## 2 Literature Review
### Urban Rail Transit Noise Assessment and Control  
Urban rail transit noise assessment conventionally employs indicators such as A-weighted sound pressure level (A-weighted SPL), Z-weighted sound pressure level (Z-weighted SPL), equivalent continuous A-weighted sound pressure level (LeqA), and 1/3 octave band center frequencies [16; 17; 18; 37; 38; 39]. A-weighted SPL reflects the sensitivity of the human ear to different frequencies by applying A-weighting to the raw sound pressure level [7]. Z-weighted SPL provides a flat frequency response from 20 Hz to 20 kHz, accurately representing the sound without any gain or attenuation at any frequency [40]. Equivalent continuous A-weighted SPL describes the average sound level over a specified period, facilitating the evaluation of the overall impact of noise [40]. The 1/3 octave band center frequency method divides the frequency range into narrower bands, offering more detailed noise information compared to broad-band analysis [41]. Each indicator suits a different scenario, with A-weighted SPL for human auditory perception, Z-weighted SPL for precise sound measurement, equivalent continuous A-weighted SPL for long-term noise exposure, and the 1/3 octave band method for detailed frequency analysis.  
Evaluation methods in the research paradigms primarily include sound level measurement, spectral analysis, and temporal analysis [7; 40; 42]. Sound level measurement employs sound level meters to capture real-time sound pressure levels at designated locations and offers instantaneous and quantitative noise indicators, which are commonly utilized for routine environmental noise monitoring or preliminary noise assessments [18]. Spectral analysis allows researchers to identify and quantify the sound pressure levels within specific frequency bands by decomposing sound into its frequency components, revealing its frequency composition and providing a foundation for the in-depth analysis of noise sources, propagation characteristics, and their potential impacts on humans [36]. The temporal analysis concentrates on the temporal evolution of sound characteristics and capturing transient or prolonged variations in sound signals, which makes it well-suited for investigating non-stationary, intermittent, or transient noise patterns [43].  
Exposure assessment is an essential subfield of urban rail transit noise evaluation research. By estimating the noise exposure levels of individuals or groups over long or short periods in specific environments, researchers can quantify cumulative noise exposure [17].  
Data collection typically involves the use of wearable sound level meters or specialized noise dosimeters [43]. Propagation simulation employs computational methods to simulate and predict noise propagation in complex environments, providing preliminary insights for experiments that are challenging to conduct in real-world settings [44]. These simulations rely on acoustic modeling software and specialized knowledge for model development, validation, and analysis, thereby offering a scientific basis for formulating noise control strategies [20].  
Despite their widespread use in urban rail transit noise research, existing studies predominantly emphasize overall noise levels and macroscopic characteristics, lacking in-depth analysis of the spectral properties and temporal variability of different noise sources, which hampers the effectiveness of current noise assessment methods and mitigation strategies in addressing complex noise environments and renders precise noise control challenging. Therefore, conducting comprehensive studies on the specific characteristics of various noise patterns and integrating spectral and temporal analyses of multi-source noise is essential for enhancing the effectiveness of noise control.