# Development and Testing of an Active Noise Control System for Urban Road Traffic Noise
## 2 Characterization of Road Traffic Noise
### Noise Characteristics Analysis  
#### 2.2.1 Overall Traffic Noise Analysis  
Continuously collecting 20 min of road traffic noise, recording various noise events, reflects the overall noise level of specific road sections or areas. This allows for an assessment of the short-term and long-term impact of traffic noise on the surrounding environment and community, enabling the implementation of corresponding noise reduction measures. Road traffic noise typically covers a wide frequency range as it comprises various sound sources, including noise from car engines, braking, tire-road contact, car horns, and aerodynamic resistance. These sound sources have different frequency ranges. Low-frequency noise typically falls within the range of 20 Hz to 200 Hz, usually including noise from large vehicle engines. Mid-frequency noise ranges from approximately 200 Hz to 2000 Hz, encompassing tire-road contact noise and noise from some vehicle engines. High-frequency noise primarily exists between 2000 Hz and 20,000 Hz and is mainly composed of car horns and braking noise [36; 37; 38; 39]. Conducting a spectrum analysis of the overall noise allows for  
Figure 1: Road traffic noise testing setup and locations, temperature, wind, distance, and height from road.  
the reflection of the amalgamation of all noise sources, accounting for various vehicle types, traffic flows, and vehicle speeds. Figure 2 illustrates the overall noise levels at five testing points and the spectrum analysis within the 1/3 octave frequency bands.  
The results indicate that vehicle speed may be a significant factor affecting the magnitude of road traffic noise. The vehicle speed on the secondary high-speed service road of the airport (Site5) is evidently higher than that at Site2, Site3, and Site4, and correspondingly, its noise level is also noticeably higher than these three sections. The vehicle speed at Site1 is also fast, but the measurement point is too far away for a direct comparison. Analysis of the noise spectrum reveals that while the spectral characteristics of different road sections generally follow the same trend, variations do exist. Peaks in the noise spectrum were observed at frequencies ranging from 50 to 80 Hz, 200 to 315 Hz, and 800 to 1250 Hz across the five testing sites. This primarily occurs due to the diverse types of vehicles and noise sources at each test point, leading to the manifestation of multi-peak characteristics in the frequency spectrum. Furthermore, at Site2, Site3, and Site4, there is a prevalence of lower-frequency noise in the range of 20 Hz to 200 Hz, surpassing the high-frequency noise between 200 Hz and 2000 Hz. This discrepancy can be attributed to the fact that these three test points are either auxiliary roads or urban expressways, where a significant number of vehicles such as electric vehicles, buses, and trucks, known to produce lower-frequency noise, pass through. In contrast, Sitel and Site5 represent urban high-speed roads where the predominant vehicles are small cars with higher speeds, resulting in fewer low-frequency noise sources. All test points exhibit peak effects in the range of 800 to 1250 Hz, mainly due to the friction noise between tires and road surfaces. Faster vehicle speeds lead to more pronounced effects of tire and road surface friction noise, explaining why Site5, the airport expressway, experiences significantly higher noise levels in this frequency range compared to the other test points.  
The above test results indicate that the type of vehicle and its speed have a significant impact on the generation of traffic noise. Therefore, an active noise control system may need to incorporate vehicle characteristic recognition capabilities to generate matching anti-noise based on the specific characteristics and speeds of different vehicles. Alternatively, it may focus on controlling the noise within a certain frequency range that is produced by various types of vehicles, such as the peak noise within the range of 800 to 1250 Hz.