Malibu Municipal Code § 6.1 Introduction

The State of California has mandated that each county and city prepare a noise element as part of its general plan. Section 65302(f) of the California Government Code requires that the element shall:

identify and appraise noise problems in the community. The noise element shall recognize the guidelines established by the Office of Noise Control in the State Department of Health Services and shall analyze and quantify, to the extent practicable, as determined by the legislative body, current and projected noise levels for all of the following sources:

The purpose of the Noise Element is to provide guidance for comprehensive local programs to control and abate excessive noise and to protect residents from adverse noise impacts. The element provides information on the existing and projected noise environment and includes, goals, objectives, policies and implementation programs to ensure an acceptable noise environment. The element also identifies criteria to be used by decision makers in evaluating the noise implications of proposed projects.

The typical community noise environment is comprised of a background noise level and higher noise levels, frequently transportation oriented. Because the background noise level is lower at night, the problems posed by higher noise levels are more pronounced at night.

The predominant noise source in Malibu is vehicular traffic from Pacific Coast Highway, the major canyon roads, and the local arterials. Stationary sources within the City include a wide range of recreational, commercial, and business activities.

Noise is most often defined as unwanted sound. Sound levels can easily be measured, but the variability in subjective and physical response to sound requires a more detailed analysis to determine the impact on people. People judge the relative magnitude of sound sensation in subjective terms such as "loudness" or "noisiness." Physically, sound pressure magnitude is measured and quantified on a scale which indicates the sound level in units of decibels (dB). All sound pressure levels discussed in this study are referenced to the standard reference pressure of 20 microPascals.

The human hearing system is not equally sensitive to sound at all frequencies. Because of this variability, a frequency-dependent adjustment called an A-weighting has been devised so that sound may be measured in a manner similar to the way the human hearing system responds. The use of the A-weighted sound level is often indicated by using the abbreviation "dBA" for expressing the sound level. An increase in the sound level by 10 dBA is judged by most people to be approximately twice as loud as the former, whereas most people are unable to detect a change in less than 3 dBA. To acquaint the reader with sound levels produced by common noise sources, measured sound levels for various sources are provided in Figure N-1.

It is desirable to describe a noise environment with a single number representing an hour or even a whole day so that easy reference and comparison can be made. Common methods which are widely used in the United States and abroad consider the average noise level and the maximum level recorded over a period of time, referred to as the Equivalent Level (Leq), the Minimum Level (Lmin), and Maximum Level (Lmax), respectively. Leq is a single-number noise descriptor that represents the average sound level in an environment where the actual noise level varies with time, Lmin and Lmax are the lowest and highest noise level that occurred during that time.

In a typical outdoor environment, the noise level varies during the day according to various activities in the community. Noise that may be acceptable during the day may be unwanted sound at night; e.g., automobile traffic on a nearby street, aircraft overflight, or dog barking. Thus, additional units of measurements have been developed to evaluate the longer term characteristics of sound. The Community Noise Equivalent Level (CNEL) and the day-night level (Ldn), along with Leq, are the measurements commonly used in California.

CNEL is a single-number noise descriptor. However, CNEL represents the average noise level over a 24-hour period and is appropriate in assessing long-term roadway noise impacts. CNEL is similar to a 24-hour Leq, but with a 10 dBA penalty added to nighttime (10 p.m. to 7 a.m.) sound levels and a 5 dBA penalty added to evening sound levels (7 p.m. to 10 a.m.) before the 24-hour average is computed. These penalties are applied to account for the increased annoyance that is generally felt by a person of normal sensitivity during the evening relaxation and nighttime sleep hours.

CNEL is the noise metric currently specified in the State Aeronautics Code for evaluation of the noise impact of airplanes. Additionally, CNEL is specified by the State Noise Insulation Standards for new multiple family dwellings. Local compliance with these standards requires that community noise be specified in terms of CNEL.

Intermittent or occasional noise, such as that associated with a stationary noise source sometimes is not loud enough to exceed the CNEL or Leq community noise standards. To account for such intermittent noise, acoustical engineers characterize noise in terms of percent noise level (L percent). The percent noise level is the level exceeded "x" percent of the time during the measurement period. For example, in an area where noise levels exceed 65 dB 90% of the time, L90 is 65 dB.