Elsevier

Applied Acoustics

Volume 127, 1 December 2017, Pages 147-159
Applied Acoustics

An experimental study on the acoustic characteristics of outdoor spaces surrounded by multi-residential buildings

https://doi.org/10.1016/j.apacoust.2017.05.037Get rights and content

Highlights

  • Acoustic characteristics in outdoor spaces of 15 apartment complexes are measured.

  • Maximum RT at 500 Hz is about 4 s, meaning high level of reverberance.

  • SPL is changed up to 17.7 dB due to different geometry.

  • An empirical method to predict RT and SPL using AutoCAD is proposed.

Abstract

A comfortable sound environment in the outdoor spaces of apartment complexes contributes to the improvement of the overall environmental quality. It is expected that the characteristics of room acoustical parameters and sound pressure level (SPL) attenuation of outdoor spaces surrounded by multi-residential buildings depends on many design factors such as the openness, volume, and building layouts, etc. The aim of this study is to clarify the influential factors determining room acoustical parameters and SPL attenuation in outdoor spaces that are surrounded by buildings with complicated topographical conditions. A series of measurements was carried out for 15 outdoor spaces in 6 apartment complexes with different building layouts. The 15 outdoor spaces were categorized into 4 types of building layouts: linear-shaped, parallel-shaped, U-shaped, and square-shaped. The result showed that reverberation time (RT) at 500 Hz and 1000 Hz is relatively long, over 4 s, with uneven RT distribution showing a non-diffuse field. With increasing source to receiver distance, the RT and early decay time (EDT) increased logarithmically. On the other hand, the Definition (D50) and rapid speech transmission index (RASTI) decreased with increasing source to receiver distances. The result for the SPL attenuation measured at a 20 m source to receiver distance in 10 outdoor spaces showed a 17.7 dB difference between the 10 spaces due to the influence of building geometry. An empirical method considering the openness, size-related parameters, and room constant is also suggested to predict the approximate RT and SPL attenuation in the outdoor spaces.

Introduction

High-rise apartment buildings have been universally built for residential purposes due to the increasing population density in urbanized cities. An apartment complex consists of several apartment buildings in a limited area of land with various types of building layouts and blocks of buildings. Outdoor spaces in an apartment complex are planned by considering many architectural, environmental, and social factors such as car parking, natural lighting, and outdoor activities [1]. Recently, the importance of the outdoor spaces for leisure and rest has also been given particular attention, especially with the increase of the available land for such uses due to underground car parking. Therefore, designing environments that have a comfortable sound environment in outdoor spaces can contribute to improving the living quality of residents.

Two approaches can be considered when designing spaces for effective noise reduction in outdoor urban environments. The first approach is to reduce background noise from external noise sources such as road traffic, and the second approach is to reduce background noise from internal noise sources such as human voices in the outdoor spaces. The majority of noise abatement schemes have mainly adopted the first approach related to reducing background noise from the external noise sources such as traffic noise by means of noise barriers as well as by the building layout such as introducing courtyards [2], [3], [4], [5], [6], [7]. This conventional approach is based on the concept that a lower background noise could help residents to feel less stressed.

Although reducing background noise in outdoor spaces is an effective noise abatement approach, it causes a relatively high signal to noise ratio (S/N) for the internal noise sources such as human conversation, pedestrian noise, and passing traffic. Therefore, it is also important to control the sound field of the outdoor space by means of acoustic materials with a high absorption coefficient such as green walls and soil which can reduce the increased sound pressure level (SPL) and reverberation time (RT) due to multiple reflections between building façades [8], [9], [10], [11].

Numerous studies have been carried out to characterize sound fields with acoustic descriptors including RT and SPL distribution. The results showed that RT and SPL distribution are useful parameters to predict transient and steady-state sound propagation in urban spaces that are influenced by complicated acoustic phenomena such as multiple reflections, diffraction, and diffusion due to surrounding buildings and obstacles. Thus, various prediction models for RT and SPL distribution have been developed for microscale urban environments to understand the effect of boundary conditions and width-to-height ratio in sound propagation [12], [13], [14], [15]. The results from these prediction models suggest that with diffusely reflecting boundaries, the RT is shorter than that with geometrically reflecting boundaries. It was also predicted that the RT in street canyons increases with increasing source to receiver distances for both diffusely and geometrically reflecting boundaries.

Several studies have also involved site and scale model measurements to examine sound propagation characteristics in urban spaces [16], [17], [18], [19], [20], [21]. Ismail and Oldham [22] investigated the role of sound reflection from building façades with irregular surfaces using physical scale models. The result suggested that the scattering coefficient is about 0.09–0.13 for urban façades in Europe. Although the scattering coefficient is small, the diffuse reflection mechanism is dominant at higher orders of reflections due to the effect of multiple reflections. Thomas et al. [23] carried out a series of measurements in 99 streets to examine the influence of geometrical parameters such as street width, average height, and façade roughness in SPL distribution by analyzing the reflection ratio, defined as the reverberant to direct sound energy ratio. The result showed that the reflection ratio strongly correlates with the street width. A model was also suggested to predict SPL according to the influence of changes in the street width and average building height in street canyons.

In comparison with street canyons and squares, the acoustic quality in outdoor spaces of residential buildings could be more important because residents require a high level of comfortable sound environments for leisure and rest in outdoor spaces and in living rooms that face outdoor spaces. This is especially important during summer when residents open their windows, because sound energy containing multiple reflections transmits through the indoor spaces of high floors [24]. Thus, it is important that architects understand how architectural design can affect the RT and SPL attenuation in outdoor spaces.

The purpose of this study is therefore to investigate the acoustic characteristics of outdoor spaces surrounded by multi-residential buildings by analyzing data measured in 15 outdoor spaces of 6 apartment complexes with different building layouts. The 15 outdoor spaces were categorized into 4 types of building layouts: linear-shaped, parallel-shaped, U-shaped, and square-shaped. Some of the measurement data (4 of 15 outdoor spaces) from the preliminary work was used for the parametric study [24]. Based on the site measurements, the RT, early decay time (EDT), and SPL attenuation were analyzed according to the source to receiver distances. The characteristics of room acoustical parameters were also analyzed using Definition (D50) and the rapid speech transmission index (RASTI), both of which are related to speech intelligibility. An empirical model using AutoCAD to predict RT and SPL attenuation is also suggested in this study.

Section snippets

Description of the studied sites

In this study, a series of field measurements was conducted to investigate the characteristics of sound propagation in 15 outdoor spaces of 6 apartment complexes in Korea. The apartment complexes were selected by taking into account the types of building layouts and building blocks. Fig. 1 shows the bird’s-eye views for each apartment complex and Fig. 2 shows the photographs for each site. Table 1 describes the site and measurement conditions for each apartment complex.

As shown in Fig. 1, Fig. 2

Impulse responses and decay curves in 15 outdoor spaces

To examine the difference in multiple reflection patterns of sound energy in 15 outdoor spaces, it is useful to compare the impulse responses and decay curves measured at the same source to receiver distance. In comparison with a short source to receiver distance, where the direct sound dominants the overall sound energy, an analysis of impulse responses measured at a rather long source to receiver distance could show distinct differences in multiple refection patterns. Thus, Fig. 6, Fig. 7

RT

It is well known from Sabine’s equation that the volume and absorption power of a space play an important role in determining RT. In comparison with the diffuse sound field in enclosed rooms, outdoor spaces have a non-diffuse sound field mainly due to the open ceiling and gaps between buildings which can be treated as surfaces with the absorption coefficient of 1.0. Generally, apartment buildings consist of concrete walls and windows with acoustically flat and reflective surfaces. Therefore, it

Discussion and conclusions

In this study, a series of field measurements for SPL attenuation and room acoustical parameters including RT, EDT, D50, and RASTI were carried out for 15 outdoor spaces in 6 apartment complexes, which were determined by considering the different building layouts, blocks, sizes, and heights for a parametric study. Based on the data from field measurements, an empirical method determining RT and SPL attenuation is also suggested.

The result for RT distribution indicated that RT is significantly

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2011-0007171). A part of this research is also the result of the research carried out in the cooperation project of the industry-academia-government in Seoul (project identification number: PSI160001) (project name: Urban problem solving technology development support project).

Cited by (0)

View full text