When we work on a project to mitigate the impact of an environmental noise source (such as a nearby highway or outdoor cooling tower) on a residence or outside use space (such as a playground), someone always asks about putting in a line of trees or shrubs to lessen the offending noise. While there is a very real psycho-acoustical phenomenon that takes place when a sound source is no longer visible, the reality is trees and other plants do little to create sound buffers.
There are many factors that affect how noise travels over distance, such as wind speed and direction, temperature and humidity. However, as a rule of thumb, a sound is reduced by about 21 dB over an open distance of 100 feet. That is if there is an open area of 100 feet between a noise source and a listener the noise will be approximately 21 dB quieter than if the listener were directly adjacent to the noise source.
One would think that a forest of trees or shrubs would help to reduce even more of that sound. However, trees and shrubs do not reflect or diffuse sound like massive and rigid noise barriers do. Instead, they absorb some of that sound energy, and therefore the denser the tree and its foliage is, the more energy will be absorbed.
Consider sound the same as a wave on the ocean approaching a marina. A solid pier or breakwater reflects that wave back into the ocean, leaving areas behind it undisturbed. But a more porous barrier, such as a line of rocks, will only provide partial protection as it breaks up that wave and absorbs some of the wave’s energy. But some of that wave’s energy will still disturb the calm marina water. Trees and foliage act the same way.
Deciduous trees (ones that seasonally lose their leaves) have a greater area between leaves and are less dense, therefore providing a greater space for the sound energy to travel around the object. However, when deciduous trees lose their leaves every autumn, they lose nearly all beneficial noise reducing properties. Conifers (like pine trees) on the other hand, are quite dense in comparison and will trap the sound energy year round. Shrubs and other plants act in the same way as trees; the greater the density, the greater the reduction in decibels they will provide.
Still, the decibels saved might not be worth the effort. On average, 100 feet of dense pine trees will only provide an additional 5 dB of noise reduction. So while there is a helpful advantage to such natural barriers, it often requires a lot of real estate for it to be an effective solution.