In cities, subway noise is often cited as a major contributor to noise pollution that impacts millions of people every day. Previous studies on this topic have shown that peak subway noise levels in some cities can be as high as 110 dB, which greatly exceeds the 70 dB level set by the World Health Organization (WHO) and EPA for safe environmental noise levels. This work aims to characterize the subway noise in Hong Kong, analyze potential source features, and make technical recommendations for consideration by government and metro companies. First, the overall noise data on all nine subway lines in the city are presented and compared with published data of other subway lines around the world. Spectra of the loudest segments are analyzed to show the effects of tunnel modes, track curvature, and other features that may play a significant role in noise radiation and reverberation. A detailed correlation study is conducted for the short-time noise level and vehicle speed. A new train speed profile that optimizes noise exposure reduction is proposed for consideration of a future auto-drive system. Finally, a study of vibroacoustic exposure by passengers is also conducted to examine the impacts beyond the audible frequency range.

In this paper, the efficacy of porous ceiling treatment to reduce noise levels inside a typical tunnel is examined with a validated modal-based prediction method. It is found that, for a point source, the effect of increasing porous ceiling thickness on sound pressure level (SPL) attenuation along the tunnel is limited. A porous ceiling with thickness of 0.3 m is comparable with an infinite porous ceiling in middle and high frequency ranges. For a line source, the effect of ceiling thickness on SPL reduc- tion in this typical tunnel is limited. Sound pressure level reduction of 4 dBA is real- ized with 0.3 m porous ceiling, which is the same as infinite ceiling and only 1 dBA smaller than the theoretically optimized value. These results suggest that, in the event only ceiling treatment is considered, 0.3 m porous material is sufficient for noise re- duction in this typical tunnel.