Summary

At the nearest residence, the average nighttime noise level, excluding the peaks, was 46 dBA. The facility has been designed to limit operational noise impacts to 5 dBA above the current background nighttime noise level at the nearest residence.

While brief quiet periods occur during the late-night hours in between car, truck and train passes, the likelihood of disturbance from a continuous, low-level noise that is only 5 dBA louder than the measured lulls is considered small compared to the high levels of intermittent and irregular noise from traffic that occur throughout the night. The nighttime noise level at the nearest residence is similar to the noise level experienced in a private office building. Therefore, noise impacts are not considered significant.

Definition of Noise

Noise is defined as unwanted sound. A new project such as the MEC may increase noise levels above the existing levels. With regard to increases in A-weighted noise level, knowledge of the following relationships will be helpful in understanding potential noise impacts:

· Except in carefully controlled laboratory experiments, humans cannot perceive a change of 1 dB.

· Outside the laboratory, a 3-dB change is considered a barely perceivable difference.

· A change in level of at least 5 dB is required before any noticeable change in community response would be expected.

· A 10-dB change is subjectively heard as approximately a doubling in loudness and would almost certainly cause an adverse community response.

In determining the daily level of environmental noise, the difference in response of people to daytime and nighttime noises must be considered. During the nighttime, exterior background noises are generally lower than the daytime levels. However, most household noise levels also decrease at night and exterior noise becomes more noticeable. Further, most people sleep at night and are sensitive to noise intrusion. This is why the plant has been designed not to exceed nighttime background levels by more than 5 dBA.

Table 1 shows the relative A-weighted noise levels of common sounds measured in the environment and in industry for various sound levels (Beranek, 1988).

Ambient Noise Levels

Existing daytime and nighttime noise levels within the study area were measured at locations representing nearby residences. These noise levels were used as a baseline for comparison with future predicted energy center noise levels. The major noise sources in the area were auto and truck traffic along Monterey Road and intermittent passenger and freight train traffic. A noise level measurement statistic called the L90 was used to measure ambient background noise levels. The L90 noise level is determined by a statistical methodology which filters out short-term periods of high noise levels (i.e., a passing train or truck) to give a better measure of the overall background noise level at a particular location.

Average nighttime L90s were 37 dBA at Encinial School (about 7,500 feet away); 43 dBA at the closest resident on the east side of Monterey Road (about 2,050 feet away); and 46 dBA at the closest residence to the plant (approximately 1,150 feet away). While the plant has been designed to exceed the values by only 5 dBA, there will be times during the normal operation of the plant that these levels are briefly exceeded. When feasible, exceedences will be scheduled during the daytime hours.

Regulatory Requirements

Generally, the controlling criterion in the design of the noise control features of the project is the minimum, or most stringent, noise level required by any of the applicable regulations of the local entities. However, since the site is being annexed into the City of San Jose and the surrounding land is already within the City, the County requirements were not considered. In this case, the controlling criterion became the most stringent criteria of the remaining entities-the California Energy Commission's significant increase criteria of 5 dBA. In addition, the City of San Jose has established a long-term outdoor noise goal of 55 DNL (average day/night noise level in decibels) and the North Coyote Valley Master Development Plan requires that at all property lines, noise produced by onsite activities shall not exceed 55 Leq (8-hour peak).