A noise barrier is a solid structure that intercepts and reduces sound within the shadow that it casts. Barrier attenuation in the open field is generally around 10 to 15 decibels due to height and width constraints, sound reflection and flanking around the edges.
The closer a noise barrier can be to the source or receiver and the higher it is, the more effective it will be. The line of sight between the source and receiver must be blocked completely by the barrier.
If a certain decibel level is to be achieved, a qualified acoustician should be used to investigate the site and measure the existing sound to be able to determine what is required and what the actual results might be.