Schools are usually placed in very noisy environments, where traffic noise is an important issue. This study has investigated the influence of noise coming from outdoors on indoor noise level and speech intelligibility based on a sample of façade sound insulation measurements, representative of the Italian façade building stock of schools.
The Italian government carried out a census of all Italian schools of different levels in order to determine whether or not they needed to be improved. Globally, more than 42,000 buildings, distributed across all Italian Regions, 55% of which built before 1976, were examined.
The results of this extensive study are referred to general aspects of the school buildings, while acoustic requirements were investigated only in a qualitative way. Figure 1 shows details of the analysis of the “qualitative” acoustic requirements with reference to three Italian Regions. Less than 10% of schools have some kind of acoustic protection against outdoor noise (such as improved acoustic insulation windows, noise barriers or other).
Figure 1: Typology of measures against noise in the schools of three Italian Regions (Credit: Simone Secchi)
The study on the effect of outdoor noise on indoor sound level of schools is based on many in-field measurements carried out on 103 façades, belonging to 64 schools of any level and built in different time periods and with different building techniques; in 45 of these cases the façade sound insulation was measured both before and after the treatment to improve acoustic insulation.
Figure 2 shows some detailed information about window frame material and type of glass before the façade acoustic improvement. In almost all the cases, the acoustic improvement required the substitution of the existing windows with windows with an aluminum frame and double laminated glass.
Figure 2: Window frame material (left) and typology of glass (right) of the windows of the selected sample of Italian Schools. (Credit: Simone Secchi)
Figure 3 shows the average value of façade sound insulation (D2m,nT) in frequency bands, before and after the treatments, with error bars equal to 1 standard deviation of the measured data. Schools with single glazed windows have typically lower insulation performances. The improvement of acoustic performances is evident especially at higher frequencies probably because of the better airtightness of the new windows.
Figure 3: Average values of D2m,nT as a function of the kind of glass installed in the windows (ante operam) and as ante and post operam global values (error bars = ± 1 standard deviation). (Credit: Simone Secchi)
Typical outdoor noise in Italian schools was analyzed with reference to a random sample of schools located in Florence and in other small towns in the surroundings. 53.5 % of the equivalent sound pressure levels (LAeq) due to traffic was lower than 60 dB and the average value was 60.9 dB with a standard deviation of 8.1 dB.
The average improvement of façade sound insulation was 11.3 dB, which, in general, allows to create a comfortable sound environment inside classrooms where the maximum value of the statistical distribution of all the indoor levels shifts from 30-40 dB to 25-30 dB, referring to an average outdoor noise level of 61 dB (Figure 4).
Figure 4: Percentage distribution of LAeq values in the sample of schools ante and post operam calculated with reference to the average outdoor sound pressure level of 61 dBA (traffic noise). (Credit: Simone Secchi)
The average value of indoor SPL due to traffic noise in the examined classrooms before the treatments is 35.7 dB, with a standard deviation of 4.5 dB; after the treatments, the average value is 27.4 dB with a standard deviation of 4.3 dB.
With reference to the limit value of 35 dB for indoor sound pressure level stated by World Health Organization, 54% of the examined schools, pre-treatment, and 3%, post-treatment, would exceed the given limit value.
Anyway, according to previous studies, younger children of primary schools seem to be more affected by ambient and background levels of outdoor noise, while the performance of older children of the same schools is more closely related to maximum noise levels due for example to sirens, lorries or motorbikes passing by the schools. For this reason, it does not seem fair to refer the limit values to the indoor equivalent SPL as in the case of the World Health Organization regulations. Instead, attention should be paid to the maximum SPL due to specific noise events that could significantly exceed the equivalent SPL.
The results of this study show that acoustic treatment of facades, that usually consists in the replacement of windows, produces good results in the abatement of indoor noise level also with respect to the analyzed short-duration noise events. On the other hand, as high reverberation times affect the intelligibility and, in general, the acoustics comfort in a classroom, it is necessary to plan façade refurbishment and acoustic treatment of classrooms together.
These findings are described in the article entitled “Effect of outdoor noise and façade sound insulation on indoor acoustic environment of Italian schools“, Applied Acoustics 126 (2017) 120–130. This work was led by Simone Secchi and Gianfranco Cellai from the University of Florence, Italy, Arianna Astolfi, Giulia Calosso, and Louena Shtrepi (Politecnico di Torino, Turin, Italy), David Cassini (Environmental Protection Agency, Florence, Italy), and Fabio Scamoni and Chiara Scrosati (Construction Technologies Institute of the National Research Council of Italy, San Giuliano Milanese, Milan, Italy).