Trees That Absorb Sound

Location

• In general, screens of trees buffer noise the closer they are placed to the source of the noise and the farther they stand from the hearer. If your neighborhood has become the unwilling host to a new highway off-ramp, building sound barriers close to the road will be more effective than planting them close to your house. This accounts for the increasing establishment of landscaping when solid noise-barrier walls are erected. Trees, correctly planted, can increase noise reduction, not just hide the wall from view.

Length

• The behavior of sound determines the length as well as the location of noise barriers. Because sound travels in waves, its trajectory can be stopped only by a solid barrier as long as the area to be shielded from noise. This behavior presents some difficulties in using trees for noise barriers. The old thinking that tree branches and leaves "soaked up" noise is inaccurate. A better analogy would be a different kind of wave, that of water. Trees planted to shield a house from the damage caused by a huge water wave would need to be planted very densely to keep the bulk of the water from leaking through. Any spaces between tree branches will let water through. Further, trees that are shorter than the height at which noise is heard or the wave flows lets noise or water sweep over the top.

Depth

• Picturing a water wave also illustrates the other challenge faced by those using trees to reduce noise. A strip of trees planted in a strip as long as intended, with trees planted very close together, must be 30 m, or 25 yards, deep to reduce noise by 5 to 10 decibels, which may seem like a small reduction when human conversation registers in the 30 to 60 decibel range and truck noise ranges between 60 and 80 decibels. To the human ear, however, a 5 to 10 decibel reduction is experienced as cutting noise in half. The difficulty is finding enough space to create a noise reduction screen composed exclusively of trees.

Supplementary Strategies

• Supplementary measures fall into two categories. The first is using trees as supplements to other noise-reduction structures. Adding trees to solid wooden fences, stone walls or high earth berms will increase the effectiveness of the structures. Second, thick shrubs and diverse trees reinforce the thickness of a tree barrier. Shrubs fill in at the bases of trees where branches may be thin. Augmenting the kind of evergreens chosen as a year-round barrier with deciduous trees and other evergreen varieties reduces the chances that disease or other damage to a single tree variety will damage the overall barrier.

Other Benefits

• While trees alone may not reduce noise to bearable levels, they offer several other benefits in noise reduction. Just as the ear perceives a small reduction in decibel level as a large one, the eye is less disturbed by nuisances it cannot see than by those it does. "Out of sight, out of mind" makes sense. A lush tree barrier can make noise seem farther away and less intrusive. Further, the "white noise" of rustling branches, the refreshing odors and improved air quality associated with planting trees may all serve to make noise seem like less of an irritant.