Noise reduction treatment of cooling towers
The noise sources of cooling towers mainly come from four aspects: fan intake and exhaust noise, water spray noise, fan reducer and motor noise, and cooling tower water pump, piping and valve noise. Among them, falling water noise is a unique noise source of cooling towers. It is a steady-state water noise generated by the large-scale continuous liquid impact of cooling falling water on the pool water in the tower.

The sound source sound level of this noise is about 80dB (A), and the spectrum is mainly composed of high-frequency (1000-16000 Hz) and medium-frequency (500-1000Hz) components, with a peak at around 4000Hz. Due to the long wavelength of sound, strong penetration ability, and insignificant sound attenuation, it is difficult to control.

On the other hand, the turbulence and friction-induced pressure disturbances generated by the air in the top guide pipe of the cooling tower, as well as the vibrations generated by the blades and the air, will radiate noise outward. These aerodynamic noises are one of the main sound sources of cooling towers.

In addition, falling water noise is also a noise source that cannot be ignored. When the circulating water of the cooling tower falls freely through the packing layer to the water trough, impact noise will be generated, and its intensity is proportional to the square of the water speed. The measurement results show that the A-level noise of the falling water reaches 70dB, which is also one of the noise sources that need to be controlled.

Distance attenuation law of sound waves
The attenuation characteristics of falling water noise with distance follow the law of hemispherical surface wave propagation, that is, as the energy distribution expands, the sound wave gradually attenuates. The distance attenuation law of this “point sound source” is that when the distance to the edge of the sound source doubles, the sound energy will attenuate by 6dB. Specifically, the sound level difference can be expressed by the formula: L1 – L2 = 20 lg(r2/r1), where L1 and L2 represent the sound level values ​​of the measuring points at different distances from the edge of the sound source, and r2/r1 represents the ratio of the distances from the far and near measuring points to the edge of the sound source.

When the distance ratio r2/r1 is 2, the value of lg(r2/r1) is 0.3010, so L1 – L2 = 6 dB. This means that as the distance increases, the sound level will gradually decrease at a rate of 6dB.

In addition, the starting position of the cooling tower as a “point sound source” is also an important consideration. Based on actual measurement data and theoretical analysis, we can estimate the starting position of the “point sound source” under different cooling tower areas. Taking the 2,000 square meter cooling tower commonly seen in my country as an example, the starting position of its “point sound source” is about 11.18 meters away from the bottom edge of the air inlet. This means that at the noise measurement point 12 meters away from the tower, we can basically regard all cooling towers as “point sound sources.”

According to the distance attenuation law of the “point sound source”, we can further deduce the sound level at different distances. For example, the sound level at a distance of 50 meters will reach 65.7 and 71.1dB (A) respectively, while the sound level at a distance of 100 meters will drop to 59.7 and 65.1dB (A). Such evaluation results provide us with a general picture of the scope and intensity of noise impact, which helps us to formulate noise reduction measures more effectively.

Noise reduction principle
During the propagation of sound waves, when encountering obstacles, three effects will occur: reflection, transmission, and diffraction. The sound barrier uses this principle to isolate and absorb direct sound waves by setting up facilities between the sound source and the sound receiving point. In this way, some sound waves will be blocked and reflected, while the other part will reach the sound receiving point through additional attenuation methods such as transmission of the screen body (extremely small transmission amount) and diffraction of the screen top, thereby effectively reducing the noise impact of the sound receiving point and achieving noise reduction effect.

For the low-frequency noise generated by the fan, it is very important to choose a suitable muffler when controlling it. Usually, a single muffler is not effective in controlling medium and low frequency noise. In contrast, although the resistive muffler has a good control effect, its frequency selectivity is too single. Therefore, it is generally recommended to use an impedance composite muffler for control.
The impedance composite muffler, a muffler that cleverly combines sound absorption and sound reflection, not only has the ability of the resistive muffler to eliminate medium and high frequency noise, but also integrates the characteristics of the resistive muffler to eliminate low and medium frequency noise, thereby achieving a wide-band noise reduction effect.

Although it is relatively easy to control the high-frequency noise of falling water, special attention should be paid to avoid affecting the heat dissipation performance. Although silencers and silencer shutters can effectively reduce noise, heat dissipation and power performance must be considered comprehensively during design. Unreasonable structural design cannot achieve the purpose of noise reduction, and excessive flow resistance will affect the normal operation of the cooling tower and reduce its cooling capacity; at the same time, poor power performance design will also increase resistance and even produce reverberation noise. Therefore, various factors must be considered comprehensively and comprehensively during the treatment process.

In addition, several common cooling tower noise reduction methods are introduced.
Acoustic guide vane method (silencer elbow)
Installing silencer guide vanes at the air inlet of the cooling tower and using their silencing effect to reduce the impact of cooling tower noise on the surrounding environment is also called the muffler method. Theoretical and experimental results show that this method can achieve a noise reduction of 35dB (A) or even higher. When the noise reduction reaches 15-20dB (A), its cost is equivalent to that of a sound barrier; and when the noise reduction exceeds 20dB (A), it becomes the only feasible solution. In addition, the muffler guide plate method has the characteristics of compact structure, no additional space and low maintenance requirements.

Noise barrier

The noise barrier is usually designed to be higher than the air inlet of the cooling tower, and the height of the barrier is equal to the distance from the barrier to the air inlet. The noise reduction effect of this design is usually between 10-15dB(A), and theoretically can reach a maximum of about 20dB(A). However, due to the problem of sound wave diffraction, the noise reduction effect of the noise barrier in the sound shadow area is better, while the noise reduction effect in the diffraction area and the sound brightness area is relatively poor. Therefore, in actual engineering, it is difficult to reduce the noise in the affected area to below 20dB(A). In addition, the noise barrier has little effect on ventilation and is relatively simple to maintain. Although the technical requirements for the construction of the noise barrier are not high, the structural requirements are quite high, and the investment cost will increase exponentially with the increase in height.

Noise barrier method and its characteristics
As a common noise reduction measure, the design concept of the noise barrier is to reduce the impact of cooling tower noise on the surrounding environment through physical isolation. Usually, the sound barrier is constructed to be higher than the cooling tower air inlet, and its height is equal to the distance from the air inlet to the barrier. This design can effectively reduce the noise level to a certain extent, usually achieving a noise reduction effect of 10-15dB(A), and theoretically even close to 20dB(A). However, due to the diffraction phenomenon of sound waves, the noise reduction effect of the sound barrier is more significant in the sound shadow area, while it is relatively weak in the diffraction area and the sound brightness area. Therefore, in practical applications, it is often difficult to reduce the noise to below 20dB(A). Despite this, the noise barrier has little impact on ventilation and is relatively easy to maintain. Although its technical threshold is not high, the structural requirements are quite strict, and the investment cost will increase significantly with the increase in height.

Structural design of sound barrier
The structure of the sound barrier can be divided into two parts: above ground and underground. The above-ground part is a huge, continuous plate-type facade with a thickness of about 20 cm, which is specially used to shield sound waves and includes diagonal braces to enhance stability. The top is designed as a fan-shaped sound absorber or an inward-inclined eaves to further absorb and block sound waves. The underground part is the basis for load-bearing and anti-overturning, ensuring the stability of the sound barrier.

Noise reduction principle of sound barrier
The noise reduction effect of the sound barrier will be affected by the diffraction phenomenon of sound waves, which is closely related to the frequency of sound waves. Therefore, the sound insulation effect of the sound barrier will vary depending on the frequency of the sound waves. For high-frequency waves, due to their short wavelength and difficulty in diffraction, the shielding effect of the sound barrier is particularly significant, and a long sound shadow area can be formed behind the barrier. However, for low-frequency waves, due to their long wavelength and strong diffraction ability, the shielding effect of the sound barrier is relatively limited. Nevertheless, by increasing the height of the barrier, the impact of diffracted sound waves on the sound receiving point can be effectively weakened. Given that the noise of falling water in the cooling tower is mainly composed of medium and high frequency components, the application of sound barriers can achieve significant noise reduction effects to a certain extent.
The noise reduction effect of the sound barrier is most significant in the local area close to the barrier in the sound shadow area, up to 25 decibels. However, the noise reduction level outside the sound shadow area will rebound due to the arrival of mid-frequency diffracted sound waves. For high-frequency waves, the attenuation is usually still 10 to 15 decibels. However, it should be noted that since the noise of falling water in the cooling tower contains mid-frequency components, its noise reduction effect will be discounted accordingly. In order to improve the noise reduction effect of the sound receiving point outside the building, it can be adjusted by increasing the height of the barrier without obstructing the air intake.

When installing the sound insulation barrier, it is necessary to ensure that it is about 1 meter away from the cooling tower louver air inlet to ensure that the ventilation air inlet of the cooling tower is not obstructed, so as to maintain its good cooling effect. In addition, in order to prevent noise diffraction from affecting the acoustic performance of the silencer guide vane, a certain length of sound barrier can be installed near the silencer guide vane to assist in noise reduction.

On the other hand, the falling water silencing method is a method of reducing the noise source from the source.
It is achieved by installing falling water energy dissipation and noise reduction materials above the water surface at the bottom of the cooling tower. This method has the advantages of low initial investment and no effect on ventilation and heat dissipation, but the noise reduction is relatively small and may require continuous investment in maintenance. In addition, attention should be paid to the problems of easy damage to components, large maintenance workload and possible blockage of condenser pipes.

The “falling water energy dissipation and noise reducer” adopts hexagonal honeycomb inclined tubes as the main structure with a layer height of 18 cm. It consists of four functional sections: vertical inlet section, silent rubbing inclined section, viscous deceleration inclined section and evacuation and sprinkler section. At the same time, equipment such as silencer pads and spring shock absorbers can also be used to further enhance the noise reduction effect.