# Development and Testing of an Active Noise Control System for Urban Road Traffic Noise
## 2 Characterization of Road Traffic Noise
### Noise Characteristics Analysis
#### 2.2.2 Noise Characterization of Different Types of Vehicles  
In response to continuously collected road traffic noise, noise data from different types of vehicles passing by were extracted. Noise characteristics such as frequency, duration, and intensity were analyzed. This allows for a more precise analysis and assessment of the impact of individual vehicles on the overall noise level of the road section. For an individual vehicle, the speed at the measurement point significantly influences the noise  
Figure 2: Noise levels and spectrum chart at test points. generation. The experimental method in this study recorded a relatively concentrated distribution of vehicle speeds. Therefore, building upon the experiments and drawing from tests conducted by the U.S. Federal Highway Administration [40], research was conducted to supplement and investigate vehicle noise data under different vehicle types and speeds. The vehicle types were classified based on the categorization by the U.S. Federal Highway Administration into large, medium, and small vehicles. Large vehicles mainly refer to trucks and buses, medium vehicles include sedans and SUVs, while small vehicles encompass compact cars and motorcycles [41]. Figure 3 presents the noise spectrum of different vehicle types at various driving speeds. The above results indicate significant differences in the noise characteristics among small, medium, and large vehicles. Vehicle noise primarily originates from the engine, exhaust, the contact between the tires and the road surface, and aerodynamic noise. When comparing different speeds, at low speeds, the noise from motor vehicles is predominantly engine noise, while at high speeds, tire noise becomes the primary contributor [42]. Consequently, as the vehicle speed increases, the sound energy of noise shifts toward higher frequencies. However, the influence of speed on the noise frequency is limited, with the vehicle type having a pronounced impact. In terms of the effect of the vehicle type on noise frequency, in the low-frequency range (20-200 Hz), the noise in this frequency band is mainly composed of engine noise and mechanical vibrations. Small vehicles typically have smaller engines and lighter body structures, resulting in lower noise levels for small vehicles, while larger vehicles generate more significant vibrations and engine noise [43]. In the 200-2000 Hz frequency range, the aerodynamic noise generated by vehicles relative to air movement during travel is most pronounced. As vehicle speed increases, the corresponding aerodynamic noise also rises. Due to the typically superior body structure and aerodynamic design of medium and large vehicles compared to small vehicles, they may produce less aerodynamic noise during high-speed driving [44]. In contrast, smaller vehicles exhibit a more noticeable increase in noise within this frequency range as speed rises. In the 1000-4000 Hz frequency range, tire and road surface friction noise may predominantly occur. Medium and large vehicles often have greater weight and a larger contact area with the road surface, leading to stronger interactions with the road surface and, consequently, higher tire-road contact noise within this frequency range. Moreover, different types of vehicles generate noise with energy concentrated in varying frequency ranges. When the driving speed exceeds 40 km/h, the peak center frequency of the noise from small cars is 1000 Hz, with energy primarily concentrated in the 600-2000 Hz range. The noise from medium-sized vehicles has a peak center frequency at 500 Hz, with energy mainly distributed within 2000 Hz. For large vehicles, the peak center frequency is 63 Hz, with the majority of energy spread under 1000 Hz. Research has  
Figure 3: The spectrum of traffic noise at different vehicle speeds for three vehicle categories: small vehicles, medium vehicles, and large vehicles. shown that on highways with a high proportion of large freight vehicles, low-frequency noise is more prominent, with the sound energy mostly distributed around 400 Hz [45; 46].