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  Figure 2. Samples of the data measured from 220 K-12 classrooms in five school districts. Top: One-minute unoccupied A-weighted equivalent levels (LAeq) were recorded to characterize the BNL in the classrooms due to ventilation noise. Ninety-one percent of classrooms do not meet the American National Standards Institute (ANSI) rec- ommended maximum of 35 dB(A). Bottom: Reverberation time was measured using a swept-sine method. Only 15% of classrooms do not meet the ANSI recommended maximum of 0.6 second.
and unoccupied conditions. They found that lower English test scores were more strongly related to higher occupied background noise levels than to unoccupied background noise levels. It is therefore necessary to diminish the background noise level in actively occupied classrooms to mitigate this effect. One other study has found that background noise levels in unoccupied classrooms correlated to student achievement scores in reading and language subject areas (Ronsse and Wang, 2013). That study did not measure levels in the occupied active classrooms though.
Limitations of American National Standards Institute/Acoustical Society of America S12.60
Many investigations have shown that although the reverberation time recommendations given in ANSI/ASA S12.60 are attainable, there are few classrooms that meet the unoccupied background noise level requirements (Knecht et al., 2002; Nelson et al., 2008; Sato and Bradley, 2008). Observations of the 220 classrooms in our study are consistent with these findings (Figure 2).
Most of the K-12 classrooms that we visited have acoustical ceiling tile covering the entire ceiling surface, with a ceiling height at or less than 11 feet (3.3 meters). These room characteristics lead to acceptable reverberation times. Figure 3 shows examples of typical classroom conditions observed in our study. If a classroom is excessively reverberant, adding absorptive materials in the space is a relatively easy task postdesign. Major renovations, however, are typically required to decrease background noise levels significantly, particularly those produced by building mechanical systems for heating, ventilation, and air conditioning (HVAC).
The best way to ensure quiet background noise levels from building mechanical systems is to design for them and not rely on postdesign solutions because the cost of replacing noisy mechanical systems is far greater than designing quiet systems in the first place. Designing a loud mechanical system is easier and less expensive than designing a quiet system, but the financial cost should not be the only cost considered when it comes to our educational facilities. There is a cost associated with designing and installing quiet mechanical systems, but the benefits experienced by the occupants of the classroom far outweigh these increased costs.
Routine approaches to mechanical systems frequently utilized in classrooms are often inconsistent with the best acoustical practices. It can be challenging to disregard routine practices in favor of less utilized, more creative design solutions. Coupling this with the fact that there are not easily enforceable acoustics requirements means that appropriate classroom sound levels are not always prioritized in the building design industry.
In the United States, the ANSI classroom acoustics standard provides a guideline for background noise levels but does not prescribe enforceable requirements. The United States Green Building Council (USGBC) introduced the Leadership in Energy and Environmental Design (LEED) green building certification system in 2000. LEED certifies
 Spring 2020, Special Issue | Acoustics Today | 9
Fall 2018 | Acoustics Today | 17 Reprinted from volume 14, issue 3

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