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Using an amplification system for teacher’s voices can dramatically improve the signal-to-noise ratio, leading to better speech intelligibility (Rosenberg et al., 1999). Although amplified solutions remain viable, most consultants in architectural acoustics will shy away from an amplified solution to mitigate inadequate signal-to- noise ratios in classrooms. Amplified systems require more substantial partition construction to ensure sufficient isolation between classrooms. Increasing the signal-to-noise ratio beyond those achievable with natural room acoustics can be counterproductive without considering isolation.
Other studies have focused on how the interaction between a classroom’s volume, geometry, and materials can result in overly reverberant conditions that negatively affect the speech intelligibility. Sounds produced in a more reverberant environment will linger longer than in less reverberant environments, increasing the overall average noise level within the space. Researchers have investigated the impact of reverberation time on speech intelligibility, often in combination with varying background noise levels (Bistafa and Bradley, 2000; Hodgson and Nosal, 2002; Wroblewski et al., 2012). For a constant signal-to-noise ratio, higher reverberation times do result in poorer speech intelligibility. Though limiting excessive reverberation is important for optimal speech intelligibility, Yang and Bradley (2009) caution that reverberation should not be eliminated because early arriving reflections from room boundaries are found to improve intelligibility by supporting the sound energy that arrives directly from the source to listeners.
Starting in the late 1990s, a growing number of measurement campaigns were undertaken to gauge the state of classroom acoustics. Acoustic conditions were documented in university lecture halls (Hodgson, 1999), preschools (Yang and Hodgson, 2005), elementary or primary schools (Picard and Bradley, 2001; Shield and Dockrell, 2004; Nelson et al., 2008), and secondary schools (Astolfi and Pellerey, 2008; Shield et al., 2015). Many of these investigations found that existing conditions did not exhibit appropriate noise levels and/or reverberation times for optimal speech intelligibility.
Recent computational developments in software and hardware have led to studies aimed at understanding classroom acoustic effects using auralization techniques (Yang and Hodgson, 2006; Hodgson et al., 2008; Neuman et al., 2010; Valente et al., 2012). Auralization refers to rendering the sound field of a built environment through modeling and simulation. For example, a recording of a teacher’s voice can be auralized in different classroom environments
so that listeners can understand how different rooms affect that listening experience. Auralization has been particularly useful for subjective testing because it allows researchers to test the effects of different acoustic environments in a more controlled laboratory environment.
Some of these laboratory studies on classroom acoustics have been shifting toward the measurement of speech comprehension that involves higher levels of cognition rather than simply recognizing words, phrases, or sentences (Klatte et al., 2010; Valente et al., 2012; Lewis et al., 2014). Results from those studies indicate that background noise and room reverberation have more detrimental effects on comprehension than on speech recognition. Work has also branched into characterizing how classroom acoustic conditions influence occupant listening effort or listening difficulty (Howard et al., 2010). Building on this concept, Prodi’s research group has focused further on quantifying “listening efficiency” as a measure of both the accuracy of speech intelligibility and listening effort (Prodi et al., 2010, 2013; Prodi and Visentin, 2015).
The American National Standards Institute/Acoustical Society of America S12.60 Classroom Acoustic Standard In 2002, the American National Standards Institute (ANSI) published the first classroom acoustics standard in the United States, ANSI S12.60 (ANSI, 2002): Acoustical Performance Criteria, Design Requirements, and Guidelines for Schools. The current standard recommends that the A-weighted sound level measured in an unoccupied classroom with ventilation (mechanical) systems on should not exceed 35 dB(A) (ANSI/Acoustical Society of America [ASA], 2010). This standard also states that the maximum reverberation time averaged over the 500, 1,000, and 2,000 Hz octave bands should be less than or equal to 0.6 second in classrooms with an enclosed volume less than 10,000 feet3 (283 meters3) or less than or equal to 0.7 second for classrooms larger than 10,000 feet3 but smaller than 20,000 feet3. The first edition of this standard provided the perceptual, educational, and developmental rationale for the recommended criteria as well as the empirical evidence from which the criteria were derived (ANSI, 2002). The rationale was that verbal communication is essential to learning, developing language proficiency, and developing cognitive skills. Verbal communication can only successfully occur when there is a high degree of speech intelligibility. Minimizing the background noise level and controlling room reverberation helps to create a clear communication channel between
Fall 2018 | Acoustics Today | 15 Spring 2020, Special Issue | Acoustics Today | 7
Reprinted from volume 14, issue 3