Field Evaluation of Reflected Noise from a Single Noise Barrier (2018) / Chapter Skim
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3 • Sound levels are higher and spectral content changes at a position between the barrier and the road, compared to the No-Barrier site, as evidenced at I-24 and SR-71. • The background sound pressure level is elevated in the presence of the noise barrier at the reference microphone position between the barrier and the road.
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4 metrics as a function of time. In turn, these metrics were combined into three different psychoacoustic measures: unbiased annoyance (UBA)
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5 • Compared to spectrograms for reflective barriers, where reflection effects are readily apparent, spectrograms for absorptive barriers reveal little indication of reflection effects. • Difference spectrograms revealed harmonically related peaks caused by comb filtering, which can be attributed to the barrier and can be perceived as the sound being buzzy or raspy.
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6 Finally, the research team suggests several recommendations and considerations for future research: • Further study reflections off sound-absorbing barriers, examining a broad range of absorptive barrier types and NRC values; • Use the measurement dataset to validate the single-wall reflections component of the new FHWA Traffic Noise Model, Version 3.0 (TNM 3.0) ; • Conduct before/after studies to assess the effect of the barrier in situ, using techniques developed in this project and others, as needed; • Incorporate some of these findings into various courses on highway traffic noise; • Evaluate time-based and time-above metrics to help understand other drivers of adverse community perception; • Further study the sound quality benefit of sound-absorbing barriers; a narrow-band analysis should be done to confirm that sound-absorptive barriers reduce comb-filtering effects; • Enhance the Barrier Reflections Screening Tool in one or more of the following ways: (1)
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7 Introduction The Problem From 1963 through 2013, nearly 3,000 linear miles of highway noise barriers had been constructed in the United States. Of these, at least 7% were sound-absorbing barriers.
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8 Another aspect of this phenomenon may be a factor that was noted in a study of a Caltrans project where sound absorption was added to a previously reflective far-side noise barrier along U.S. 101 in San Rafael, California (Menge and Barrett 2011)
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