Page 8 - Special Issue
P. 8

Building a Sound Future for Students
 Figure 1. The spatial decay of the sound pressure level in the rever- berant field of a classroom. The sound pressure level at 3 feet (1 me- ter) from a talker is 65 dB(A). Blue area, region within +15 dB sig- nal-to-noise ratio (SNR); red area, levels lower than the background noise level (BNL). Where the blue and red areas intersect, listeners at that distance from the talker will experience lower than +15 dB SNR. Top: When the BNL is 45 dB(A), only students sitting within 9 feet of the teacher will hear the lesson with a sufficient SNR. Middle: A 40 dB(A) background noise level can meet the desired +15 dB SNR in classrooms where the largest dimension is less than 30 feet. Bot- tom: Designing classroom background noise levels to be at most 35 dB(A) ensures that students sitting anywhere within a classroom will experience a SNR of at least +15 dB, even with talkers that produce slightly lower voice levels.
teachers and their students, and doing so can be particularly important for children as they are still developing their language skills (Klatte et al., 2010).
Classroom speech levels are an important factor in determining the maximum recommended background noise levels. The American Speech-Language-Hearing Association (1995) recommends a signal-to-noise ratio of at least +15 dB to ensure high speech intelligibility for children with language and hearing impairments. Bradley and Sato (2008) found that an even higher signal-to-noise ratio of +20 dB was preferable for the youngest students in their study (grade 1) to attain near-ideal speech communication. A study by Pearsons et al. (1977) showed that the A-weighted sound level of teacher’s speech is typically 67 dB(A) at a distance of 1 meter in a quiet classroom. Because sound levels are expected to decrease approximately 3 dB per doubling of distance in a classroom, the levels of the talker could diminish to be as low as 55 dB(A) in the rear of a typical classroom in the United States (Figure 1). To conservatively ensure a minimum +15 dB signal-to-noise ratio everywhere in the classroom, the ANSI standard set the recommended background noise level to not exceed 35 dB(A). Meeting this recommended maximum noise level achieves a suitable signal-to-noise ratio for high speech intelligibility, thereby positively influencing student learning.
An underlying assumption has been that improving speech intelligibility results in improved student achievement. However, only a few studies before the introduction of ANSI’s standard in 2002 showed a direct link between noise levels and actual student learning outcomes (Bronzaft, 1981). Investigations completed after the introduction of the standard have provided more evidence that poor classroom acoustic conditions correlate to worse student performance.
For example, in situ studies focusing on aircraft noise in the classroom have shown that greater exposure to such noise is related to lower reading scores for elementary students (Stansfeld et al., 2005; Klatte et al., 2017).
Moreover, Shield and Dockrell (2008) surveyed classrooms with noise sources more commonly found at elementary schools (e.g., traffic, ventilation systems) in both occupied
   8 | Acoustics Today | Spring 2020, Special Issue
16 | Acoustics Today | Fall 2018 Reprinted from volume 14, issue 3

   6   7   8   9   10