Solution to Wind Turbine Noise
The causes of noise generated by fans are numerous; only by identifying the true cause of the noise can an effective solution be implemented. Essentially, a fan is a type of volumetric compressor, consisting of a pair of involute blades rotating at high speeds in opposite directions to transport gas. The main noise produced during the operation of a fan unit includes aerodynamic noise (i.e., airflow noise), transmission gear noise, motor noise, and pressure regulator noise. Among these, the aerodynamic noise is the most intense and impactful. It encompasses two components: the periodic exhaust noise caused by the cyclic pressure pulsations as the fan blades exhaust gas, and the vortex noise generated when gas vortices split at the interface of the fan blades. The intensity of the exhaust is primarily related to the blade speed, the exhaust flow rate of the fan, and static pressure, with the noise spectrum often presenting low to medium frequencies and certain noise peaks. Vortex noise, on the other hand, depends on the shape of the fan blades, the relative velocity of the airflow to the body, and the flow pattern, typically resulting in a continuous frequency spectrum of high-frequency noise.
When operating under certain conditions, high-intensity noise is radiated from the intake and exhaust ports of the fan, the housing, and the pipelines, among other locations. Under actual operating conditions, the fan's exhaust port is often connected to the gas pipeline and sealed. Consequently, the noise emitted from the intake port appears particularly loud, causing significant environmental disruption. Measurement shows that the noise level from the fan's exhaust port ranges from approximately 105 to 135 decibels for the "A" setting, while the "C" setting registers around 110 to 140 decibels.
The wind turbine not only exhibits a high noise intensity but also has complex and continuous frequency components. To quantitatively describe the distribution of its noise energy across the entire audio frequency range, octave band filters are commonly used in noise measurements. These filters analyze noise across eight frequency bands ranging from 63 Hz to 800 Hz, enabling spectral analysis tests to obtain the noise spectrum characteristics of the sound source, i.e., the relationship curve between octave band sound pressure levels and frequency. The characteristics of the noise spectrum curve include:
The noise spectrum is broad, meaning there is high noise across a wide frequency range, primarily composed of low to medium frequency noise. When the static pressure is low, or the load is small, the peak frequency often occurs around 125 Hz at low frequencies. When the pressure is increased to the rated static pressure, a new noise peak around 500 Hz in the medium frequency range appears. This indicates that as the working pressure of the fan increases, there will be a significant increase in medium to high frequency noise.
The noise intensity and spectral characteristics of a fan are not only related to the working static pressure of the fan but also significantly influenced by the fan's flow rate and rotational speed. As the flow rate increases, so does the noise level. Additionally, doubling the rotational speed can lead to a noise increase of approximately 5 to 10 decibels, with a more pronounced increase in mid-to-high-frequency noise.
