The Greenbelt Station developer has sent the study report below on reflected noise from the Greenbelt Station development. The College Park City Council was considering to ask the developer to conduct the noise reflection study at tonight’s Council meeting. Please let me know if you’ve any questions.
Figure below shows an aerial photo of the site. The aerial photo shows the two Metrorail tracks (closer to the Greenbelt Station site), the two CSX railroad tracks (farther from the Greenbelt site), some new roads built on the Greenbelt Station site, and neighboring houses across the railroad in the Hollywood neighborhood.
The approximate locations of the proposed noise wall and three evaluation points labeled A, B, and C are also marked in Figure 1. It can be seen that there are many houses in the Hollywood community to the northwest including those on Huron Street and Iroquois Street. The land
immediately to the west of the Greenbelt Station site is an undeveloped forest bounded by 51st Avenue and Huron Street, with an industrial area further south. We have assumed that the only concern of this analysis is the residential community, and not the forest or industrial area. Locations A, B, and C as labeled in Figure 1 represent the houses on 51st Avenue and Huron Street that are closest to the Greenbelt Station site, as well as a house somewhat farther away.
Previously, we measured sound levels on the Greenbelt Station site in two locations for nearly four days.
Sound levels were extrapolated to other locations on the Greenbelt Station site based on the assumption that sound levels vary solely as a function of distance from the railroad. Based on the predicted noise levels, a noise wall has been proposed to mitigate railroad noise reaching the Greenbelt Station townhouses.
It was determined for freight trains sound levels dropped off at a rate of approximately 6 dB per doubling of distance with the acoustical centerline fairly close to the near edge of the near pair
of CSX tracks, while for Metro trains sound levels dropped off at a rate of approximately 4.5 dB per doubling of distance with the acoustical centerline roughly between the two pairs of Metro tracks. We used this same approach to estimate the Day-Night Average Sound Level (DNL) at locations A, B, and C in Figure. The resulting predictions are that the DNL are 74.6 dB, 66.9 dB, and 74.9 dB at locations A, B, and C, respectively. These sound levels are quite high.
In order to estimate whether the proposed noise wall would amplify railroad noise at locations A, B, and C we used the computer program SPM9613. The SPM9613 algorithms are based on the ISO standards 9613 Parts 1 (1993) and 2 (1996) which consider geometric divergence of sound, attenuation of sound by the atmosphere, and interaction of sound with the ground. The input to the noise model included:
• Three dimensional coordinates of the railroads. The CSX railroad and Metrorail tracks were
each modeled as a series of noise sources located along the centerlines between the pairs of
tracks. The sound power levels were arbitrarily set to 100 dB in all frequency bands; since the
focus is a change in noise level, the absolute sound power levels are not that important. The
source height was 0 up to 15 feet above the tracks.
• Three dimensional coordinates of the top and bottom of the proposed noise wall. It was
conservatively assumed that 100% of the sound hitting the noise wall bounces off (i.e., it has a
reflection coefficient of 1.0).
• Three dimensional coordinates of points A, B, and C, with a nominal height of 5 feet above the
ground, and a medium-hardness ground near them and midway to the railroad.
The resulting prediction is that the noise wall would only increase sound levels 0.1 dB at locations A and B, and 0.0 dB at location C. A change of 3 dB would be just barely perceptible. A change of 0.1 dB is completely imperceptible. In other words, there will be no noticeable reflections and noise levels will not increase noticeably due to the noise wall.
There are two reasons why reflections off the noise wall will not be significant. First, and most
importantly, the noise wall height is not that great relative to the horizontal distance between the noise wall and houses. Second, the noise wall does not extend as far north as the Hollywood houses. If the wall extended past the storm water management pond, the increase in noise level would be somewhat higher.