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Administrative Draft Environmental Impact Report Gill Medical Center Project Emergency Sirens Residential receptors in the immediate vicinity of the Project would experience periodic exposure to siren noise.The potential adverse effects of noise associated with the use of emergency vehicle sirens on the quality of life of nearby residents is often a concern in development of new hospitals and emergency facilities. Federal regulation limits emergency siren noise at 123 dBA at 10 feet. Factoring an attenuation rate of approximately 6 dBA per doubling of distance from the source equates to a noise level of approximately 103.5 dBA at 100 feet. Since emergency vehicle response is by nature rapid, the duration of exposure to this peak noise level is estimated to last for a maximum of 10 to 20 seconds as emergency vehicles enter and exit the Project site. Thus, receptors would be exposed to very short-duration high noise levels for approximately 10 to 20 seconds for each emergency response event. Further, it is typical practice for ambulances to use sirens to break traffic at intersections or warn drivers of the emergency vehicle approach when traffic is congested. It is not unlikely in minor emergency scenarios that a siren is not used. Responses to nighttime emergency calls,when nuisance noise is most noticeable, routinely occur without the use of sirens. It is also noted that the manner in which older homes in California were constructed generally provides a reduction of exterior-to-interior noise levels of about 20 to 25 dBA with closed windows (Caltrans 2002). The exterior-to-interior reduction of newer residential units is generally 30 dBA or more (HMMH 2006). A key focus of analysis with regard to noise is the potential for long-term exposure to higher noise levels (i.e., continuous, involuntary exposure for many hours per day over a long period of time)that may adversely affect human health.As a result of this emphasis, noise standards focus on increases in long- term exposure to ongoing average noise levels rather than infrequent short-duration peak effects. Siren noise from intermittent emergency vehicle trips sourced from the Project site would not substantially change the Ldn or CNEL for the Project vicinity as the intermittent siren use would not constitute a significant change in the existing noise environment.Additionally, per Section 9-1025.9, Noise, of the County's Municipal Code any mechanical device, apparatus or equipment used, related to, or connected with, emergency activities or emergency work shall be exempt from noise standards. Helicopter Noise The most prominent noise source on the Project site would be that of helicopter activities. Most of the noise from a helicopter is generated by the main rotor located on the roof of the helicopter.The main rotor is comprised of rotary wings (rotor blades) and a control system that generates the aerodynamic lift force that supports the weight of the helicopter, and the thrust that counteracts aerodynamic drag in forward flight.There is also a significant amount of noise that is generated from the tail rotor located on the tail of the helicopter. The tail rotor provides a counteracting force to the helicopter's main rotor and allows the pilot to steer the helicopter around its vertical axis by adjusting the pitch of the rotor blades. According to the Helicopter Association International (1983), smaller helicopters are generally quieter than larger ones and sound levels tend to increase approximately three decibels per doubling of helicopter weight. Per information from the International Civil Aviation Organization (2015), the approach case (landing) is normally the loudest flight condition for a helicopter due to the sound produced by the Noise 4.15-29 October 2021