Acoustic performance of balconies having inhomogeneous ceiling surfaces on a roadside building facade Xu Wang, Dongxing Mao * , Wuzhou Yu, Zaixiu Jiang Institute of Acoustics, Tongji University, No. 1239 Siping Road, Shanghai 200092, China article info Article history: Received 29 March 2015 Received in revised form 24 June 2015 Accepted 26 June 2015 Available online 4 July 2015 Keywords: Balcony Insertion loss Inhomogeneous impedance surface abstract Balconies provide noise screening effects in residential buildings even with the balcony door opened for natural ventilation. However, the screening effect of a balcony was found to be canceled due to the reflection from the ceiling. This paper describes a balcony whose ceiling is made from materials of inhomogeneous impedance which eliminates this drawback. The nonuniform impedance affects wave behavior by altering the direction of energy flux away from the region of a balcony as it reflects on the ceiling. A proposed realization of the balcony ceiling comprises a closely spaced array of progressively tuned hollow narrow tubes which create a phase gradient. The acoustic performance of a balcony with an inhomogeneous ceiling surface is examined theoretically by a ray-based model. All of the results pre- dicted by the theory fit well with numerical simulations using a two-dimensional finite element method. Balconies with the proposed ceilings have the potential to be widely used in a roadside multi-residential building against the exterior traffic noise. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Balconies are architectural features functioned as a buffer zone to provide a comfortable environment for residents, and recently regarded as one of the green and innovative features in residential building [1]. Due to the high density of population and the scarcity of building land in metropolis, it is common that multi-residential buildings are located close or even next to traffic roads and hence exposed to severe exterior noise. Existing sound protecting treat- ments, such as sound barriers, are limited in their ability to protect a roadside building, especially the upper stories, against the road traffic noise [2]. However, balconies were found to be effective in providing the noise screening effect in residential blocks even with the balcony door opened for natural ventilation [3]. Therefore, there have been a diversity of studies on investigating the screening effect of balconies. Mohsen and Oldham [4] investigated a closed balcony by computer simulation and measurements on a scale model, and derived an empirical equation to predict the perfor- mance of a closed balcony. May [5] observed a significant increase in sound level on high-rise balconies close to freeways by field measurement, and the sound absorption treatment of the ceiling was found to provide effective noise reduction. Boundary element method (BEM) was also used to study the performance of balconies in a tall building close to a road [6]. Kim et al. [7] investigated a special type of balcony, i.e. balconies fitted with windows, by using in-situ measurement. To predict the sound field inside balconies, which are partially covered by absorptive materials, Kropp and Berillon [8] developed a theoretical model by using the Green's function for rigid walls and replacing the non-rigid areas by monopole sources. Cheng et al. [9] carried out a theoretical study on windows with lintels, which are structures similar to balconies, by combining Macdonald's diffraction theory and the image receiver method. As a modification of the standard prediction scheme CRTN [10], a methodology based on the geometrical ray theory was developed for the prediction of sound field inside a balcony [11]. Furthermore, the form of balcony was also attracted much atten- tion: A balcony opened to the street but enclosed on all other sides was investigated by means of scale models by Hammad et al. [12]. Hossam El-Dien and Woloszyn [13,14] tested the influence of bal- cony configurations, which include the ceiling inclined angle, bal- cony depth and parapet form, on the acoustic performance of building facades close to roadways using pyramid tracing simula- tions and scale model measurements. In addition, a study on the scattering effects of balconies has also been carried out using a scale model [15]. The studies mentioned above defined exterior noises solely as traffic noise and considered the balcony effect on a single * Corresponding author. E-mail address: dxmao@tongji.edu.cn (D. Mao). Contents lists available at ScienceDirect Building and Environment journal homepage: www.elsevier.com/locate/buildenv http://dx.doi.org/10.1016/j.buildenv.2015.06.027 0360-1323/© 2015 Elsevier Ltd. All rights reserved. Building and Environment 93 (2015) 1e8