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The noise in hydraulic transmission systems is generated by the closed loop composed of the motor, hydraulic pump, valve accessories, and hydraulic pipes. In this closed-loop system, the hydraulic fluid in operation generates fluid noise due to the pulsations in flow rate and pressure, which cause cavitation, turbulence, and vortices; this noise is then transmitted to the support components, inducing vibrations that result in mechanical noise. Noise sources and their influencing factors in hydraulic transmission systems: Generally speaking, the noise generated by hydraulic transmission systems includes fluid dynamic noise, mechanical vibration noise, pipeline resonance noise, as well as resonance noise from components such as support structures and oil tanks caused by vibrations and noise. The motor of the electric machine is the power source, and motor noise is the main factor contributing to mechanical noise. The noise generated by the motor of an oil pump comes mainly from the aerodynamic noise produced by the cooling fan, as well as from vibrations caused by uneven rotation of the rotor, electromagnetic vibrations, the commutator and rectifier, as well as various components such as the housing and end caps. Noise of hydraulic pumps and hydraulic motors: mechanical noise. During the operation of hydraulic pumps and hydraulic motors, periodic fluctuations in oil pressure within each working chamber, along with sudden periodic changes in the pressure in those chambers, cause vibrations that generate noise. Insufficient oil supply leads to cavitation and erosion, resulting in intense vibrations. In gear pumps, where such forces are irregular, the noise generated by gear meshing increases as the load, speed, and surface roughness of the gears increase; this noise is closely related to the viscosity of the oil used. Fluid noise. During operation, both hydraulic pumps and hydraulic motors generate energy fluctuations to varying degrees; when these fluctuations encounter resistance, they are converted into pressure fluctuations, thereby causing forced vibration in the containers. Liquid noise is formed, which has a wide noise spectrum; its pulsation frequency is proportional to the number of flow rate fluctuations per unit time. Furthermore, the high-frequency noise of hydraulic pumps and hydraulic motors is caused by eddies, cavitation, or shocks that occur during operation; it has no regular pattern and its intensity is low. Hydraulic valve noise: Hydraulic valves with various valve bodies are moving components, and the spools of many valves rely on springs to function; as a result, vibrations occur easily, leading to mechanical noise. The mechanical vibrations generated based on the different functions within their system vary as well; the shock sound produced when the directional control valve changes direction is instantaneous ； The high-frequency vibration generated by the spool when the relief valve releases oil is continuous; if an external disturbance has a frequency that matches the damping frequency under those conditions, resonance will occur, increasing noise levels. The high-frequency noise caused by resonance has a frequency range of 1–8 kHz, a sound pressure level of over 90 dB, and distinct peaks. In hydraulic transmission pipeline systems, unstable flow of the oil occurs frequently. Such unstable flow is caused by certain components within the system or by external disturbances. Generally speaking, in a well-designed hydraulic system, this unstable state is only temporary; it will eventually give way to a new stable state after a short period, during which shock waves are generated inside the pipes. Under certain conditions, the combination of a pipeline with a pump or valve can result in continuous vibration of the fluid within the pipeline system. When the length of the pipeline is exactly equal to the resonance length, intense high-frequency noise is generated. Tank noise: The tank is a device with a large radiation area, and it is the main component responsible for transmitting and amplifying noise. Its noise is often generated by other devices, and it is relatively loud; if the hydraulic pump motor is mounted directly on the tank cover, the vibration of the pump or motor will cause vibration in the oil. For noise control in hydraulic transmission systems, it is necessary to select motors and hydraulic pumps appropriately. Motors and hydraulic pumps are the main sources of noise in hydraulic transmission systems. When designing hydraulic transmission systems, it is necessary to understand their performance characteristics and noise properties, and take appropriate measures. To reduce motor noise, it is necessary to select the appropriate fan shape in accordance with China’s standards for motor noise limits. Using a smaller fan can achieve better noise reduction. Generally speaking, changes in fan diameter are related to variations in noise. To reduce oil suction resistance, the oil filter should be cleaned and inspected regularly to prevent clogging. Depending on the region and season, different grades of working oil should be selected, or a proper preheating device should be used. Attention should also be paid to the configuration of the oil pipes; when arranging the suction ports of dual-pump systems in hydraulic systems, relevant guidelines should be followed. Otherwise, the oil is likely to flow toward the high-capacity pump and the low-pressure pump, which can lead to cavitation in the low-capacity pump. Additionally, if the fuel level in the tank is low and the oil filter screen is above the surface of the oil, the hydraulic pump will draw in a large amount of air in the form of vortices; therefore, the openings of the oil pipes should be submerged in the oil. To compare noise coefficients, it is generally possible to do so by changing the fan diameter. For example, reducing the outer diameter of the fan can lower noise. Additionally, using a single-backward-sloping airfoil plastic or a fan made of an alloy with high internal damping also yields good results. Noise caused by resonance between the blades and the number of air duct grooves can also be addressed by changing the number of blades. Secondly, it is important to reduce electromagnetic noise. The main measures include selecting motors with high precision to stabilize the power supply voltage, and promptly tightening loose components such as fan cover screws to prevent resonance. Sound suppression and insulation measures can be employed again; sound-insulating enclosures or reactive silencers can be used to reduce noise without affecting the motor’s ventilation and heat dissipation. Oil discharged from a pump with low pulsation still contains pulsations, and such pulsating flow can induce mechanical vibrations that result in noise; therefore, it is necessary to choose a pump with as low pulsation as possible. For example, for the same gear pump, internal gear pumps have much lower pulsation than external gear pumps. Try to prevent the formation of cavitation in the system. Cavitation in hydraulic transmission systems occurs mainly due to air dissolving in the oil or small bubbles forming and mixing within the oil. When the local pressure drops to the oil’s air separation pressure, a large amount of air dissolved in the working oil separates out, forming voids. If such bubbles enter a high-pressure area, they will burst, generating high-frequency shock pressures of large amplitude in that localized area, which causes high-frequency noise. This cavitation phenomenon is caused by excessive suction resistance of the pump; therefore, it is necessary to use a hydraulic pump with good suction performance, and at the same time make the suction pipe as short, thick, and straight as possible. Reducing or avoiding sudden changes in the oil suction pipe, as well as designing the tank structure properly, are effective ways to remove air bubbles from the oil. Usually, oil returning to the tank contains bubbles. As long as there is enough volume such that it is greater than or equal to the system’s maximum flow rate over two minutes, the time that the oil stays in that area can be increased intentionally, allowing the bubbles in the oil to dissipate fully. Preventing vibration in pipeline systems: The vibration of pipeline systems is related to its vibration characteristics and amplitude, as well as the pipeline length, diameter, material, type of support, and method of connection. To prevent resonance, it is necessary to avoid the natural frequency of the piping system, usually by keeping the excitation frequency within a certain range outside that frequency. In most cases, the excitation frequency cannot be changed arbitrarily; therefore, it is necessary to adjust the natural frequency of the piping system. For example, adding supports in appropriate locations to change the installation position of the valve body, and so on. For approximate calculations of the natural frequencies of piping systems, refer to relevant manuals; however, pipe supports should be designed to be mounted on sturdy frames as appropriate to the actual conditions. To prevent vibration in pipeline systems, cushioning, sound insulation, and vibration isolation measures can also be employed, such as using rubber hoses to separate the oil discharge pipes and their supports from adjacent components, covering the pipelines with sound-insulating materials, and increasing the rigidity of the pipes. Reduce the fluid noise of valves in the system: When the directional control valve changes direction, it causes the oil to shut off and open rapidly. Due to inertia, a hydraulic shock will be generated, whose magnitude depends on the rate of pressure change. Therefore, reducing the moving speed and appropriately extending the commutation time can reduce vibration. Installing tapered commutation valves, adding buffer devices, and using electro-hydraulic commutation valves can mitigate shock forces. The flow velocity noise of the relief valve is related to the geometric shapes of components such as the main spool and the main seat of the valve body. The jet flow passing through the throttle hole first loses part of its kinetic energy due to impact force compensation; therefore, an increased diameter helps enhance the ability to suppress the jet flow. The difference is also related to flow velocity noise. As the difference increases, the liquid flow injection angle also increases. In this way, the horizontal flow velocities of the liquid flow from the annular throttle orifice and the central jet can partially cancel each other out before rushing toward pressure compensation. Loss of some kinetic energy can reduce flow velocity noise, but overly sharp edges on the throttle orifice can increase noise; therefore, a low-noise relief valve with a cone-shaped valve core can be used. Due to its smaller size, it can effectively prevent turbulence at the throttle opening, thereby reducing noise. Due to changes in fluid force and flow velocity, the flow control valve causes the throttle valve to produce noise of varying intensity. If the pressures and flow rates before and after the throttle valve change significantly, in addition to an increase in flow-related noise, significant vortex noise will also be generated. Hierarchical throttling or the use of appropriate flow control check valves can help; vibrations and impacts mainly occur, and their intensity is related to the spring – if the spring is too stiff and the adjustment force is high, the vibration and impact noises will be louder as well. To this end, an appropriate spring stiffness should be selected to reduce the flow rate through the check valve, thereby lowering noise. Reducing noise from structural vibrations: When designing hydraulic transmission systems, in order to keep the structure compact and reduce the space occupied, pump motors are often installed on the oil tank. In this way, the vibration of the pump and motor can easily cause the oil tank to generate significant noise. To reduce vibration, it is best to place the motor and the pump separately, or to take sound-insulating and vibration-damping measures during installation. For example, rubber pads can be placed between the motor/pump and the cover to isolate vibrations; the oil pipes connected to the pump can be replaced with rubber pipes or pipes covered with sound-insulating materials, and elastic supports can be installed at appropriate distances to secure the pipes and increase the rigidity of the fuel tank. Conclusion: The operating conditions of hydraulic transmission systems are relatively complex. Since various hydraulic transmission systems differ greatly in terms of performance specifications, transmission methods, and structural features, the level of noise generated also varies significantly. Therefore, theoretically analyzing the causes of noise generation and control methods in hydraulic transmission often fails to yield expected results. It is necessary to adopt a variety of noise-reduction measures in a targeted manner, based on the specific structure of the hydraulic transmission system as well as its actual conditions of operation and use, in order to effectively control the noise in such systems.