**1 Introduction**
In a pneumatic system, the intake and exhaust of the cylinder are regulated by an air distribution valve. The noise generated when air is discharged through the valve’s exhaust port into the atmosphere is often less intense than the noise produced within other parts of the system. However, this noise can still pose serious risks to workers and the surrounding environment, causing harm and pollution [2]. Given the widespread nature of this issue, its severe consequences, and the urgency for effective solutions, extensive experimental research has been conducted to study noise characteristics. This work specifically analyzes the challenges and trends in muffler performance for noise control, aiming to clarify misconceptions and develop practical strategies for industrial applications.
**2 Air Distribution Valve Exhaust Noise Characteristics and Muffler Requirements**
**2.1 Classification of Exhaust Noise**
Exhaust noise is categorized as aerodynamic noise. Depending on the fluctuation patterns, it can be classified into three types: stable, periodic, and intermittent. Stable noise remains relatively constant throughout the exhaust process, such as from an air compressor venting through a valve. It is typically high-frequency dominated. Periodic noise changes in intensity over time, as seen in internal combustion engines, and is usually low-frequency dominant. Intermittent noise features sudden, sharp pulses, with long quiet intervals between them, commonly found in cylinder exhaust via air distribution valves. For optimal performance, the pump suction port should be large enough to prevent submersion and ensure smooth operation.
**2.2 Characteristics of Air Distribution Valve Exhaust Noise**
Taking the 20MN hot forging press clutch cylinder’s air distribution valve as an example, measured noise results (as shown in Figure 1) reveal that higher cylinder pressure leads to greater noise levels across all frequencies. The spectrum shows a saddle-shaped curve, with low-frequency noise from monopole sources and high-frequency noise from quadrupole sources. The initial noise is the highest, gradually decreasing until it disappears. Although low-frequency noise is significant, the A-weighted sound level remains below 80dB, while high-frequency levels are much higher. Since A-weighted levels are widely used for assessing human ear damage, the noise from air distribution valves is primarily high-frequency. Thus, noise control must focus on high-frequency components.
**2.3 Muffler Requirements for Air Distribution Valve Exhaust Noise**
Based on the noise characteristics and real-world conditions, mufflers for air distribution valve exhaust must meet several criteria:
(1) Excellent high-frequency noise reduction capability.
(2) Good aerodynamic performance—low resistance and no excessive delay in exhaust.
(3) Strong resistance to harsh environments, including water, oil, and dust.
(4) Durable structure, cost-effective, easy to maintain, and long-lasting.
**3 Commonly Used Noise Control Mufflers and Their Issues**
**3.1 Orifice Injection Muffler**
The theory of small-hole injection, proposed by researchers like Professor Ma Dao, suggests replacing a large orifice with multiple small holes to shift high-frequency noise into inaudible ranges. The formula ΔL = 27.5 – 30lgD indicates that halving the hole diameter increases noise reduction by 9dB. Holes should be spaced 5–10 times their diameter apart, with a recommended hole size of 1–3mm. However, many factory mufflers fail to meet these standards, resulting in poor performance and frequent removal due to inadequate noise reduction.
**3.2 Porous Diffusion Muffler**
A domestic muffler with 40 holes of 5.5mm diameter was found to have a small spacing (b/d = 1.36), leading to poor noise reduction (only 2.5dB). While porous sintered and multi-layer wire mesh types offer good mid-to-high frequency noise absorption, the former is prone to clogging, requiring regular cleaning. The latter is more durable but also needs maintenance.
**3.3 Resistive Muffler**
Resistive mufflers use absorbing materials to reduce noise, but they often underperform due to high airflow velocity, structural vibrations, and contamination from oil and water. Many tests showed that actual noise reduction was far below design expectations, sometimes even amplifying noise.
**3.4 Anti-Muffler**
Made of metal plates, anti-mufflers are resistant to oil and water but struggle with high-frequency noise and turbulence. They can cause structural vibrations and flow instability, making them unsuitable for intermittent exhaust noise.
**4 Conclusion**
(1) Air distribution valve noise exceeds national standards, posing serious risks to workers and the environment. Effective control is essential.
(2) Exhaust noise is classified into stable, periodic, and intermittent types, with air distribution valves producing intermittent noise.
(3) Noise levels increase with pressure, and high-frequency components dominate.
(4) Mufflers must provide good high-frequency noise reduction, aerodynamic efficiency, environmental resistance, and durability.
(5) Small-hole injection and porous diffusion mufflers are most effective, but require proper design and regular maintenance.
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