How Do Trees Reduce Noise Pollution?

Noise barriers made from trees and other plants can provide a respite from unwanted noise pollution. When strategically placed by a roadway, in a backyard, or in a park, trees help reduce pesky noises by absorbing, deflecting, refracting, or masking sound waves. A tactically designed 100-foot-wide tree barrier will reduce noise by 5 to 8 decibels (dBA), according to the USDA.

Noise pollution is defined by the EPA as “unwanted or disturbing sound.” In broader terms, it involves consistent exposure to elevated sound levels, which may cause adverse health and environmental effects. Because sound is not something we see directly, it is often overlooked as an environmental pollutant.

The 1972 Noise Control Act was the first federal regulation of environmental noise pollution in the U.S. While still technically in effect today, the Noise Control Act lost funding in the 1980s, rendering it ineffective. Today, noise pollution is regulated under Title IV of the Clean Air Act.  

How Do Trees Contribute to Sound Attenuation?

Trees are able to reduce or attenuate sound by intercepting sound waves and changing their behavior. Different plant parts reduce noise by absorbing, deflecting, or refracting sound waves depending on their physical characteristics. Tree sound barriers may also create their own sounds or attract wildlife visitors to mask unnatural sounds.

Absorption

Noise is absorbed when sound wave energy is taken in by an object and some of the energy is dissipated.

The structure of a tree, including height, branching structure, leaf shape and density, bark texture, and wood density, determines how effective it is at absorbing sound. A study published in Applied Acoustics found that, of 13 conifer and deciduous species, larch tree bark was the best at absorbing sound waves because of its rough texture. Conifers in general, the study concluded, absorbed more sound than deciduous trees.

Most of the sound absorbed within tree buffers is absorbed by the ground between trees. The presence of trees creates conditions better suited to absorbing sound waves, as roots keep the soil loose, dead organic matter adds a spongy top layer, and the tree canopy helps the soil retain moisture. 

Deflection 

Sound deflection or reflection occurs when sound waves bounce off a surface back towards the source of the noise. The level of sound deflection depends on the interfering object’s density, with harder objects deflecting more sound.

Leaves, branches, and trunks all contribute to the deflection of sound waves by creating a physical barrier. Large, rigid tree trunks are by far the best sound deflectors, especially those with dense bark, like oak, for example. In addition to bouncing back toward the noise source, deflected sound waves can change direction and interfere with each other. This destructive interference has a noise-canceling effect. 

Refraction

Noise is refracted when sound waves change direction as they pass through different mediums. For example, an empty room without carpet will experience echoes because sound waves are reflected off hard, bare surfaces. Adding soft textures, like carpet or curtains, will dissipate sound waves and dampen the noise in the room. 

Similarly, the complex structures of tree crowns can dampen noise pollution. And the more textures in leaves, branches, vines, and bark, the more noise will be refracted.

Masking

Unlike absorption, deflection, and refraction, masking does not interfere with the sound waves emitted by noise polluters. Instead, masking helps offset noise pollution by creating sounds that are more pleasant to the human ear.

Trees can be selected for the sounds they make in response to wind or for the animals they will attract. Species with thick or papery leaves, like quaking aspen or oaks, rustle in even a small breeze. Bamboo is another option for a white noise-generating plant—however, non-native bamboo species can quickly spread out of control. The presence of vegetation can also attract wildlife, such as songbirds and crickets, that make pleasant sounds and allow one to feel more fully immersed in nature. 

How to Create a Sound Barrier With Trees and Plants

The best noise barriers have diverse structures that prevent gaps and add varied textures to the environment. So, in addition to trees, effective sound barriers will include shrubs, bushes, vines, and herbaceous plants.

The width of a vegetation barrier and its distance from the source of the noise plays a key role in its noise blocking effectiveness. According to the USDA, "a 100-foot wide planted buffer will reduce noise by 5 to 8 decibels (dBa)." A buffer planted closer to the source of noise will do more to block noise than a buffer farther back. For example, a 100-foot wide tree buffer planted 100 feet from a road will block about 10 decibels more noise than the same buffer planted 200 feet away.

Broadleaf trees are most effective at deflecting sound. However, when broadleaf trees drop their leaves in winter, the sound barrier is lost. Evergreen trees provide a consistent buffer against sound because they keep their needles or leaves across seasons. Evergreens are also fast-growing and can be planted close together, which creates a denser vegetation barrier. 

How Are Plants Affected by Noise?

Noise pollution can have detrimental effects on nearby vegetation by changing how plants and animals interact. Many tree species, like oaks, depend on animals to disperse their seeds by moving them away from the parent tree to locations where they are more likely to survive. 

Human-made sounds can also change animal behavior, causing them to avoid unfamiliar noises. While this does not have an immediate impact on trees and other vegetation, it can lead to changes in tree composition over generations. And the effects of noise pollution on plant-animal interactions can continue long after noise is removed.

A study published in Proceedings of the Royal Society B found that, in areas with 15 or more years of persistent noise pollution, plant communities did not recover after the source of the noise was removed. Instead, they saw a shift in community composition away from masting species—those that produce huge amounts of seeds every few years—to animal-dispersed species that produce seeds annually or to wind-dispersed species.

Noise pollution, however, is not all bad for plants. A different study, also published in Proceedings of the Royal Society B, determined that rates of pollination may actually increase in areas with noise pollution. Their research looked specifically at hummingbirds, which were previously shown to nest more often in noisy areas, and found that they visited flowers more often in areas with artificial noise.

The research exploring how noise pollution affects plants is limited. Evidence suggests, however, that noise has cascading effects throughout plant communities with potentially long-term or permanent consequences.