Field Evaluation of Reflected Noise for Sensitive Receptors Across from a Noise Barrier
Community & Cultural Concerns, Environmental Process
Research Idea Scope
It has been the experience of a number of state transportation departments, that there is a strong potential for residents on the opposite side of a highway from a noise barrier to report that construction of the noise barrier has increased noise levels in their area due to reflected noise from the new noise barrier. In many situations, the increase in noise levels is small, as described with standard highway noise metrics; the reflected sound does not intuitively support the reported perceptions by the affected receptors (exceptions would be cases where sound shielding experienced by the direct sound path is bypassed by the reflected sound path, raising the sound level an amount that is commonly thought to be perceptible). The cause and nature of the perceived and/or actual increase in noise levels is not fully understood, and there are not readily available analytical tools to fully assess the single reflection phenomenon that is at the heart of this problem.
Comprehensive studies of noise level magnitude, annoyance, and in particular fractional octave-band frequency spectrum of the noise source in the presence of a single barrier on the opposite side of the highway are recommended; cases with both reflective (non-absorptive) and absorptive barriers should be examined. In addition to standard highway noise metrics, other metrics should be investigated to help understand the perception of this reflected sound; these may include: a frequency weighting other than A-weighting and narrow-band spectral analysis (narrower than 1/3-octave band). These quantitative analyses will then be used to determine if the degree/magnitude and/or composition of the noise level actually changes, or whether the complaints are triggered by the psychological phenomenon of “barrier envy” (nearby residents are not receiving a noise barrier when their neighbors do).
Several state Departments of Transportation (DOT) should be canvassed where residents have complained about an increase in noise due to the presence of a recently constructed noise barrier across the highway. Using information about specific sites provided by the DOT’s, study sites should be selected that ideally would provide equivalent cross-sectional topography and traffic operations for both target (across the highway from the reflective barrier) and reference (no barrier) locations. Data for comparative analyses should be simultaneously collected at various setback distances up to 500-1000 feet from the roadway. Data should be collected for target locations near the center of the barrier, and near the end of the barrier. Simultaneous data should be collected for reference locations under the same traffic conditions. A detailed record of meteorological conditions, e.g. wind speed and atmospheric stability, should be maintained during data collection. Using annoyance metrics to identify image events and especially 1/3 octave-band frequency analysis and meteorological data, traffic noise source and reflected spectra should be analyzed. Results should be assessed against current prediction methodologies. The analysis should identify whether or not a measured shift in the overall noise level and/or frequency content occurs for a given set of traffic operations. Evaluations should also consider if the presence of a phenomenon varies with distance, or is influenced by geometric or physical parameters such as barrier height, pavement surface roughness and porosity, and by vehicle type and under variable traffic mix scenarios (i.e. high vs. low truck percentages , low vs. high speed traffic, etc.).
Urgency and Payoff
To address the reflections from barriers (with the limited analytical tools currently available), many states have resorted to near universal specification of sound-absorbing surface treatments to all noise barriers where reflections may affect noise sensitive receptors. The potential for increased cost associated with absorptive barrier surfaces, aesthetic concerns and limitations, and questions of long-term durability of absorptive surfaces would suggest the need to fully understand the actual need for such treatment, and how best to assess effectiveness and design parameters required for the barrier system.
Data from this research would be useful in development and refinement of reflection calculation algorithms in the FHWA TNM prediction model. The results from this study will also provide those dealing with the public, insight into the factors and underlying conditions that influence public reaction and attitudes.
Ken Polcak, Maryland State Highway Administration