branch (c), dashed blue curve, is a standard air filled resonator for comparison. The halving of the frequency of noise reduction in the metamaterial is clearly visible.
Space-Saving Devices
Slowing down the sound by half has the effect of halving the frequency of the noise that can be cancelled with the same size of resonator, or halving the size of the resonator that operates on a particular frequency, because of the relationship: speed of sound = wavelength x frequency
When the speed of sound is halved, either the length of the resonator, or the frequency of the sound it reduces must also be halved. This allows significant space savings and opens up the possibility of building more practical, size efficient devices for the reduction of low frequency noise.
Read the full paper here: doi.org/10.1063/1.5022197
(1) European Environment Agency Report No. 10/2014: www.eea.europa.eu/publications/noise-in-europe-2014
When the speed of sound is halved, either the length of the resonator, or the frequency of the sound it reduces must also be halved. This allows significant space savings and opens up the possibility of building more practical, size efficient devices for the reduction of low frequency noise.
Read the full paper here: doi.org/10.1063/1.5022197
(1) European Environment Agency Report No. 10/2014: www.eea.europa.eu/publications/noise-in-europe-2014
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