CIR: Design improvement to the building envelope to reduce exterior sound transmission. Tarkington Elementary School is a new three-story school with a total building area of 134,165 square feet. The school is composed of (6) 1200 sf classrooms, (27) 900 sf classrooms, and (4) 600 sf classrooms. Special amenities include art, music, and science classrooms, full serving kitchen and dining facilities, and a multipurpose room. The total capacity of the school will be approximately 1000 students. The building site is 9.82 acres and is situated on the north side of 71st Street within Marquette Park, west of Kedzie Avenue in Chicago, Illinois. This site is located within 8 miles from Chicago Midway Airport. Acoustical Study An exterior noise impact study was conducted by an acoustic engineering firm to study the level of noise due to aircraft-flyovers to the nearby Midway airport and vehicular traffic from the street intersections. Sound level meters and microphones positioned on the site observed a maximum sound level of 74 dBA on two different days (April 28 and May 13, 2003). Recommended Design Criteria Normal "Industry Standard" criteria for classrooms, as published by ASHRAE, is a background A-weighted decibel sound level of about 40 dBA, equivalent to an NC-35. There are no specific criteria set for the level of exterior noise intrusion in the standard but a commonly used acoustical rule of thumb is that exterior noise level should not exceed the interior background sound level by greater than 5 to 8 dBA so as not to become obtrusive and distracting. Thus, the maximum exterior noise intrusion level goal was established as 45 to 48 dBA. Design Improvement to the building envelope to reduce exterior sound transmission Exterior windows Windows being the weak point in sound transmission, special window assembly (1" assembly thickness: laminated glass-airspace-plate glass) will be used to improve the Sound Transmission Coefficient (STC) from 35 of a standard window assembly (1" thickness: plate glass-airspace-plate glass) to 40. Standard 1" window assembly with an STC-35 would result in maximum aircraft and traffic noise levels in the 49 to 52 dBA range, which is higher than ideal for good speech intelligibility between students and teacher. Special window assembly with and STCof 40 will result in maximum exterior noise intrusion level in 43-47dBA range, which is within the design goal for this project. Exterior Walls Assembly Face brick - 11/2" airspace - 1 +" insulation - 10" CMU will achieve an STC of 53 and will transmit exterior noise within acceptable levels as established in the design criteria. Roof Assembly
The proposed target of 45-48 dBA for the classrooms is not sufficient for an innovation point. Standard voice level (approximately 45-50dB) would be competing with ambient noise. According to the Acoustical Society of America, "Studies have shown that, in classrooms having a signal-to-noise ratio of less than +10 dB, speech intelligibility is significantly degraded for children with average hearing. Children with some hearing impairment need at least a +15 dB S/N ratio." In order to earn an acoustical performance innovation point for a school, documentation should demonstrate compliance with ANSI S12.60-2002, "Acoustical Performance Criteria, Design Requirements, and Guidelines for Schools." Although not applicable to this project, note that relocatable classrooms ("portables") would not be held to this requirement. For relocatable classrooms, unducted HVAC units would not be permissible, and substantial performance would need to be achieved. Applicable Internationally.