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4 Environmental Analysis 4.13 Noise Term Definition Propagation and Attenuation Propagation is the decrease in amplitude of an acoustic signal due to geometric spreading losses with increased distance from the source.Additional sound attenuation factors include air absorption,terrain effects,sound interaction with the ground,diffraction of sound around objects and topographical features, foliage,and meteorological conditions,including wind velocity,temperature,humidity,and atmospheric conditions. 4.13.1.2 Vibration Metrics and Terminology Vibration is an oscillatory motion that is described in terms of displacement, velocity, or acceleration. Velocity is the most common descriptor used when evaluating human perception or structural damage. Velocity represents the instantaneous speed of movement and more accurately describes the response of humans, buildings, and equipment to vibrations. Peak particle velocity (PPV) and root mean square velocity are typical metrics used to describe vibration levels in units of inches per second in the United States. However, to evaluate annoyance to humans, the vibration dB (VdB) notation is commonly used. The decibel notation acts to compress the range of numbers required to describe vibration. In the United States, the accepted velocity reference for converting to dB is 1x10-6 inches per second. The abbreviation "VdB" is used for vibration dB to reduce the potential for confusion with sound decibels. In contrast to airborne noise, ground-borne vibration is not an everyday occurrence for humans. The background vibration velocity levels within residential areas are usually 50 VdB or lower, which are well below the human perception threshold of approximately 65 VdB. However, human response to vibration is not usually significant unless the vibration exceeds 70 VdB. For a significant impact to occur, vibration levels must exceed 72 VdB during frequent events, 75 VdB for occasional events, and 80 VdB during infrequent events (FTA 2018). Outdoor sources that generate perceptible ground-borne vibrations are typically construction equipment, steel-wheeled trains, and traffic on rough roadways. Table 4.13-3 provides typical vibration sources as well as the human and structural response to ground-borne vibrations. Table 4.13-3. Typical Levels of Ground-borne Vibration ResponsePPV Velocity Level Typical Sources i feet from source) Minor Cosmetic Damage,Fragile 0.4 100 Blasting from Construction Projects Buildings 0.17-0.2 92-94 Heavy Tracked Construction Equipment Difficulty with Tasks,Such as Reading a 0.125 90 Computer Screen 0.074 85 Commuter Rail,Upper Range Residential Annoyance,Infrequent 0.04 80 Rapid Transit,Upper Range Events 0.013 75 Commuter Rail,Typical 0.023 72 Bus or Truck Bump Over Residential Annoyance,Frequent 0.013 70 Rapid Transit,Typical Events Approximate Threshold of Human 0.007 65 Perception 0.005 62 Bus or Truck,Typical 0.0013 50 Typical Background Vibration Levels Griffith Energy Storage Project 4.13-3 Tetra Tech/2022120675 Draft Environmental Impact Report August 2023