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Chapter 3. Fundamentals of Traffic Noise <br />The following is a brief discussion of fundamental traffic noise concepts. For a <br />detailed discussion, refer to the Caltrans Technical Noise Supplement (TeNS), a <br />technical supplement to the Protocol that is available on the Caltrans website <br />(Caltrans 2013). <br />3.1. Sound, Noise, and Acoustics <br />Sound can be described as the mechanical energy of a vibrating object transmitted by <br />pressure waves through a liquid or gaseous medium (e.g., air) to a hearing organ <br />(e.g., a human ear). Noise is defined as loud, unexpected, or annoying sound. <br />In the science of acoustics, the fundamental model consists of a sound (or noise) <br />source, a receptor, and the propagation path between the two. The loudness of the <br />noise source and obstructions or atmospheric factors affecting the propagation path to <br />the receptor determine the sound level and characteristics of the noise perceived by <br />the receptor. The field of acoustics deals primarily with the propagation and control <br />of sound. <br />3.2. Frequency and Hertz <br />Continuous sound can be described by frequency (pitch) and amplitude (loudness). <br />A low -frequency sound is perceived as low in pitch. Frequency is expressed in terms <br />of cycles per second, or Hertz (Hz) (e.g., a frequency of 250 cycles per second is <br />referred to as 250 Hz). High frequencies are sometimes more conveniently expressed <br />in kilohertz (kHz), or thousands of Hertz. The audible frequency range for humans is <br />generally between 20 Hz and 20,000 Hz. <br />3.3. Sound Pressure Levels and Decibels <br />The amplitude of pressure waves generated by a sound source determines the <br />loudness of that source. Sound pressure amplitude is measured in micropascals (µPa). <br />One µPa is approximately one hundred billionths (0.00000000001) of the normal <br />atmospheric pressure. Sound pressure amplitudes for different kinds of noise <br />environments can range from less than 100 to 100,000,000 µPa. Because of this huge <br />range of values, sound is rarely expressed in terms of µPa. Instead, a logarithmic <br />scale is used to describe sound pressure level (SPL) in terms of decibels (dB). <br />Fairview Street Improvements from 9th Street to 16th Street 11 <br />and Bridge Replacement Project Noise Study Report <br />