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Noise abatement, such as noise walls, are only considered when there are noise impacts.
There are two types of noise impacts as defined by federal regulations:
When noise levels approach (within 1 decibels) or exceed the Noise Abatement Criteria (NAC). So for Category B, residential, a receiver is considered “impacted” if future noise levels are 66 dB or more.
When future noise levels, usually 20 years in the future, are predicted to be 15 decibels or more above existing noise levels.
