facades and the ground are assumed to be perfectly smooth by the ISM, which has contributed to the fundamental understanding of the behavior of the sound field in streets [16]. However, there are inevitable diffuse reflections due to irregularities in building facades in actual streets, which lead to considerable errors [21]. To simulate the diffuse reflections, RTM and ARM have been widely used. In RTM and corresponding hybrid models, a random process is commonly applied to model the scattered sound. A random number between {0,1} is compared with the scattering coefficient to decide whether a reflection is specular or diffuse [22]. Therefore, a large number of sound rays are needed to obtain an approximation, which leads to a great time cost when simulating sound propagation in large urban spaces. To simulate the diffuse reflection process, the radiosity method was introduced from the field of thermodynamics [23]. By dividing the surfaces into smaller patches, it converts the diffuse reflection on surfaces into energy transfer among patches. Due to the simple geometry of street canyons, ARM is easy to apply and useful in revealing the fundamental differences between sound fields formed by diffusely reflecting boundaries and geometrically reflecting boundaries [18; 24]. The original ARM considers only diffuse reflections, which is in contrast to the image source method (ISM) [25]. However, the actual building facades act between these two extreme conditions, which are neither completely diffuse nor completely specular [19]. Therefore, a series of hybrid models were proposed by merging the image source model (ISM) with the radiosity model [19; 20; 26]. The major problem related to the hybrid model is how different methods are merged to cover all the reflection components. In the hybrid model proposed by [PERSON] and [PERSON], the entire sound energy transfer process was modeled by multiple consecutive specular reflections and the first-order diffuse reflection [26]. Then, in [PERSON]'s model [19] and [PERSON]'s model [20], high-order diffuse reflections were considered. A similarity among these models is that the entire sound energy transfer is divided into two separate parts: a specular reflected component and a diffusely reflected component. The pure specular reflected component is first modeled by the ISM, and the received energy of each patch is saved as the initial energy for the following diffusely reflected component modeled by ARM. As suggested by [PERSON], this scheme could lead to considerable errors with the increase of the scattering coefficient and reflection order, especially for acoustic parameters determined by high-order reflections [26]. Another key problem is model validation, especially in actual street canyons. Due to the complexity of real urban areas, only a few measurements have been implemented [27; 28; 29; 30]. Furthermore, it is difficult to use the measured data directly for model validation because of the lack of accurate parameters of surfaces, especially scattering coefficients. It is common to use a series of hypothetical scattering coefficients in calculation models, and the result is a range of acoustic indexes, such as sound level attenuation. Therefore, model validation can only prove that the measured data is in the range of the simulation result instead of analyzing the detailed accuracy of the calculation model. This leads to difficulties in applying the model in actual street canyons with many surface conditions. A scale model has been developed and widely used to study sound propagation in street canyons [31; 32; 33; 34; 35; 36]. However, because of the difficulty in reproducing the full-scale phenomena using scale models (due to absorption, scattering, etc. ), scale models are also challenging to use to examine the accuracy of a calculation model. In this paper, an iterative model combining the radiosity method and image source method (IMCRI) is developed that allows both diffuse reflection and specular reflection in any reflection order.