# Development of a Metasilencer Considering Flow in HVAC Systems
## 1 Introduction  
In recent years, the e-mobility industry, which is a means of transportation utilizing electricity (considered to be an environmentally friendly power source), has expanded globally, and the use of electric vehicles, which is one of the representative fields of the e-mobility industry, has been rapidly increasing. Although the level of noise of electric vehicles is lower compared with conventional internal combustion engine vehicles, now it seems that noise, vibration, and harshness (NVH) problems associated with electric drives as well as heating, ventilation, and air conditioning (HVAC) systems are emerging. Thus, it is crucial to reduce the noise of the HVAC system, which is one of the main causes of interior noise.  
Various studies have been conducted to reduce noise in HVAC systems. [PERSON] et al. [1] utilized design changes such as fan casings and blade spacing in HVAC systems to reduce noise, whereas [PERSON] et al. [2] changed the internal shape of the air conditioner. [PERSON] et al. [3] studied sound absorbing material (SAM) attached to HVAC systems using natural materials such as jute felt. These studies focused on how to change the internals of heater and evaporator systems. Another way to dampen the noise level of an HVAC system is to install a silencer on the HVAC duct. [PERSON] et al. [4] produced a silencer with a Helmholtz resonator structure to reduce the noise of an HVAC system. A Helmholtz resonator was installed and tested in the HVAC system, but the silencer was developed without considering the flow noise. To reduce duct noise in HVAC systems, there is a need for a method of reducing noise using structures other than Helmholtz resonators.  
One method of reducing noise by attaching a silencer to a duct involves the use of acoustic metamaterials. Among acoustic metamaterials, there is a method that can manipulate the direction of sound waves called a metasurface [5; 6; 7; 8; 9]. The metasurfacemay have a structure similar to those of Helmholtz [10; 11; 12], quarter wave [13; 14], or hook-like [15; 16] resonators. Other metasurfaces include labyrinthine structures such as tapered [17] or space-coiling [18] labyrinthine types. These metasurfaces can manipulate the sound wave direction at the target frequency using the generalized Snell's law and phase delay [19].  
In this study, a metasilencer was developed using an acoustic metasurface to reduce noise in HVAC systems. The metasilencer was designed to restrain the travel direction of the target frequency sound wave into a U-shaped configuration using a metasurface. To make the installation space of the metasilencer advantageous, the unit cell was set in the labyrinthine structure, and the length and interval of the unit cell were determined using the generalized Snell's law and phase delay. Considering the influence of the flow noise of the HVAC system, the inlet of the unit cell has a curved structure, and the interior of the unit cell is filled with a small amount of SAM. The acoustic analysis results of the metasilencer confirmed the refractive effect of the noise reduction frequency domain and metasurface. In the speaker test, the noise reduction effect was confirmed by comparing the test results before and after the metasilencer installation, and an HVAC test was performed to confirm the noise reduction effect of the silencer, even in the case of the flow.