Noise control or noise mitigation is a present technology that reduces sounds. Active noise reduction is a method for reducing sound that you don’t want by the addition of another sound specifically designed to cancel the other one you don’t need. The sounds are created through digital generators that travel into a space using speakers that people do not notice. It does not cancel or eliminate noise but adds a sound in the background so that you won’t be distracted by the sound you don’t want to hear. Currently this technology is being used in offices, hospitals, libraries and in other spaces where quiet spaces are needed.

 

Sound walls are presently constructed using concrete, metal, wood and other materials that last a long time. Sounds from these walls can bounce off the surface (reflected), pass though the wall to the other side (transmitted) or absorbed. “According to metrics by the Department of Transportation, in order to achieve a 5 decibel reduction of noise, a barrier must be built at the line of sight (and hearing) of the average individual. Every additional meter above the line of sight reduces the sound by an additional 1.5 decibels. This works through reducing the amount of sound energy able to pass over the wall via diffraction.”  Soundwaves can diffract over the top of these walls. The size of the source will affect how much diffraction happens.

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Berms are another way to block out noise from a highway. Berms are mounds of soil, sand or construction material that is pushed together to create a natural hillside. These barriers require a lot more space than sound walls. This structure has a lower cost but often needs to have plants added to the structure which adds an additional maintenance cost.

 

Another natural form of sound reduction is the use of plants in an area of road noise.  Sound is absorbed by all parts of the plants such as leaves, branches, twigs, and wood.  Evergreen plants do better at absorbing sound because they do not lose their leaves like deciduous plants.  Large shrubby trees scatter sound waves. The denser the plants are, the greater its scattering possibility. Plants and trees are not typically used for sound deflection and reflection but used because they do absorb some sound and fit well in the natural world.