text: Several different materials may be used for sound barriers. These materials can include masonry, earthwork (such as earth berm), steel, concrete, wood, plastics, insulating wool, or composites. Walls that are made of absorptive material mitigate sound differently than hard surfaces. It is also possible to make noise barriers with active materials such as solar photovoltaic panels to generate electricity while also reducing traffic noise.
  A wall with porous surface material and sound-dampening content material can be absorptive where little or no noise is reflected back towards the source or elsewhere. Hard surfaces such as masonry or concrete are considered to be reflective where most of the noise is reflected back towards the noise source and beyond.
  Noise barriers can be effective tools for noise pollution abatement, but certain locations and topographies are not suitable for use of noise barriers. Cost and aesthetics also play a role in the choice of noise barriers. In some cases, a roadway is surrounded by a noise abatement structure or dug into a tunnel using the cut-and-cover method.