Noise Barrier Design Handbook
In accordance with the technical definition, a 5-dB penalty is added to evening operations when computing the Lden
noise metric. The 5-dB penalty, expressed in terms of a weighting factor, is equivalent to 3.16 not 3. However, in Title 21 Subchapter 6, 5001 of California state law, a factor of 3 is used. Since the State of California is the primary user of the Lden metric, the computation of the metric is consistent with state law, rather than the traditional technical definition.
All photographs contained in this document are catalogued in a data base (each photo has a data base number)which can be found on the FHWA Highway Noise Barrier Design CD-ROM. The CD-ROM contains these and additional related photographs with detailed information including barrier material, location, special features, etc.
The reference microphone positions shown in Figures 256 and 257 are preferred. If the barrier is located more than 30 m from the source, and hence, probably closer to the receiver positions, locate the reference microphone at a distance of 15 m from the source and at a height such that a line from the near edge of the source to the microphone makes an angle greater than 10 degrees with the nominal ground plane.
Due to multiple reflections between source and barrier and/or edge diffraction at the top of a barrier, a 0.5 dB correction factor to reference microphone sound levels in the AFTER case may be applied. Good engineering judgment, based on repeatability through measurements, should be used to determine the magnitude and necessity of this correction. For example, if for several runs (i.e., greater than six), a consistent repeatable difference at the reference microphone position in the BEFORE and AFTER case occurs, and it can be proven that the traffic during both cases were equivalent, then the difference can be attributed to edge diffraction effects. The edge diffraction correction factor will be a negative value which is added directly to the sound level measured at the reference microphone in the AFTER case. Note: Larger corrections due to parallel barriers may be necessary.
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