Lessons Learnt and Future Perspectives on Classroom Acoustics

Abstract: Classrooms’ design is aimed at addressing various premises that impact both teaching and learning activities. Beyond ergonomic and functional criteria, classrooms should support effective teacher-to-student communication, adapting to the changes in teaching methods. The acoustic design of classrooms then comes to play a key role as it has a significant impact on both teachers and students. Poor acoustics, i.e., excessive noise and reverberation, can strain teachers' voices. For students, especially the younger ones, poor acoustics can lead to learning delays, reduced attention, and lower academic performance, with secondary effects that affect social, relational, and psychological well-being. Thus, investing efforts in shaping a good architectural, system and acoustic design of classrooms ensures a healthy occupational environment for teachers and optimizes students' cognitive and learning abilities. This summary reflects the key findings of recent classroom acoustics studies, detailing how acoustically well- or bad-designed spaces can either support or hinder the teacher-student communication process, eventually influencing the learning outcomes.

Keywords: classroom acoustics, learning, voice production, reverberation, noise, fan noise.

Introduction

The architectural design of classrooms should consider a variety of factors that influence the activities within them. For instance, the ergonomics of desks and chairs should be appropriate for the student's age and adapt as they progress through different school levels. Additionally, shading systems should be integrated with windows to prevent brightness from affecting visual tasks. Beyond these elements, classrooms should also be acoustically designed to support the teacher-to-student communication process, eventually adapting to the teaching methodologies that continuously evolve. But what does this entail?

The last decades have fixed significant milestones around the understanding of the extent to which classroom acoustics has effects on occupants. On the one side, teachers are affected in terms of vocal effort required and recovery time when physical diseases occur, which may be from light (e.g., sore throat) to severe (e.g., polyps or nodules at the vocal cords) [1]. On the other side, students are influenced in a significant and permanent way [2, 3], as their cognitive resources may be stressed under noise and reverberation. This causes primary effects like learning delays, annoyance, reduced attention and learning achievements, being more detrimental for younger pupils at earlier stages of education. Further effects can then involve among others the social, relational and psychological spheres.

Classrooms are thus an interesting study environment to understand the effectiveness of a good building design (i.e., sound insulation), a proper acoustic treatment (i.e., sound absorption), and a proper heating, ventilation and air conditioning (HVAC) system design on the interaction with occupants, from which derives the occupational health of teachers and optimal scenarios to support students’ learning.

This contribution is intended as a summary of the main outcomes on classroom acoustics studies carried out in the last decades and particularly in the very recent years. First, an exploration of how “good” and “bad” classroom acoustics can be defined is briefly given. Then, the subsequent paragraph is presented as a focus on the most relevant aspects involved in the design of classrooms from an acoustic point of view, highlighting the positive and negative effects on the teacher-to-student communication process.

How can “good” and “bad” classrooms acoustics be defined?

The student’s perspective

There are milestone studies on classroom acoustics that agree on specific thresholds of some acoustic parameters to either support or not learning. Bradley et al. [4] highlights that room acoustics parameters have less impact on speech intelligibility than the signal-to-noise ratio (S/N), suggesting that reducing ambient noise to achieve an S/N ratio above 15 dB is more critical than perfecting RT. Yang and Bradley [5] emphasized the relationship between noise and reverberation in a typical classroom setting where noise is generated by nearby talking and moving children. Existing a positive correlation between these two parameters, reverberation time should range between 0.3 s and 0.9 s, with an optimal value around 0.7 s, to allow speech levels to increase as reverberation time increases while noise levels remain constant. This outcome corroborates a study by Hodgson and Nosal [6], who showed that non-zero reverberation times are optimal when noise sources like children speaking are considered. They suggest to first minimize noise and then adjust reverberation time to enhance speech levels. Furthermore, they recommend a range of acceptable reverberation time values rather than a maximum limit, as insufficient reflected sound can be costly and lead to vocal strain. Bradley et al. [7] and Picard and Bradley [8] suggest that a reverberation time between 0.4 s and 0.5 s is preferable, as it allows for more background noise tolerance.

A thorough literature review by Astolfi et al. [9] and by Minelli et al. [10] identified key factors affecting students' performance, which were used to evaluate the acoustical quality of primary school classrooms through in-field measurements. In Astolfi et al. [9], noise, room acoustics, and intelligibility indices were measured in 29 first-grade classrooms of 13 Italian typical schools. Cluster analysis divided the classrooms into Bad Acoustics (BA) or Good Acoustics (GA). Findings confirmed that classrooms with reverberation times of around 0.8 s (not longer), speech clarity (C50) values above 3 dB at mid-frequencies and useful-to-detrimental ratio (U50) values of 1.0 dB at 1 kHz provide excellent intelligibility and belong to the GA cluster.

The teacher’s perspective

From the teacher’s point of view, classroom acoustics is also a crucial issue. As highlighted by Astolfi et al. [1], teachers are among the professionals most affected by voice disorders due to their continuous vocal demands, proving the need for preventive measures to protect their vocal health. Indeed, on the one side, teachers are subject to vocal effort and load; on the other side, they can exhibit vocal fatigue. To better understand these concepts, it is important to define them. Vocal effort is a physiological measure of the adjustments in voice production in response to external factors (e.g., distance from the listener, background noise, physical environment), as defined in the ISO 9921 [11]. Vocal effort is strictly correlated with vocal load, which can be defined as the amount of voicing over time [12]. Vocal fatigue, instead, is considered as the vocal adaptation that occurs because of prolonged voice use, typically having detrimental effects on the talker’s vocal health and happening especially in critical conditions [13].

Based on these definitions, research has focused on validating voice monitoring techniques in-field using devices that detect vocal fold activity unobtrusively and can be worn for extended periods. Indeed, in agreement with Puglisi et al. [14], teachers’ vocal behaviour should be regularly monitored during working hours in realistic environments to detect any changes that could worsen vocal health over time. Furthermore, vocal use during non-working periods is also necessary to assess potential differences in voice usage during teaching. Gaskill et al. [15] monitored primary school teachers over two five-day workweeks and found that vocal dosimetry helped reduce vocal load, though no statistically significant changes in vocal behaviour were observed. Hunter and Titze [16] tracked the vocal activity of teachers for two weeks in both occupational and non-occupational settings. They found that the average sound pressure level during work was 2.5 dB higher than during non-work periods, and that the fundamental frequency during work was 10 Hz higher than non-work settings. Two comprehensive monitoring campaigns conducted by Puglisi et al. [14] and by Calosso et al. [17] involving primary and secondary school teachers during plenary lessons in Italian classrooms, respectively, revealed an average speech level of 71 dB at 1 m from the mouth, corresponding to a vocal effort between “Raised” and “Loud” based on the vocal effort ranges provided in the standard ANSI S3.5 [18]. Additionally, a notable increase in speech level by up to 5 dB was observed in the afternoon compared to the morning. Secondary school teachers working in poor classroom acoustics experienced a 2 dB increase in vocal effort by the end of the school year compared to its start.

To conclude this overview on the effects of classroom acoustics on teachers’ voice production, it is important to recall the findings related to the so-called Lombard effect. It is the involuntary tendency to raise voice level as background noise increases, which primarily happens to enhance speech intelligibility [19]. In studies by Bottalico and Astolfi [20], Puglisi et al. [14] and Calosso et al. [17], during plenary lessons in primary and secondary schools the Lombard effect was observed with slopes ranging from 0.4 to 0.7 dB per dB of noise. In the study by Calosso et al. [17] such effect diminished by the end of the school year, proving a greater vocal fatigue as the voice levels of teachers increased under similar noise conditions.

Key architectural and environmental aspects that affect the teaching and learning process in classrooms

Following the overview related to the effects of classroom acoustics in general on teachers and on students, this section focuses on the key acoustical and environmental aspects that have an influence on the teaching-learning process. This brief introduction to specific measurable parameters, can help in the designing process.

Reverberation time

Research studies have widely investigated on the effects of reverberation time in classrooms on the teaching-learning process, both focusing on the teacher’s and on the students’ premises. Some remarkable outcomes to understand what this means can be found in Puglisi et al. [14] and Calosso et al. [17]. These two studies highlighted the relationship between vocal effort (measured as teacher’s mean sound pressure level at 1 m from the mouth SPLmean,1m in dB) and the classroom’s reverberation time at mid frequencies in occupied conditions (T300.25-2kHz,occ in s). This mutual relationship is shown in Figure 1 and constitutes an important step forward in the available knowledge that drives classrooms’ design, as both studies remark an optimal reverberation time around 0.7 s to allow for the reduction of vocal effort. Indeed, both lower and higher reverberation times can bring teachers to raise their voice, leading to vocal fatigue. Furthermore, as previously mentioned [9], this optimal central value of reverberation time is advisable to guarantee optimal speech intelligibility towards listeners too.

Figure 1. Best-fit quadratic regression between the teachers’ mean sound pressure levels at 1 m from the mouth (SPLmean,1m) and the reverberation times at mid-frequencies in occupied classrooms (T300.250-2kHz,occ). Left figure is from Puglisi et al. (2017) and refers to primary school teachers. Right figure refers to secondary school teachers at the beginning (Stage 1) and at the end (Stage 2) of a school year [17].

Noise from the inside: anthropic sources

An interesting reference for this key aspect can be recognized in the work by Astolfi and Pellerey [2]. They administered questionnaires to secondary school students, asking for their preferences on several aspects related to the overall environmental quality of classrooms. As far as the perception of noise from indoor sound sources is concerned, they reported the highest (i.e., worst) scores in relation to the perceived disturbance when students talk in the classroom, and also when they both talk and move.

In a subsequent study by Astolfi et al. [21], noise inside the classroom caused by students talking was proved to be a complex interferer because its effects on intelligibility scores were not affected by a reduction in reverberation time. Therefore, an acoustical treatment exclusively based on sound absorption might not be as effective in the reduction of chatting noise as expected, mainly because such noise is generated by sources (students talking) that are close to the listeners and characterized by a direct component rather than a reverberated one. A possible strategy to reduce noise from the inside due to anthropic sources might be the use of monitoring systems such as the one proposed by Di Blasio et al. [22], which is based on lighting feedback (i.e., “green”, “orange”, “red”, for low, increased but not hampering and excessive chatting noise levels, respectively) that engages and motivates students to significantly lower noise levels in the room.

Noise from the inside: ventilation systems

An adequate degree of indoor air quality (IAQ) is crucial in educational settings, as many studies proved that it affects students' cognitive abilities, cognitive functions, and overall health [23-26]. Furthermore, providing proper ventilation in educational environments has become even more important since the Covid-19 pandemic, as enhancing ventilation was critical to reduce viral transmission. A proper ventilation rate to reduce CO₂ concentration in classrooms typically allows for good IAQ to be reached, and it can be fostered either by opening windows or using mechanical systems. In fact, CO₂ can be assumed as an effective proxy for other contaminants associated with occupants, and it is generally easier to be measured compared to other indoor pollutants.

Although they might bring to similar positive effects on IAQ, the use of either natural ventilation systems (i.e., opening windows) or mechanical ones (i.e., setting systems in the classrooms) has different consequences on the acoustic environment. Indeed, though in different ways, they increase students' exposure to louder and more frequent task-irrelevant sounds, potentially impacting both learning and well-being. Mechanical systems, in particular, may bring to detrimental consequences on learning under different points of view, as highlighted in a recent systematic review on the effects of ventilation-related sounds on students by Pellegatti et al. [27]:

·         Effects on speech perception. The small number of available studies that account for the effect of mechanical systems (i.e., fan noise) on speech perception (i.e., quantified in terms of students’ performance) consistently reveal a decline in the achievements as signal-to-noise ratios and speech transmission index values decreased. Furthermore, Valente et al. [28] and Peng et al. [29] also examined the combined effect of fan noise and classroom reverberation time, proving that the influence of noise is higher in worst (i.e., longer) reverberation time conditions.

·         Effects on language skills. Negative effects have been reported on students' performance. Valente et al. [28] involved participants in a classroom-learning task where they listened to a teacher and students reading a story, then answered questions. The experiment was conducted under two positive signal-to-noise ratios, and the results showed that task accuracy decreased significantly as noise levels increased, with fewer correct answers at the lower SNR. Ronsse and Wang [30] focused on chronic exposure to fan noise and its effect on reading comprehension, finding a negative correlation between noise levels and students' scores on a standardized test.

·         Effects on attention. In the review by Pellegatti et al. [27], only one study was reported to have a focus on the effects of fan noise on students’ attention [31]. A significant correlation between the increase in noise level and the reduction of attention was proved, particularly in terms of increase in reaction times and decrease of accuracy.

·         Effects on memory. With respect to this issue, research has mainly focused on episodic and semantic memory [32] and on working memory [31]. Findings highlight that when fan noise is present in classrooms, the accuracy of the episodic memory tasks are significantly impaired. As far as working memory is concerned, the study combined fan noise at different levels and different environmental temperature conditions. Findings reveal that an increase in noise levels is associated to the decrease in task accuracy at all indoor temperatures.

Noise from the outside

In school buildings, noise from the outside can be associated to a variety of sound sources that may imply different consequences on the teaching-learning process. Astolfi and Pellerey [2] found that students suffer from disturbance from traffic noise and from other noises coming from outside the building, especially when classrooms’ windows are opened. This highlights the complexity of the designing process, as either choosing to support mechanical or natural ventilation in classrooms can lead to reduced acoustic comfort and increase disturbance, especially towards students. In practice, noise from the outside can actually be reduced through a proper acoustical treatment or early-stage design based on the use of absorbent surfaces, as this strategy ensures the reverberation time decrease. Indeed, Astolfi et al. [21] proved the negative effects of exposure to traffic noise on intelligibility scores, and found that they diminish when reverberation time decreases from 1.6 s to 0.4 s. Particularly, they found that such soundproofing determines a 10% constant increase in intelligibility scores in a signal-to-noise ratio range of -15 dB to +6 dB.

Conclusions and hints on an effective classroom acoustics design

To conclude this general discussion on the lessons learnt around classroom acoustics, it is worth trying to summarize some advisable future perspectives for the design of educational environments accounting for the presented acoustical premises.

First, a key aspect is to consider the educational level: kindergarten and primary school classrooms rely on direct teacher-student interaction, requiring the environment to passively support speech. In contrast, university lecture halls, designed to accommodate hundreds of students, often require active speech support via amplification systems. Second, classroom acoustics should enhance speech intelligibility. Therefore, design strategies must focus on reducing but not eliminating reverberation time (i.e., minimizing sound reflections) and lowering noise levels, as these factors have been shown to strain teachers' vocal effort and diminish students' ability to understand speech. The recently published Italian Standard UNI 11532-2 [33] is the first attempt to list and standardize these requirements for optimal acoustics in teaching-learning environments. It provides thresholds or range values to be fostered at the design stage but also when considering a case of refurbishment. Most of all, UNI 11532-2 introduces a range of “categories” (from A1 to A6) that cluster school environments based on the main task that takes place there and on the listeners’ typologies, providing such specific optimal thresholds or range values.

From a practical point of view, acoustic treatments for the reduction of reverberation time in classrooms are often applied after a school building has been completed, yet it is crucial to integrate them from the outset of the project. Regardless of when they are implemented, anyway, these treatments typically involve using sound-absorbing materials on walls or ceilings. However, recent studies and updated standards suggest that both absorbent and diffusive surfaces should be used to support both the teaching and the learning tasks. This is because overly reducing sound reflections (i.e., having a very short reverberation time due to extensive absorption) can hinder speech and increase the vocal strain on teachers. Therefore, achieving optimal classroom acoustics requires a balance between absorption and diffusion, accomplished by strategically placing different surfaces throughout the room. Diffusive surfaces, in fact, help redirecting sound energy in a non-specular way, preventing acoustic defects like echoes while preserving early sound reflections, which are useful for the enhancement of speech intelligibility even in the back of the room. So far, only a few studies have examined and implemented this approach in real classrooms, with most focusing on simulated environments. Therefore, more research needs to be carried out on the topic to obtain significant evidence and to bring together up-to-date design premises.

As far as the possible HVAC strategies to guarantee adequate indoor air quality in classrooms are concerned, the use of mechanical systems implies fan noise that has an overall negative effect on the learning process, as it has been proved to affect students' speech perception, cognition, and comfort. Noise coming from outside the school building due to natural ventilation, instead, has been seen to affect students mainly in terms of perceived disturbance, but the learning tasks were less influenced. To help practitioners in such complex designing tasks, the mentioned Italian standard UNI 11532-2 introduces limits in terms of noise levels for systems installed both inside and outside the classroom (Lic,int, Lpu,c), but still within the school building. It also considers the overall noise level (Lamb), which includes noise coming from outside. A mixed and integrated approach to HVAC and natural ventilation design in classrooms should then be fostered, first of all considering the exposure of the building, its surroundings and its capability to implement passive strategies.

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[33]   UNI 11532-2, Caratteristiche acustiche interne di ambienti confinati - Metodi di progettazione e tecniche di valutazione - Parte 2: Settore scolastico (Acoustic characteristics of indoor environments – Design methods and evaluation techniques – Part 2: school sector) (Ente Italiano di Normazione, 2020).