The working principle of a steam silencer

Typical steam silencers feature a large number of small holes drilled in the wall at the end of the exhaust pipe; these holes replace a single larger opening while maintaining the total area of the outlet. When air flows through these holes, the spectrum of the jet noise shifts towards higher frequencies or ultra-high frequencies, which significantly reduces the audible components in the spectrum. This, in turn, lessens the disturbance and harm caused to humans – and this is what constitutes noise reduction

Now I understand. Thank you. It means that only the audible components have been reduced, and the sound hasn’t been completely eliminated

Could anyone recommend some books or standards related to silencers? I am extremely grateful

There are many types of silencers, but based on their noise reduction mechanisms, they can be divided into six main categories: resistive silencers, reactive silencers, impedance-combined silencers, microperforated plate silencers, orifice silencers, and active silencers. 　　Resistive silencers primarily use porous sound-absorbing materials to reduce noise. Fixing acoustic absorption materials on the inner walls of the airflow channel or arranging them in the ducts in a certain manner constitutes a resistive silencer. When sound waves enter a resistive silencer, part of the acoustic energy is converted into heat through friction within the pores of the porous material, thereby dissipating it and reducing the intensity of the sound waves passing through the silencer. A resistive silencer is like a pure resistive circuit in electricity, with the sound-absorbing material functioning similarly to a resistor. Therefore, people call this type of silencer a resistive silencer. Resistive silencers are effective at reducing noise in the medium and high frequency ranges, but less effective at reducing noise in low frequency ranges. 　　A reactive silencer is composed of tubes and chambers with discontinuous interfaces, functioning like an acoustic filter. Similar to electrical filters, each small chamber containing a tube represents a element of the filter; the mass of air in the tube corresponds to inductance and resistance in electrical circuits, and is referred to as acoustic mass and acoustic impedance. The volume of air in the chamber is equivalent to the electrical capacitance, and is referred to as acoustic compliance. Similar to electrical filters, each chamber with a tube has its own natural frequency. When sound waves containing various frequency components enter the first short tube, only those sound waves with frequencies near the natural frequency of the first mesh can pass through the mesh and reach the opening of the second short tube; sound waves with other frequencies cannot pass through the mesh and can only reflect back and forth within the chamber. Therefore, we refer to this structure that has a filtering function for sound waves as an acoustic filter. By selecting appropriate tubes and chambers and combining them, noise with certain frequency components can be filtered out, thereby achieving noise reduction. Resistant silencers are suitable for eliminating medium and low-frequency noise. 　　By combining resistive and reactive structures in a certain way, an impedance composite silencer is formed. 　　Micro-perforated plate silencers are generally made from pure metal sheets with a thickness of less than 1 mm; holes are drilled in these sheets using drills with a diameter of less than 1 mm, with a perforation rate of 1% to 3%. By selecting different perforation rates and cavity depths with varying plate thicknesses, it is possible to control the spectral performance of the silencer, enabling it to achieve good sound attenuation within the desired frequency range. 　　The structure of a perforated silencer is a straight tube with a closed end, on the wall of which many small holes are drilled. The principle of the orifice silencer is based on the spectrum of jet noise. If the total area of the nozzle is kept constant but replaced with many small nozzles, as the airflow passes through these small openings, the spectrum of the jet noise shifts towards higher frequencies or ultra-high frequencies, thereby significantly reducing the audible components in the spectrum and thus minimizing disturbance and harm to humans. 　　The basic principle of an active noise suppressor is to use electronic devices to generate another sound wave in the existing sound field, one with a magnitude equal to that of the original sound pressure but with an opposite phase, so that it can cancel out the original sound field within a certain range. This type of silencer is a set of instrumentation devices, mainly consisting of a microphone, amplifier, phase-shifting device, power amplifier, and speaker.

The steam silencer used by Nantong Jianghan Petrochemical features both resistive and absorptive mechanisms: the resistive mechanism relies on sudden changes in cross-sectional area, as well as the refraction and reflection of sound waves as they pass through small holes, thereby dissipating energy; the absorptive mechanism involves the use of sound-absorbing material to absorb energy. The impedance composite type has a noise reduction level of over 35 decibels.