## 5 Conclusions  
The propagation rule of the structure-borne sound caused by the indoor distribution transformer in a building has been analyzed and the prevention and control method is put forward. The following conclusions can be drawn.  
1. Indoors, the vibration propagates mainly upward along columns and beams. The floor structure is relatively stable, while the amplitude of the surrounding walls is relatively large. The vibration of the edge is stronger than that of the center, for the stability of the central structure is higher, and the vibration will be transmitted to the edge. When the transformer is located at the central location, it impacts most on the structure that is right above it and less on the surrounding structure but still cannot be ignored. When the transformer is located at one side edge of the house, it impacts more on all surrounding structures, but much less on the central position.
2. For the whole building, the vibration in the basement structure can propagate to the distant areas, and maintain a certain amplitude. The place where the vibration is strongest is in the transformer-centered small area, changing with the transformer's position. It has large attenuation during upward transmission, and only affects the boundary region, while having less influence on the central region which is more stable.
3. As the story rises, propagating up a layer, the maximum of the vibration of the upper floor will be reduced by 20% to 50% relative to the lower layer. Therefore, the attenuation in this direction is relatively rapid. In the horizontal direction, the closer to the transformer, the stronger the vibration is. The attenuation of vibration is about 50% less through a wall.
4. In the field test, after installing the vibration absorber base to the transformer, each harmonic component in the spectrum changes greatly, and the fundamental wave is inhibited clearly, basically decreasing 30% to 50%. The installation of the base will significantly suppress the structure-borne noise from the source.  
From our perspectives, the best position for the transformer is the central area of the basement for the whole building. However, in the two-layer model, when the transformer is at middle position of the model, the impact of vibration on the upper house is more extensive. From an overall point of view, when the transformer is at the edge position, the impact range of the vibration on the whole wall of  
Figure 38: The measured vibration curves of the coil after the base is installed: (**a**) No-load; (**b**) Short-circuit; (**c**) Load.  
the building is larger. Therefore, from the scope of the impact on the whole building, we consider that the central area is the best position. Besides, it is useful to control the vibration and structure-borne sound transmission caused by the indoor distribution from the vibration source. It is effective to take vibration isolation measures.  
There are still some problems to be further studied in this paper. The vibration propagation models are simplified, only involving two-layer and six-layer house structures, as well as the indoor structure. In addition, in the simulation experiment, only the single transformer is studied, which may be different from the actual situation. When multiple transformers work at the same time, the optimization of the layout and the structure-borne sound propagation laws are worthy of further study.  
This work was supported in part by the National Natural Science Foundation of China (Grant Number 51507114) and in part by the National Natural Science Foundation of China (Grant Number 51507122).  
[PERSON] and [PERSON] conceived and designed the simulation; [PERSON] wrote the paper; [PERSON] contributed the field test; [PERSON] provided the professional guidance; [PERSON] and [PERSON] revised this paper.  
The authors declare no conflict of interest.