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Article

Facade Design and the Outdoor Acoustic Environment: A Case Study at Batna 1 University

by
Sami Hamouta
1,2,*,
Noureddine Zemmouri
3,4 and
Atef Ahriz
5,6
1
Department of Architecture, 8 MAI 1945 Guelma University, Guelma 24000, Algeria
2
Laboratory of Civil Engineering and Hydraulics (LGCH), 8 MAI 1945 Guelma University, Guelma 24000, Algeria
3
Department of Architecture, University of Biskra, Biskra 07000, Algeria
4
Laboratory of Design and Modeling of Architectural and Urban Forms and Ambiances (LACOMOFA), University of Biskra, Biskra 07000, Algeria
5
Department of Architecture, University of Tebessa, Tebessa 12000, Algeria
6
Applied Civil Engineering Laboratory (LGCA), University of Tebessa, Tebessa 12000, Algeria
*
Author to whom correspondence should be addressed.
Buildings 2024, 14(11), 3339; https://doi.org/10.3390/buildings14113339
Submission received: 18 August 2024 / Revised: 16 October 2024 / Accepted: 18 October 2024 / Published: 22 October 2024
(This article belongs to the Section Building Energy, Physics, Environment, and Systems)
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Abstract

:
The relationship between architectural design and outdoor acoustic environments remains underexplored, particularly in educational spaces where noise levels impact comfort and usability. This study investigates the impact of building facade height on the outdoor acoustic environment in university courtyards. Acoustic measurements were conducted in two courtyards at Batna 1 University, each surrounded by buildings with distinct facade heights. Key acoustic parameters, including reverberation time (RT), early decay time (EDT), rapid speech transmission index (RaSTI), Definition (D50), and sound pressure level (SPL) attenuation were evaluated at specified source-receiver distances. The results reveal a strong correlation between RT20 and distance at higher frequencies due to building facade reflections, while lower frequencies are more influenced by geometric configuration and material absorption properties. The results demonstrate that RT and EDT increase logarithmically or polynomially with distance, especially at higher frequencies (2000–4000 Hz), due to the decrease in direct sound energy and increase in reflected sound amplitude. Taller building facades lead to longer RT and EDT values compared to lower heights. D50 and RaSTI decrease polynomially with increasing source–receiver distance, with lower values observed in the courtyard with taller facades, indicating reduced speech clarity. The SPL attenuation is influenced by surrounding geometry, with the least reduction in the courtyard with lower facade heights, followed by the taller facade courtyard, contrasting with semi-free field conditions. These findings highlight the significant role of building facade height and architectural elements in shaping the acoustic characteristics of outdoor spaces, providing valuable insights for designing acoustically comfortable urban environments, particularly in educational settings.

1. Introduction

Tranquil outdoor spaces play a crucial role in urban environments, particularly in educational settings such as university campuses. These spaces offer numerous benefits, including reduced anxiety, cognitive improvements, and heightened engagement [1,2,3]. Moreover, they provide restorative experiences that are essential for mental well-being [4]. However, the tranquility of these spaces is increasingly threatened by noise pollution, which has become the second most significant cause of environmental pollution due to rapid urbanization, growth in the building sector, and increased transportation activities [5].
The impact of noise pollution is particularly concerning in university campuses, where it can negatively affect students’ behavior and comprehension. Loud environments are not conducive to learning, making instruction laborious and generating annoyance and difficulty focusing [6,7,8,9,10,11]. While outdoor spaces in universities aim to preserve students’ restorative experiences and relaxation [12], they are often exposed to noise reflected from surrounding buildings, creating complex acoustic effects that influence both transient sound levels (reverberation time) and continuous noise levels (sound pressure level (SPL)) [13].
The acoustic characteristics of these outdoor spaces are influenced by various factors, including dimensions, irregularity, material characteristics, architectural arrangement, and ground surfaces. These elements affect auditory comfort not only in open-air environments, but also in indoor facilities like classrooms, libraries, and laboratories. Consequently, creating outdoor spaces with pleasant acoustics is crucial for enhancing the overall quality of life for educational, instructional, and relaxation purposes.
Built environment morphology plays a crucial role in shaping the acoustic characteristics of outdoor spaces [14,15,16,17,18]. Key features such as building layout shape and arrangement can significantly influence noise levels and alter various acoustic parameters within the environment. Previous studies have explored how these built environment features, particularly building layout shapes, impact the acoustic ambiance of outdoor spaces [19,20,21]. Research has demonstrated that different morphological elements of building layouts affect the acoustic environment. Specifically, studies [13,22,23] have investigated the influence of building configuration and disposition on acoustical parameters like reverberation time (RT), early decay time (EDT), Definition (D50), rapid speech transmission index (RaSTI), and SPL attenuation, underscoring the importance of building arrangement in shaping acoustic outcomes.
Furthermore, site measurements of sound propagation in outdoor spaces have been conducted to assess the impact of buildings on RT and noise levels [13,19,22,23,24,25,26,27,28,29]. These measurements indicate that buildings in urban environments contribute to increased noise levels and RT due to multiple reflections. These reflections are influenced by various factors, including the width of streets and the acoustic characteristics of building facades and ground surfaces, such as absorption, diffusion, and diffraction.
Recent research has explored the relationship between building facade height and the sound environment in urban settings. Studies have investigated the influence of height-to-width (H/W) ratios on sound attenuation [30], the relationship between street width-to-building height ratio and noise pollution [20], and the effects of different facade configurations on acoustic comfort [19,31,32,33]. Additionally, research has examined the impact of building height on noise reflection and aeroacoustics performance [34,35], as well as speech interference levels on residential balconies exposed to road traffic noise [36].
Despite these valuable contributions, there remains a gap in our understanding of how building facade height specifically affects the outdoor acoustic environment in university courtyards. While previous studies have primarily focused on street contexts, the unique layout and function of university courtyards warrant further investigation. The complex interplay between building height, the surrounding landscape, and the enclosed nature of courtyards may result in acoustic characteristics distinct from those observed in street settings.
This study seeks to bridge the knowledge gap regarding the impact of building facade heights on outdoor acoustic characteristics by conducting a detailed analysis of two distinct courtyards at Batna 1 University. Each courtyard is characterized by different building facade heights, providing a controlled comparison to assess how architectural design influences acoustic behavior. The aim is to generate insights that are particularly applicable to the design and acoustic management of university campuses.
The investigation focuses on categorizing the courtyards based on their building heights and evaluating key acoustic parameters such as reverberation time (RT), early decay time (EDT), and sound pressure level (SPL) attenuation. Using in situ measurements taken at specified locations within the courtyards and analyzing the distances between sound sources and receivers, we provide a comprehensive understanding of the acoustic environment in these spaces. Additionally, room acoustical parameters, including the rapid speech transmission index (RaSTI) and the Clarity Index (D50), are examined to assess speech intelligibility within these outdoor areas.
By focusing on university courtyards and explicitly investigating the role of building facade height in this context, our study aims to contribute valuable insights to the field of architectural acoustics and campus design. The findings from this research will not only enhance our understanding of acoustic behavior in enclosed outdoor spaces, but also provide practical implications for creating more acoustically pleasant and functional university environments.

2. Methods

2.1. Description of the Case Study

This study aims to investigate the impact of the height of building facades on the behavior of the sound environment through two outdoor spaces within the campus of Batna 1 university, situated in Batna city (Aures region), in the northeast of Algeria.
As seen in Figure 1 and Figure 2, the study focuses on two outdoor areas designed as courtyards, each situated by university buildings and frequently used by students. The selection of these courtyards was based on their architectural similarities, including an almost square shape measuring approximately 36 m by 36 m, concrete walls, large glass windows, and being enclosed by buildings on all sides.
The key distinction between the two courtyards is the height of the surrounding building facades. One courtyard is bordered by buildings with six levels, while the other is surrounded by buildings with two–three levels. These differences in facade height contribute to variations in acoustic properties. Specifically, the height of the surrounding buildings influences sound reflection, which can lead to longer RT and increased SPL compared to an open area. This setup allows us to investigate how variations in facade height impact acoustic characteristics within similarly shaped and enclosed outdoor spaces.

2.2. Measurement Protocol

Figure 3 displays the workflow of this research to analyze the impulse response, including RT, EDT, D50, and RaSTI and the SPL. This study utilized ISO-3382 guidelines [37], which are primarily designed for room acoustics, to guide our outdoor acoustic measurements. ISO-3382 is intended for environments with defined volumes, which presents certain challenges when applied to outdoor spaces that lack such boundaries.
To address these challenges, this study is based on precedents set by previous research that applied room acoustic parameters to outdoor environments. Notably, studies by Yang et al. (2013) and Yang et al. (2013) [13,22], have successfully explored acoustic characteristics in outdoor spaces surrounded by buildings and in apartment complexes. These studies demonstrate that room acoustic parameters can offer valuable insights even in open or semi-enclosed environments.
To measure SPL attenuation with distance, white noise was used as the sound source. Its broad frequency spectrum and consistent power distribution across all audible frequencies ensure clearer and more accurate measurements. The uniformity and repeatability of white noise make it ideal for controlled measurements, providing reliable and comparable data when assessing the acoustic properties of outdoor spaces.
The white noise was produced by a directional speaker at a height of 1.5 m. The signal-to-noise ratio (S/N) for the measurement was 56 dB (unweighted values) at a distance of 1 m from the source. This indicates that there was enough sound power to accurately measure the SPL attenuation at distances up to 40 m between the source and the receiver. The S/N ratio, measured at a distance of 40 m from the source, was around 28 dB (unweighted values).
Reverberation time is defined as the duration it takes for the sound level in a space to decay by 60 dB after the sound source has been turned off [38]. Impulse sources, such as a signal gun and balloon popping, are often used in practice to measure reverberation time [13,22,39]. For this study, the impulsive signal was assessed using a starting clapper as the sound source. This decision was based on the clapper’s sharp onset, wide frequency spectrum [40], and its ability to minimize the influence of background noise [22]. These characteristics are essential for accurately measuring reverberation time, as they allow for precise identification of the sound’s beginning, capture the full range of audible frequencies, and provide a clearer signal for analysis.
The acoustical characteristics, such RT, EDT, D50, and RaSTI, were examined using the EAZERA PRO (V 1.2) software from AFMG. The decision to use EASERA is based on its noise compensation algorithm, which minimizes the impact of background noise on the computation of RT and its proven effectiveness in environments where background noise and reflections can be controlled, such as the semi-enclosed courtyards examined in our research. These conditions are critical for ensuring the accuracy of acoustic measurements.
EASERA is designed to handle complex acoustic environments, including outdoor spaces with large reflective surfaces and potential for significant reverberation. The software includes predefined templates for various settings, such as “stadium” locations, which share acoustic characteristics with our study areas. Specifically, the courtyards analyzed in this study exhibit similar challenges to those found in stadiums, including a mix of direct sound and reflections from surrounding structures.
The measurement was conducted using the Scarlet Solo Focusrite audio interface, a ½ inch measuring microphone (Dayton audio type EMM-6), and a real-time analyzer (RTA) inside the ESERA software developed by AFMG. Both the source and the receiver were positioned at a height of 1.5 units above the ground. Each measurement represented the average value obtained from five consecutive claps performed in succession. Prior to each measurement, the microphone underwent calibration. The measurements were made under the meteorological conditions specified in Table 1. The measurements were conducted on the same day during winter to ensure consistency and avoid variability in weather conditions between sites. The uniform conditions help to ensure that the results are comparable between the measurement zones.
The number and position of the source and recipient points in each measurement zone are outlined in Table 2. Figure 1 depicts the positions of source-to-receiver points in the two regions, with a total of 10 points used for measuring impulse responses and SPLs. Although the source sound, namely a starting clapper for RT and a speaker for SPL attenuation, remained constant in all outside areas, the positions of the reception points, which were microphones, were adjusted along their respective lines of sight. The distance between the source and receiver for each measurement zone was obtained by taking into account the size of the outdoor spaces. The source receiver distance was logarithmically scaled within a range of about 40 m in two areas. This was carried out in order to study the distribution of RT and the attenuation of SPL in these outdoor spaces.
The INR was found to be 20.40 dB at 125 Hz, 30.1 dB at 250 Hz, 39.6 dB at 500 Hz, 54 dB at 1000 Hz, 56.4 dB at 2000 Hz, and 63.5 dB at 4000 Hz at a distance of 36 m, which is considered to be the greatest source-to-receiver distance.
For accurate RT measurement of reverberation time measured over the decay of 20 dB (RT20) and reverberation time measured over the decay of 30 dB (RT30), respectively, ISO 3382-2 recommends an INR of at least 35 dB and 45 dB. Based on the INRs in the one band displayed above, the RT calculation method was based on RT20 (−5 dB to −25 dB) in one-octave bands from 500 Hz to 4000 Hz for source-to-receiver distances within 36 m. The RT20 at the low frequencies (125 Hz and 250 Hz) are not included in this analysis because of the insufficient INR (less than 35 dB).

3. Results and Discussion

3.1. Impulse Response

In order to analyze the effect of the height of the building’s facade on variations in sound energy reflection patterns, Figure 4 plots the pressure squared impulse responses, recorded at the same source–receiver distance (20 m), across two outdoor spaces. Figure 4 visualizes that the impulse responses include subsequent peaks coming later to the direct sound arrival, indicating reflections of the sound wave off the building facades. The sound energy that bounces back produces an increase in SPL and RT, which could cause acoustic defects including echoes. Despite the measurement being conducted at equal distances between the source and receiver, the reflection patterns of impulse responses vary throughout the two outside areas. The reflection pattern is impacted by several design aspects, including building facade height. An outdoor space encircled by six stories reflects sound more strongly and for a longer duration than an outdoor space surrounded by two or three stories. Figure 5’s decay curve adds credence to this finding.

3.2. General Features and RT Distribution

For the following octave bands: 500 Hz, 1000 Hz, 2000 Hz, and 4000 Hz, Figure 6 shows the general averaged RT20 for maximum, average, and minimum values assessed at the two measurement areas. For an accurate measurement, The RT20 at the low frequencies (125 Hz and 250 Hz) are not included in this analysis because of the insufficient INR (less than 35 dB). Despite the INR being sufficient (greater than 35 dB) at 8000 Hz, we did not include the results in this analysis because the ISO 3382 standard recommends excluding the 8000 Hz frequency due to the human ear’s reduced sensitivity and the strong air attenuation at this frequency.
RT20 presents variations in patterns across the band frequencies. Comparing RT20 values across frequencies, it can be noted that values decrease along with increasing frequencies’ bands. For instance, RT20 decreases from 3.68 s at 500 Hz to 1.97 s at 4000 Hz. At low frequencies, where diffraction plays a significant role in sound propagation, the absence of substantial gaps between buildings restricts the paths through which sound can diffract. As a result, more sound energy tends to remain within the enclosed space rather than escaping through diffraction around obstacles. However, at a high frequency, the energy of sound diminishes to non-considerable values due to the atmosphere’s absorption and diffusion.
The highest value in Figure 6, with RT20 measuring 3.68 s at 500 Hz, depicts the reverberant sound field in outdoor situations.
Considering a variety of frequencies in octave ranges from 500 Hz to 4000 Hz, Figure 7 shows the highest, average, and lowest RT20 values reported at each measurement region. The overarching goal of this study is to assess how widespread RT20 is in outdoor environment settings.
The results show that RT20 varies significantly between the greatest and lowest values across several measurement regions, indicating that RT20 is not uniformly distributed in the outdoors.
The different maximum, average, and minimum values of RT20 determined by the measuring areas demonstrate the substantial influence of building heights on RT20. At 500 Hz, 1000 Hz, 2000 Hz, and 4000 Hz, the RT20’s durations are much longer than at other frequencies. For a structure with six stories and a duration greater than 3.68 s, the maximum RT20 occurs at a frequency of 500 Hz.
Figure 8 displays the results of the RT20 measurements taken in the two measurement spaces using various sources and receivers. Even if the source–receiver distance is constant, the results show that RT20 varies significantly across different measurement zones. This provides further evidence that building height and other architectural details may affect RT20.
To sum up, RT20 may be affected by architectural design elements such as building heights, according to the results pertaining to its dispersion and general characteristics.

3.3. Acoustic Parameters

3.3.1. RT

In urban environments, the distance between the sound source and receiver plays a crucial role in determining the RT. Figure 9 illustrates the measured RT as a function of the distance between the sound source and receiver for two outdoor areas characterized by different building facade heights. The graph displays regression curves and correlation coefficients (R2) especially pertaining to the frequency of 500 Hz. The selection of the calculating approach for the regression curve is based on choosing the correlation coefficient with the highest value. The equation of the second order is used for the polynomial regression curve.
The correlation coefficients (R2) observed between these phenomena range from 0.81 to 0.0.94 at 2000 Hz and 4000 Hz in both outdoor spaces, signifying a strong correlation. This can be attributed to several factors. High-frequency sound waves have shorter wavelengths, which are more easily reflected by building surfaces, especially smooth, hard materials like concrete or glass. These reflections contribute significantly to the reverberant sound field, making the RT20 more sensitive to changes in distance from the source. Additionally, higher frequencies are less affected by diffraction, meaning that sound energy tends to remain more concentrated in direct and reflected paths, leading to clearer patterns of reverberation as the distance increases.
Despite the strong correlation between RT20 and distance at higher frequencies, differences emerge at lower frequencies (500 Hz and 1000 Hz) between the two outdoor spaces. In the (A) building with six levels of height, the correlation is moderate at 500 Hz with an R2 value of 0.1654 and strong at 1000 Hz with an R2 value of 0.9867. In contrast, in the (B) building with two–three levels of height, the correlation is strong at 500 Hz with an R2 value of 0.7456, but moderate at 1000 Hz with an R2 value of 0.3526. This variation can likely be attributed to the longer wavelengths of lower frequencies, which are less influenced by facade height and more significantly affected by factors such as the overall geometric configuration, material absorption properties, skyward diffraction effects, and ground interactions within the space.
At the frequency with a high correlation, the regression curves across both typologies show a consistent pattern, increasing logarithmically or polynomily with distance. This trend likely results from the decrease in direct sound energy at shorter distances, while the amplitude of reflected sound grows at longer distances. Notably, this occurs in the (A) building with six levels of height.
It is worth noting that the RT20 values at the same source–receiver distance varied significantly among the two outdoor spaces. For instance, in the (A) building with six levels of height, the RT20 values were relatively strong compared to the (B) building with two–three levels of height. This difference can be attributed to the amount of surface of facades surrounding the outdoor space. In the (A) building with six levels of height, where the outdoor space is enclosed by a large surface of facades, the sound reflections and reverberations are more pronounced, causing longer RT20 values. On the other hand, in the outdoor area surrounded by (B) building with two–three levels of height, which have fewer surfaces surrounding facades, the sound reflections and reverberations are less prominent, leading to shorter RT20 values. These variations in RT20 values demonstrate the significant influence of the architectural design element, such as the height of building facade surrounding structures on the acoustic characteristics of the outdoor space. Therefore, this finding is crucial for designing outdoor spaces where acoustic comfort is a priority.

3.3.2. EDT

Within Figure 10, The EDT for each measurement context is shown based on the source-to-receiver distances at a frequency of 2000 Hz. The findings indicate that the EDT tends to exhibit a logarithmic rise as the distance between the source and receiver increases. This trend is comparable to the behavior of the RT20 in outdoor environments. Like RT20, the correlation coefficient also ranges from 0.81 to 0.92, indicating a robust and evident link between the variables. This strong correlation suggests that as the sound travels further from the source, the decay rate is increasingly influenced by reflections from surrounding structures. Additionally, it is evident that at the same distance between the source and receiver, the EDT is comparable to the RT20. However, their values fluctuate because of the varying heights of the building facades around the outside area. Taller facades lead to more pronounced reflections, resulting in longer EDTs. Understanding these fluctuations in EDT is crucial for the design of outdoor spaces, particularly in applications where specific acoustic characteristics are desired. For instance, in public gathering places, ensuring voice clarity might require minimizing excessive reverberation, which is influenced by both RT20 and EDT. Conversely, performance venues might benefit from enhanced sound projection and controlled reverberation, which also necessitates careful consideration of EDT. By accounting for these variances, architects and acoustic engineers can tailor outdoor environments to meet specific auditory requirements.

3.3.3. D50

Figure 11 illustrates the D50 values at different distances between the source and receiver for the two outdoor regions. In both types of spaces, the D50 value decreases (in a polynomial manner) as the distances rise. As distances increase, the sound becomes less clear. The correlation coefficient of regression curves in the settings of RT20 and EDT ranges from 0.81 to 0.98, demonstrating a strong link between the variables. Each outdoor location has different D50 values at the same distance. For instance, when the distance between the source and receiver is 20 m, the D50 value is 0.2 in the building with six levels of height and 0.6 in the building with two to three levels. This disparity in D50 values can be attributed to the larger surface area of the facades in the six-level building, which increases the number of reflections and thus diminishes speech clarity. The larger and more reflective surfaces create a more reverberant environment, reducing the direct-to-reverberant sound ratio, which is crucial for maintaining clarity.
The practical implications of these findings are significant for the design of outdoor spaces, particularly those intended for communication, such as public squares or outdoor lecture areas. In environments where speech intelligibility is critical, the height and surface area of surrounding facades should be carefully considered. Taller buildings with extensive facade areas may require additional acoustic treatment or design modifications to enhance clarity and ensure effective communication in the space. By understanding the impact of facade height on D50, designers can make informed decisions to optimize the acoustic environment of outdoor spaces.

3.3.4. RaSTI

Based on the data shown in Figure 12, the RaSTI tends to decrease as the distance rises in both typology situations. This tendency is similar to the results of D50. The reason for this is that when the source and receiver are close together, the direct sound has a stronger impact on the initial sound energy of the impulse response, resulting in a shorter RT20. Nevertheless, when the distance increases, the intensity of the direct sound diminishes, resulting in an increase in the RT20.
The evaluation is determined by five unique levels, with each level corresponding to a particular range. The level of 0–0.3 is classified as extremely bad, 0.3–0.45 as poor, 0.45–0.6 as fair, 0.6–0.75 as good, and 0.75–1.0 as exceptional [22]. The RaSTI measurements differ in each outdoor area, even when the distance is the same. The RaSTI is classified as “extremely poor” for a building with six floors of height, with a value of 0.2. However, a building with two or three floors falls inside the acceptable range, with a value of 0.6. The disparity in RaSTI values can be attributed to the varying number of reflective surfaces provided by the surrounding building facades. In the case of the six-floor building, the greater surface area increases the reverberant sound energy, reducing speech intelligibility as measured by RaSTI. Conversely, in the two- or three-floor setting, the reduced facade surface area results in less reverberation and better speech clarity.
These findings underscore the importance of considering the height and surface area of surrounding facades when designing outdoor spaces where speech intelligibility is critical. The RaSTI metric provides valuable insights into how architectural features influence acoustic performance, particularly in semi-enclosed outdoor environments. Designers should aim to minimize excessive reverberation in such spaces, possibly through the use of acoustic treatments or careful planning of facade geometry, to ensure that speech remains clear and intelligible even at greater distances.

3.3.5. SPL Attenuation

According to the inverse square law, sound pressure levels typically decrease by approximately 6 dB for every doubling of the distance in an open, free-field environment. However, in our case, the presence of boundaries, such as surrounding buildings, leads to reflections and diffraction, which influence the sound propagation. This results in deviations from the expected 6 dB reduction, and creates a more complex sound attenuation pattern.
Figure 13 displays the SPL attenuation findings in relation to the reference SPL, which was measured at a distance of 1 m between the source and receiver, in three outdoor locations. The results suggest that SPL decreases as the distance between the sound source and the listener grows in all outdoor spaces. This is due to the characteristics of the non-diffuse field. There is little variance in sound pressure level (SPL) decrease across the three outdoor locations when the source and receiver are within a distance of 1–5 m. This is due to the significant influence of the direct sound.
However, in the far field, at the same position where the sound source and receiver are located, it can be seen that the SPL decreases differently depending on the outdoor area’s features, such as the surrounding geometry.
While the expectation was for similar SPL across both spaces, and there was a higher SPL at the highest height, The outdoor environment surrounded by a building facade ranging from two to three levels of height exhibits the least SPL reduction. This is followed by the outdoor-surrounded building facade with six levels of height. Finally, the semi-free field environment shows the highest SPL attenuation. This discrepancy arises due to the presence of building facade elements like balconies, resulting in a lower amount of reflected energy compared to an outdoor environment with a building facade ranging from two to three levels. This discrepancy can be attributed to the architectural elements of the facades, such as balconies and other protrusions, which influence the reflection and absorption of sound. In environments with lower building heights, these elements may contribute to more efficient sound reflection and diffusion, resulting in less attenuation of SPL. In contrast, taller facades, while providing a larger surface area for reflection, may also lead to increased scattering and absorption, reducing the overall SPL.
These results highlight the significant impact of building design on the acoustic environment of outdoor spaces. The variation in SPL attenuation across different settings suggests that architectural features, particularly the height and surface complexity of facades, play a crucial role in shaping the acoustic experience. This has practical implications for urban design, particularly in managing noise levels and ensuring comfortable soundscapes in outdoor areas frequented by students or the general public.
Understanding these acoustic dynamics can guide architects and urban planners in designing outdoor spaces that mitigate unwanted noise while maintaining the clarity and intelligibility of desired sounds. For instance, strategic placement of reflective surfaces or the inclusion of acoustic treatments on facades could be considered to control SPL attenuation and enhance the overall acoustic quality of outdoor environments.

4. Conclusions

The present research assessed SPL attenuation and room acoustical parameters including RT, EDT, RaSTI, and D50 in outdoor spaces characterized by different building facade heights. These insights provide valuable understanding for the acoustic design of urban outdoor environments, especially in educational and public spaces.
RT20 showed variations across frequency bands, with the highest value of 3.68 s at 500 Hz, reflecting the outdoor reverberant sound field. These results emphasize that RT20 is not uniformly distributed in outdoor settings and is affected by the architectural design of surrounding buildings.
The findings demonstrate a strong correlation between RT20 and distance at higher frequencies (2000 Hz, 4000 Hz), largely due to the reflective properties of building facades and the minimal impact of diffraction. In contrast, lower frequencies (500 Hz and 1000 Hz) show a weaker correlation, indicating that the architectural design, such as regarding facade height, has a reduced influence, while other factors like the overall geometric configuration, material absorption, skyward diffraction, and ground interactions play a more prominent role. These insights emphasize the frequency-dependent nature of sound behavior in outdoor environments and underscore the importance of considering both high- and low-frequency characteristics when designing acoustically comfortable urban spaces.
At higher frequencies (2000 Hz, 4000 Hz), the regression curves of RT across both outdoor spaces exhibit a consistent pattern, increasing logarithmically or polynomially with distance. This indicates that while direct sound energy decreases at shorter distances, reflected sound amplitude increases at longer distances, an observation applicable to similar urban spaces with reflective building surfaces.
The findings also suggest that EDT follows a similar logarithmic trend with increasing distance between the source and receiver. This behavior is likely to apply in other outdoor environments surrounded by buildings of varying facade heights, emphasizing the impact of architectural design on both RT and EDT values.
D50 and RaSTI decreased as source–receiver distance increased, highlighting the significant role of building configurations, particularly facade height, on speech intelligibility in outdoor spaces. These trends are likely relevant to other outdoor public spaces where speech clarity and intelligibility are important.
SPL attenuation patterns demonstrated a significant influence of surrounding geometry, including balconies and overhanging architectural elements, with the least reduction observed in areas surrounded by lower-height buildings, followed by higher-height buildings. Protrusions such as balconies contribute to variations in SPL attenuation due to the complex reflective surfaces they introduce. This trend contrasts with the expected behavior in semi-free field conditions and is likely applicable to outdoor environments with similar geometric features.
The study’s findings underscore the importance of building facade height and geometry in shaping outdoor acoustic conditions. These insights can inform urban planners and architects aiming to optimize acoustic comfort in public, recreational, and educational settings, where controlling noise and enhancing acoustic comfort are essential for improving the quality of life.
Design Recommendations for Optimizing Outdoor Acoustic Environments in Educational Settings:
1. Building Height and Facade Surface Area:
-
Lower building heights (two–three stories) over taller facades (six stories) to minimize excessive sound reflections and reverberation in outdoor spaces.
-
Implement strategies to break up large, continuous facade surfaces to reduce the overall reflective area and improve acoustic comfort.
2. Facade Articulation and Geometry:
-
Utilize guidelines on incorporating facade articulation techniques, such as balconies, protrusions, or angled surfaces, to diffuse sound and reduce the impact of reflections on speech intelligibility and overall acoustic comfort.
-
Follow recommendations on the optimal depth and spacing of facade elements to achieve the desired acoustic effects based on our findings.
3. Material Selection:
-
Use sound-absorbing materials on facades, particularly at lower heights, to minimize early reflections and improve speech clarity (D50 and RaSTI).
-
Utilize a combination of reflective and absorptive materials to balance sound projection and control excessive reverberation in outdoor spaces.
4. Outdoor Space Configuration:
-
Follow guidelines on the optimal dimensions and proportions of courtyards and other outdoor spaces to minimize the impact of building height on reverberation time (RT) and sound pressure level (SPL) attenuation.
-
Implement strategies for integrating landscaping elements, such as vegetation, to further enhance the acoustic comfort of outdoor environments.

Author Contributions

S.H.: conceptualization, methodology, formal analysis, investigation, writing—original draft, writing—review and editing, formal analysis, visualization, funding acquisition. A.A.: conceptualization, methodology, writing—review and editing, validation, supervision, formal analysis. N.Z.: conceptualization, methodology, writing—review and editing, validation, supervision, formal analysis. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data will be made available on request.

Acknowledgments

The authors would like to express their gratitude to the following organizations and individuals for their valuable contributions to this research: (1) Ahnert Feistel Media Group (AFMG) company for generously providing a free version of the EASEA Pro software, which was instrumental in conducting the measurements and analyses for this research. This software significantly contributed to the outcomes of our study, and we appreciate AFMG’s support in facilitating our work. (2) The Laboratory of Child, City and Environment (LEVE) directed by Dib, B., for generously providing equipment for conducting field measurements. (3) M.C. Mansouri, A. Mestiri, S. Haddad and T. Benfifi for providing assistance with data collection. We also extend our appreciation to the faculty and staff at Batna 1 University for their support and cooperation throughout this study.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Batna 1 University, Master Plan and location of measurement stations.
Figure 1. Batna 1 University, Master Plan and location of measurement stations.
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Figure 2. Photographs of each measurement station. (A) Building with six stories; (B) building with two–three stories.
Figure 2. Photographs of each measurement station. (A) Building with six stories; (B) building with two–three stories.
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Figure 3. Investigation workflow.
Figure 3. Investigation workflow.
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Figure 4. Impulse responses at 1000 Hz for each of the two outdoor sites measured at a source-to-receiver distance of about 20 m.
Figure 4. Impulse responses at 1000 Hz for each of the two outdoor sites measured at a source-to-receiver distance of about 20 m.
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Figure 5. Decay curves at 1000 Hz for each of the two outdoor sites measured at a source-to-receiver distance of about 20 m.
Figure 5. Decay curves at 1000 Hz for each of the two outdoor sites measured at a source-to-receiver distance of about 20 m.
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Figure 6. General averaged RT20 for maximum, average, and minimum values measured at the two outdoor areas.
Figure 6. General averaged RT20 for maximum, average, and minimum values measured at the two outdoor areas.
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Figure 7. RT20 values, including the maximum, average, and minimum, with their corresponding frequencies, measured at the two outdoor places, at 500 Hz; 1000 Hz; 2000 Hz; 4000 Hz.
Figure 7. RT20 values, including the maximum, average, and minimum, with their corresponding frequencies, measured at the two outdoor places, at 500 Hz; 1000 Hz; 2000 Hz; 4000 Hz.
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Figure 8. The general RT20 at a frequency of 500 Hz, measured at two distinct areas using varying source–receiver distances.
Figure 8. The general RT20 at a frequency of 500 Hz, measured at two distinct areas using varying source–receiver distances.
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Figure 9. RT20 measured based on source–receiver distance for the two different types of building layouts, with regression curves and correlation coefficients R2 at the following: 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz.
Figure 9. RT20 measured based on source–receiver distance for the two different types of building layouts, with regression curves and correlation coefficients R2 at the following: 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz.
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Figure 10. Measured EDT at 2000 Hz with different source-to-receiver distances for the two different types of building heights.
Figure 10. Measured EDT at 2000 Hz with different source-to-receiver distances for the two different types of building heights.
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Figure 11. D50 with different source-to-receiver distances for the two different types of building heights.
Figure 11. D50 with different source-to-receiver distances for the two different types of building heights.
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Figure 12. RaSTI with different source-to-receiver distances for the two different types of building heights.
Figure 12. RaSTI with different source-to-receiver distances for the two different types of building heights.
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Figure 13. SPL attenuation according to source-to-receiver distance.
Figure 13. SPL attenuation according to source-to-receiver distance.
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Table 1. Weather conditions of each measurement zone.
Table 1. Weather conditions of each measurement zone.
Weather Condition(A) Building with Six Levels of Height(B) Building with Two–Three Levels of Height
Temp. (°C)12.712.7
Humidity (%)1010
Wind speed (m/s)6.256.25
Table 2. Number and position of the source and receiver points at every measurement area.
Table 2. Number and position of the source and receiver points at every measurement area.
Building Height (Level)Number of SourcesNumber of ReceiversSource–Receiver Distance(m)Measurement Parameter
SPL AttenuationImpulse Response
(A) building with six levels of height01051-5-10-20-36xx
(B) building with two–three levels of height01051-5-10-20-32xx
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Hamouta, S.; Zemmouri, N.; Ahriz, A. Facade Design and the Outdoor Acoustic Environment: A Case Study at Batna 1 University. Buildings 2024, 14, 3339. https://doi.org/10.3390/buildings14113339

AMA Style

Hamouta S, Zemmouri N, Ahriz A. Facade Design and the Outdoor Acoustic Environment: A Case Study at Batna 1 University. Buildings. 2024; 14(11):3339. https://doi.org/10.3390/buildings14113339

Chicago/Turabian Style

Hamouta, Sami, Noureddine Zemmouri, and Atef Ahriz. 2024. "Facade Design and the Outdoor Acoustic Environment: A Case Study at Batna 1 University" Buildings 14, no. 11: 3339. https://doi.org/10.3390/buildings14113339

APA Style

Hamouta, S., Zemmouri, N., & Ahriz, A. (2024). Facade Design and the Outdoor Acoustic Environment: A Case Study at Batna 1 University. Buildings, 14(11), 3339. https://doi.org/10.3390/buildings14113339

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