This study investigates the real acoustic conditions of 26 primary school classrooms during lessons. There was a specific focus on how classroom activity type, student age, and room acoustics shape both speech levels and activity noise. Using Gaussian Mixture Models, the researchers separated teacher and student speech from non-speech activity noise across 93 homogeneous activity periods.
Activity Noise as a Dominant Sound Source
Across lessons, student activity noise averaged 50 dB(A), with considerable variation between classes linked to behaviour and acoustic conditions. This value sits close to thresholds known to negatively affect children’s academic performance, particularly in literacy and numeracy tasks. Importantly, activity noise increased classroom sound levels by more than 5 dB above baseline, after an initial 10 dB rise simply from occupation.
The above scatter plots illustrate the relationships between activity noise levels and (a) baseline noise, (b) speech clarity C50. Each point represents the mean value of the activity noise levels monitored in a class during different activities.
Speech Levels and Pedagogical Activity
Speech levels were strongly shaped by activity type: interactive tasks (teacher-led lessons, group work) produced the highest levels, while quiet work produced the lowest. Younger students required and produced higher speech levels, with a 5 dB decrease from Grade II to Grade V. These dynamics also influenced signal to noise ratios (SNRs), which often failed to reach recommended values, especially in younger grades.
The above scatter plot shows the relationship between activity noise and speech levels [dB(A)] measured in classrooms. Each point represents an activity for a specific class. The solid line indicates the fitted linear regression, and the shaded area denotes the 95 % confidence interval.
Understanding the complex mixture of signals and noise in group activities
There was no significant difference in overall sound levels between teacher‑led lessons and group work. This is likely because group work was structured in pairs or small groups under close supervision, helping keep noise controlled. However, the nature of sound sources differs greatly between these activities. In teacher‑led lessons, one speaker acts as the primary signal for the whole class. Whereas group work involves multiple simultaneous conversations, where a voice can be a signal for one group but unwanted noise for another. Even the teacher’s voice may serve as meaningful information for one group while becoming background noise for others. This overlapping of sound roles makes SNR (signal‑to‑noise ratio) difficult to interpret during group work. Although overall levels are similar, the complex mixture of signals and noise in group activities may still create disturbance for students trying to focus.
The Critical Role of Speech Clarity (C50)
A key contribution of the study is demonstrating that higher speech clarity (C50) is consistently associated with lower activity noise. C50 shows the balance of direct sound plus early reflections relative to late reflections and reverberation. The findings show that activity noise, generated by students in close proximity, is governed mainly by direct sound and early reflections, not by late reverberant energy. This explains why reverberation time (T30) showed no significant relationship with activity noise, whereas C50 did.
The above scatter plot shows the relationship between reverberation time T30 (s) and speech clarity C50 (dB). The data points indicate measured values in occupied conditions, while the line represents the theoretical relationship.
Why C50 Is a Crucial Parameter
C50 gives a fuller understanding of the acoustic experience in busy classrooms, improving C50 directly supports both speech intelligibility and noise control. Classrooms with better clarity were measurably quieter, suggesting that targeted acoustic treatments. This can support better groupwork speech communication, reduce teacher vocal strain, and support positive classroom management.
- C50 (Clarity index at 50 ms) measures the ratio of early sound energy (within the first 50 milliseconds) to late reverberant energy. It’s expressed in decibels (dB) and indicates how clear speech or music sounds in a space. A higher C50 means better clarity.
- D50 (Definition at 50%) is a related parameter representing the percentage of early arriving sound energy (within 50 ms) relative to the total energy (early + late). It’s expressed as a percentage (%), showing how much of the sound is perceived as direct or early reflections versus overall reverberation.
For more information about this study, the full paper is available here.
The authors are Chiara Visentin, a Fixed-Term Researcher (Type A, Italy) at the University of Ferrara, and Nicola Prodi Associate Professor at the University of Ferrara, Italy . Contact Chiara here. References/ acknowledgements and links
Previous article about this study can be read below:
