Barriers such as walls or screens will act to create an acoustic shadow by obstructing the free flow of sound energy. The reduction in sound level within this shadow zone is dependent on frequency. At high frequencies the effect of the barrier is most pronounced, whereas at low frequencies much diffraction occurs at the edges so the shadow effect is diminished.
There are a number of quite computationally intensive methods for calculating this reduction, however a simple method that suits our purposes is as follows:
- R = the reduction (dB) over the inverse square law,
f = the frequency (Hz),
d = the path difference between diffracted and direct sounds, and
c = the speed of sound (usually taken as 343m/s).
NOTE: This prediction is based on the assumption that any transmission through the wall or screen itself is negligible.
The exact placement of a barrier between the sound source and a receiver position will determine its effectiveness. To work best, the barrier must make the sound travel as far as possible from the direct route in order to traverse it, and change direction by the greatest angle. For a barrier of a given height, this can be achieved by placing it either directly next to the source or directly next to the receiver. Halfway between the two is the worst position. As all barriers will allow some small amount of direct transmission through them, it is best to place the barrier closest to the source so that any transmitted sound still has to travel the maximum distance to the receiver.
- The best position for a barrier is nearest the noise source.
- The second best position would be nearest the receiver.
- The least effective position is midway between the two.
Vegetation as a Barrier
It appears that there is a widely held belief that vegetation is an effective controller of sound. However, research by Fricke (1984) and others shows that vegetation is only effective at higher frequencies (above 2000Hz). The impedance of the ground is the dominant factor at lower frequencies (125-500Hz) with vegetation having little effect on sound propagation at mid-frequencies (500-2000Hz). It is suggested by Aylor (1972) that scattering, rather than absorption, is the more important phenomenon at mid-frequencies. At higher frequencies, however, absorption takes over as the dominant phenomenon.
The literature generally suggests that the principal effect of plantings is psychological. By removing the noise source from view, plantings can reduce human annoyance to noise. The fact that people cannot see a highway can reduce their awareness of it, even though the noise remains. Another effect to consider is increased wind noise generated by the leaves of large trees, effectively masking other mid-high frequency sounds.
Large areas of vegetation can be effective, but only in belts greater than 50m thick.
Traffic Noise Attenuation as a Function of Ground and Vegetation by Rudoff Hendricks (CALTRANS, office of Materials Engineering and Testing, 5900 Folsom Blvd., Sacramento, CA. 95819)(July 1995).[TD100:CA 95-23] This report concluded that vegetation is usually not an effective highway noise mitigation measure. In this research project, the term "vegetative barriers" refers to shrubs and trees planted in relatively narrow and dense strips along highways for the primary purpose of landscaping. As used in this report, vegetative barriers do not include the specially designed "green" or "living" noise barriers that incorporate vegetation and structural materials for the specific purpose of noise abatement.
Measure traffic noise attenuation rates as a function of distance from source, height above ground, and absorptive characteristics of six ground types, ranging from reflective paved surfaces to soft, plowed dirt and ground covers. Measure traffic noise attenuation provided by four species and three heights or thicknesses of vegetation belts alongside highways, such as ivy covered fences, dense oleander and other shrubbery. Establish improved traffic noise attenuation rates and shielding values to be used as inputs for Caltrans noise prediction methods, based on findings in this study. Develop guidelines for use of evergreen vegetation belts (barriers) in Caltrans noise abatement procedures, if effectiveness were proven in this study.
A continuous strip of oleander or equivalent shrubs, at least 2.4 m (8 ft) high and 4.5 to 6 m (15 to 20 ft) wide, planted along the edge of a highway shoulder, provides noise attenuation of 1-3 dBA at distances of up to 15 m (50 ft) from the rear edge of vegetation. A single line of pine trees planted about 7.5 m (25 ft) from the edge of a highway shoulder, 12 m (40 ft) tall, 9 m (30 ft) in diameter, spaced 3-6 m (10-20 ft) apart, low branches intertwined and touching ground, provides noise attenuation of 0-1 dBA at distances of up to 18 m (60 ft) from the rear edge of vegetation.
A combination of oleander, planted 11 m (35 ft) from the edge of a highway shoulder, 2.7 m (9 ft) high and 3 - 4.5 m (10 - 15 ft) wide, and redwood trees, equally spaced at 9 m (30 ft) in the oleander strip, 15 m (50 ft) tall and 6 m (20 ft) in diameter provides noise attenuation of 0 to 1 dBA at distances of up to 21 m (70 ft) from the rear edge of the oleander strip.
Vegetative barriers (as defined in this study) are not an effective highway noise mitigation measure to use on a routine basis. In some borderline cases (to mitigate or not mitigate), thick oleander (at least 4.5-6 m, or 15 - 20 ft wide and 3 m, or 10 ft high) may have some value by avoiding conventional noise barrier construction, if adequate right-of-way is available.
Trimming or removal of shrubs and trees along highways by maintenance or construction does not cause perceptible noise level increases to nearby residences. However, the sudden visibility of highway traffic previously shielded visually by the vegetation, and the possibility of a shift in sound frequencies, may bring on a renewed awareness of the presence of noise sources. This may result in additional noise complaints.