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4.6 – Noise Draft Environmental Impact Report February 2021 14800 W. Schulte Road Logistics Center 4.6-1 4.6 Noise This section describes the existing noise conditions of the 14800 W. Schulte Road Logistics Center (Project) site and vicinity, identifies associated regulatory requirements, evaluates potential impacts, and identifies mitigation measures related to implementation of the Project. In addition to the documents incorporated by reference (see Section 2.7 , Documents Incorporated by Reference, of Chapter 2, Introduction, of this Environmental Impact Report), the following analysis is based, in part, on the noise calculations and specifications prepared by Dudek in January 2021 (Appendix E). 4.6.1 Existing Conditions 4.6.1.1 Noise and Vibration Fundamentals and Terminology Noise and Sound Noise is commonly defined as unwanted sound. Vibrations, traveling as waves through air from a source, exert a force perceived by the human ear as sound. Sound pressure level (referred to as sound level) is measured on a logarithmic scale in decibels (dB) that represent the fluctuation of air pressure above and below atmospheric pressure. Frequency, or pitch, is a physical characteristic of sound and is expressed in units of cy cles per second or hertz. The normal frequency range of hearing for most people extends from approximately 20 to 20,000 hertz. The human ear is more sensitive to middle and high frequencies, especially when the noise levels are quieter. As noise levels get louder, the human ear starts to hear the frequency spectrum more evenly. To accommodate for this phenomenon, a weighting system to evaluate how loud a noise level is to a human was developed. The frequency weighting, called “A” weighting, is typically used for quieter noise levels, which de-emphasizes the low- frequency components of the sound in a manner similar to the response of a human ear. This A -weighted sound level is called the “noise level” and is referenced in units of dBA. Because sound is measured on a logarithmic scale, a doubling of sound energy results in a 3 dB increase in the noise level. Changes in a community noise level of less than 3 dB are not typically noticed by the human ear (Caltrans 2013). Changes from 3 to 5 dB may be noticed by some individuals who are extremely sensitive to changes in noise. A 5 dB increase is readily noticeable. The human ear perceives a 10 dB increase in sound level as a doubling of the sound level (i.e., 65 dBA sounds twice as loud as 55 dBA to a human ear). An individual’s noise exposure occurs over a period of time; however, noise level is a measure of noise at a given instant in time. The equivalent continuous sound level (Leq), also referred to as the average sound level, is a single number representing the fluctuating sound level in A-weighted decibels (dBA) over a specified period of time. It is a sound-energy average of the fluctuating level and is equal to a constant unchanging sound of that dB level. Community noise sources vary continuously, being the product of many noise sources at various distances, all of which constitute a relatively stable background or ambient noise environment. Noise levels are generally higher during the daytime and early evening when traffic (including airplanes) and commercial and industrial activities are the greatest. However, noise sources experienced during nighttime hours, when background levels are generally lower, can be potentially more conspicuous and irritating to the receiver. To evaluate noise in a way that considers periodic fluctuations experienced throughout the day and night, a concept termed “community noise equivalent level” (CNEL) was developed, The CNEL scale represents a time-weighted 24-