**1 Introduction**
In a pneumatic system, the intake and exhaust processes of the cylinder are regulated by the air distribution valve. The noise generated when compressed air is released into the atmosphere through the exhaust port of this valve is often less intense than the noise produced within other parts of the system. However, this noise can still cause significant harm to workers and the surrounding environment [2]. Given the widespread occurrence of such noise, its severe impact on health and the urgency of noise control, the author conducted extensive experimental research to better understand noise characteristics. In particular, the challenges and trends in muffler design for noise reduction were analyzed, aiming to clarify the vague understanding of mufflers used in noise control. This study seeks to provide practical solutions for effective noise management in industrial settings.
**2 Air Distribution Valve Exhaust Noise Characteristics and Muffler Requirements**
**2.1 Classification of Exhaust Noise**
Exhaust noise is a type of aerodynamic noise. Depending on the fluctuation of noise levels during the exhaust process, it can be categorized into three types: stable, periodic, and intermittent. Stable exhaust noise remains relatively constant throughout the process, such as the noise from an air compressor venting to the atmosphere. This type is dominated by high frequencies. Periodic exhaust noise, on the other hand, varies in intensity over time, typically observed in internal combustion engines, with low-frequency dominance. Intermittent exhaust noise features sudden, sharp pulses with long intervals between them, commonly found in cylinder exhausts via air distribution valves. It is important to ensure that the pump suction port is sufficiently large and does not extend below the liquid surface to avoid cavitation.
**2.2 Noise Characteristics of Air Distribution Valve Exhaust**
Taking the 20MN hot forging press clutch cylinder’s air distribution valve exhaust as an example, the measured noise results (as shown in Figure 1) reveal that higher cylinder pressure leads to greater noise levels across all frequencies. The sound pressure level increases, particularly in low- and high-frequency ranges. The spectrum exhibits a saddle-shaped curve. Low-frequency noise originates from a monopole source, while high-frequency noise comes from a quadrupole source. The initial noise is the highest, gradually decreasing until it disappears. Although low-frequency noise is more intense, the A-weighted sound level is below 80dB, while high-frequency noise reaches very high levels. Since A-weighted sound levels are widely used globally to assess human ear damage, the dominant noise from the air distribution valve is high-frequency, and thus, high-frequency noise control is essential.
**2.3 Muffler Requirements for Air Distribution Valve Exhaust Noise**
Based on the noise characteristics and real-world conditions of pneumatic equipment, the muffler should meet the following requirements:
(1) Excellent high-frequency noise reduction performance.
(2) Good aerodynamic performance, ensuring minimal resistance and no excessive delay in exhaust after installation.
(3) Strong resistance to harsh environments, including water, oil, and dust.
(4) Durable structure, cost-effective, easy to maintain, and long-lasting.
**3 Common Types of Noise Control Mufflers and Their Issues**
**3.1 Orifice Injection Muffler**
The theory of small-hole injection was proposed by researchers at the Institute of Acoustics, Chinese Academy of Sciences. By replacing a single large orifice with multiple small ones, high-frequency noise—less perceptible to the human ear—is reduced. The formula ΔL = 27.5 – 30lgD indicates that halving the hole diameter increases noise reduction by about 9dB. Holes of 1–3mm are generally preferred. If the holes are too large or spaced too closely, the noise reduction effect decreases. The center-to-center distance should be 5–10 times the hole diameter. For optimal performance, the open flow area should be 1.5–2 times the valve area. However, many factory-installed mufflers fail to meet these standards, leading to poor performance and frequent maintenance.
**3.2 Porous Diffusion Muffler**
A common type used in domestic factories, this muffler has a large number of small holes that break up the exhaust jet into smaller streams, reducing noise. However, if the holes are too large or closely spaced, the noise reduction is minimal. For instance, a muffler with 40 holes of 5.5mm diameter showed only a 2.5dB reduction. These mufflers are not recommended due to their limited effectiveness.
**3.3 Micro-porous Sintered Muffler**
This type uses sintered materials with tiny pores to absorb noise. While effective, they are prone to clogging due to impurities in the compressed air, reducing their lifespan. Regular cleaning is necessary, but even then, performance degrades over time.
**3.4 Resistive Muffler**
Resistive mufflers use absorbing materials to reduce noise. However, high-speed airflow can damage the material, causing it to degrade quickly. Additionally, turbulence and structural vibrations can increase noise rather than reduce it. In some cases, resistive mufflers may even amplify noise instead of suppressing it.
**3.5 Combined Resistive and Reactive Muffler**
These mufflers combine both resistive and reactive elements for better noise control. However, they require careful design to avoid issues like turbulence and structural vibration, which can reduce their effectiveness.
**4 Conclusion**
(1) The noise from air distribution valves exceeds national standards, posing serious risks to workers and the environment. Effective noise control is urgently needed.
(2) Exhaust noise can be classified into stable, periodic, and intermittent types. Air distribution valve noise is typically intermittent.
(3) The noise is dominated by high and low frequencies, with peak levels occurring early in the exhaust process.
(4) Mufflers must have good high-frequency noise reduction, aerodynamic performance, environmental resistance, and durability.
(5) Several muffler types exist, with small-hole injection and micro-porous diffusion being the most effective. However, regular maintenance is required to ensure long-term performance.
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