This paper discusses a noise reduction structure with a thin Helmholtz resonator array integrated in louver elements. Using theoretical analysis, we derived the propagation constant and characteristic acoustic impedance with consideration for sound wave attenuation in the clearance between two planes for a thin cavity within the louver. The experiments with variation in three conditions were also performed: number of necks, internal thickness of the cavity, and number of partitions. Another experiment was conducted to verify that sound reduction characteristics change depending on the number of holes. The results closely matched with those calculated with consideration for attenuation, demonstrating the validity of the theoretical analysis. With respect to changes in the internal thickness of the cavity, experimental results showed that attenuation increased for clearances of 2 mm, causing significant differences in the sound reduction characteristics. At clearances of 8 mm, there was almost no attenuation in the clearance. Louvers with thin internal dimensions, in which holes diameter equal to the internal thickness of the cavity had been formed, created a larger effect than those with normal open end correction length. This resulted in a reduction in sound reduction frequency. Additionally, when cavities seen from the neck are created on the end face of the louver element as acoustic tubes, the properties of the acoustic tubes are added to the properties of the air spring. This achieved a significant reduction in the sound reduction peak frequency without increasing the volume of the cavity.