Hearing the Forest for the Trees: Foliage Acoustics and Green Noise Barriers
Written by: Alex Odom
Aside from the popular philosophical question asking if a tree falling in a forest makes a sound, the most common arboreal inquiry I get as an acoustical consultant get is: “Can we block the noise with trees instead of a noise barrier?” The short answer is “no”, trees are not particularly effective when compared to a noise barrier, especially when there is only a single line of trees. The long answer, drifting over to “sort of”, is driven by the psychological impact of plants, and it reveals some of the reasons why it is such a common suggestion by non-acousticians.
WHAT MAKES A NOISE BARRIER WORK?
It is helpful to understand the basics of noise barriers to see why trees aren’t particularly good on their own, but how they can effectively supplement traditional noise control to benefit the acoustic environment. The three fundamental things that a noise barrier does to reduce sound is 1) block sound, 2) absorb sound and 3) diffract sound.
To effectively block sound, the barrier wall must be made of a dense material (~ 3 lb/ft2). This performance is measured with Sound Transmission Class (STC). A noise barrier should meet STC 25 and have no gaps in the surface between panels. Gaps between the barrier and the ground should also be minimized.
The large gaps between individual trees is the main limitation on their performance as noise barriers. The effect of a single row of trees is almost negligible, so a much larger space is required for a tree belt to achieve significant noise reduction. For example, a 100-ft deep optimized belt of dense trees can provide similar attenuation to a 10-ft tall traditional barrier wall. But even this performance is largely dependent upon the soil characteristics between the source and receiver, revealing the importance of the two other factors that drive barrier performance, absorption and diffraction.
To prevent reflections and the “build-up” of sound, a sound absorptive finish is often applied on the side of the barrier that faces the noise source, whether that is a highway or mechanical equipment on a rooftop. Absorption is measured in terms of a noise reduction coefficient (NRC), a value between 0-1, where a NRC of 0.0 represents a perfectly reflective (“acoustically hard”) surface, and a NRC of 1.0 represents a perfectly absorptive (“acoustically soft”) surface where all sound energy is absorbed. An absorptive noise barrier finish should meet a minimum NRC of 0.75.
Grass, soft foliage, and leaves provide some absorption of sound, particularly at higher frequencies, whereas the bark of the tree is quite acoustically hard. This is why trees that maintain their leaves year-round, like conifers, would be preferable for tree belts acting as noise barriers. To add absorption to a traditional noise barrier wall with greenery, vegetation can be applied directly to the surface of the barrier or a single belt of trees can be placed directly behind the barrier. These are particularly beneficial for highway noise barriers, but may not be achievable when seeking absorption on the interior of an equipment well. In addition to the vertical surfaces of the barrier wall, attention must be paid to the top of the wall where diffraction occurs.
Once the direct pathway of sound is accounted for, much of the attenuation provided by a noise barrier comes from diffraction: how sound bends around corners and over the tops of walls. Depending on the shape of the top of the sound barrier, sound will diffract differently. A complex barrier edge, such as a Y-shape or a cylindrical edge can provide up to a 5 dB improvement over a plain barrier of identical height. Vegetative barrier caps designed with this geometry in mind can achieve these performance increases with added greenery while adding the needed absorption. This phenomenon explains why the complicated geometries of a natural forest with random arrangements of trees, branches, and leaves will provide a complex scattering sound.
You might think that forests are highly absorptive environments as they often can seem quiet, but they can be very reverberant. Sound bounces off branches and leaves, so their removal can provide more clarity to birdsong. While trail running, I’ve often heard gun noise from a nearby shooting range echo through Harold Parker State Forest in Massachusetts. Acentech’s Ben Markham shared a field report of a reverberant forest on hike:
The acoustic environment of the outdoors will change throughout the year. The winter can be peaceful after a fresh snowfall, and deer eating through ground vegetation can reduce the diffraction and absorption typically provided by leaves and shrubs. But despite these changes, people always tend to think positively of the sounds of nature throughout the year.
SOUNDSCAPES AND GREEN PSYCHOLOGY
Trees don’t just block sounds, they produce sounds too, like the rustling of leaves or the creaking of the trunks in heavier winds. In rare cases when trees dry out they also produce a sound inaudible to humans from cavitation (for more on cavitation, see Pop Science). In general, people prefer these kinds of sounds to industrial or other anthropogenic sounds. Even if urban parks aren’t always “quiet”, the addition of natural sounds like crickets chirping, birdsong, or running water can improve the perceived quality of the soundscape and give the feeling of tranquility.
This positive association with greenery is so powerful that it can provide a psychological reduction of noise. One survey found that 90% of subjects believed that plants reduced noise and 55% of subjects inflated the effectiveness of the attenuation provided, which explains why our clients often suggest them as a cheaper alternative to traditional noise control. Even if trees may not be particularly effective noise barriers, they are helpful at softening the urban environment and keeping industrial sources out of sight and out of mind.