Among them, exhaust noise and fan noise are the two main sound sources. Understanding their composition mechanism and control methods is crucial to optimizing equipment design and reducing environmental interference. This article analyzes the composition of the two types of noise from a technical perspective and provides practical noise reduction solutions.
1. Exhaust noise: "Acoustic bomb" of high-temperature airflow
Exhaust noise is the most energetic part of the noise of
generator, and is mainly composed of the following four parts:
1. Airflow pulsation noise
When the engine is working, the exhaust gas is ejected from the exhaust manifold at high speed in the form of pulses, forming periodic pressure fluctuations. This pulsation is transmitted to the muffler through the exhaust pipe. If the muffler structure is not designed properly, the high-frequency noise will be directly radiated into the environment.
2. Combustion vibration noise
When the mixture explodes in the cylinder, the rapid change in pressure will excite the vibration of the engine housing and propagate outward through the exhaust system. This low-frequency noise (usually below 500Hz) has strong penetration and is the main source of noise when running at low speeds.
3. Eddy noise
When the exhaust gas flows in the exhaust pipe elbow, valve and other parts, turbulence is generated due to the sudden change of flow velocity, forming high-frequency howling. For example, a right-angle elbow without a diffuser may increase the noise by 8-12dB (A).
4. Mechanical radiation noise
The structural noise generated by the thermal expansion and contraction or vibration of the exhaust manifold, turbocharger and other components is transmitted to the unit casing through the connector, forming secondary radiation.
2. Fan noise: a "duet" of pneumatic and mechanical
Fan noise is a by-product of the unit's cooling system, and its composition is more complex:
1. Aerodynamic noise (accounting for more than 70%)
Rotation noise: The fan blades periodically cut the air to produce discrete frequency noise, frequency = number of blades × speed (RPM) / 60. For example, the base frequency of a 4-pole fan at 1500RPM is 100Hz.
Eddy noise: The separation of the blade boundary layer and the shedding of the wake vortex cause broadband noise, and the peak value is usually between 2000-5000Hz.
Resonance noise: When the blade passing frequency coincides with the natural frequency of the unit, it triggers aeroelastic resonance, and the noise increases sharply by 15-20dB (A).
2. Mechanical noise
Friction/impact noise caused by bearing wear and belt loosening is manifested as continuous "buzzing" or intermittent abnormal noise.
The electromagnetic vibration of the motor is transmitted through the fan bracket, forming low-frequency modulated noise.
III. Noise superposition effect and control difficulties
Exhaust and fan noise do not exist in isolation. The two are superimposed in the following ways:
Frequency coupling: The low frequency band of exhaust noise (100-500Hz) and the mid-frequency band of fan rotation noise (500-2000Hz) form a continuous spectrum, which increases the difficulty of noise reduction.
Spatial propagation: The exhaust port and cooling fan are usually located on the same side of the unit. The sound waves reverberate after reflecting on the wall, which increases the noise level on site by 3-5dB (A).
4. Scientific noise reduction scheme: from source to propagation path
1. Exhaust system optimization
Resistance muffler: adopt expansion cavity + perforated tube structure, impedance mismatch design for specific frequency bands (such as 250Hz, 500Hz), noise reduction can reach 25-35dB (A).
Flexible connection: install bellows between exhaust manifold and muffler to isolate vibration transmission.
2. Fan system upgrade
Bionic blade design: refer to the sawtooth structure of the leading edge of owl wings to reduce eddy current noise by 5-8dB (A).
Frequency control: automatically adjust the fan speed according to the load, and the noise reduction effect is significant at low load.
3. Whole machine soundproof cover
Adopt multi-layer composite structure (outer galvanized plate + middle layer damping coating + inner layer micro-perforated plate), sound insulation ≥30dB (A), and reserve intake and exhaust channels to prevent heat accumulation.
Conclusion
Generator noise control needs to take into account multidisciplinary knowledge of aeroacoustics, vibration engineering and thermal management. By optimizing exhaust pulsation suppression, fan aerodynamic design and sound insulation measures. If you are troubled by unit noise problems, it is recommended to entrust a professional team to conduct spectrum analysis. The customized noise reduction solution is more than 40% more efficient than general products.
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