Application of a micro-perforated panel absorber to reduce the curve squeal noise of railways

A micro-perforated panel absorber (MPPA) can reduce noise without the use of sound-absorbing materials. It is made of various materials and is poten- tially applicable in a variety of fields. However, the application of MPPAs is limited owing to the narrow absorption band. In a previous study, to enhance the sound- absorption performance of MPPAs, we proposed a parallel arrangement of multi- layer MPPAs. Based on the results of this previous study, we carried out the present study to apply an MPPA to the curve squeal noise of railways. Squeal noise, which occurs at approximately 500 to 5000 Hz, has high-level peaks. We designed and fabricated an MPPAwith the proposed structure according to the characteris- tics of the squeal noise. The noise reduction was analyzed after an MPPA barrier was installed in the field with a curve radius of 120 m. The reduction in noise level was approximately 17 dB(A) on the first floor and approximately 7 dB(A) on the fourth floor of the building. At 630 to 5000 Hz, the noise reduction level was at least approximately 5 dB(A). To analyze the noise absorption effect of the MPPA bar- rier, a simulation was carried out and subsequently verified by comparing the measurement results with the simulation results. In the simulation, the difference in noise reduction level between the MPPA and reflective barrier was analyzed. The noise reduction level of the MPPA barrier was approximately 7.5 dB(A) higher than that of the reflective barrier on the second and third floors of the build- ing at 630 to 5000 Hz. These results support the viability of MPPA application in re- ducing noise from curve squeal noise.