Square duct muffler

Product Model	Rectangular Resistive Disc Muffler
Category	PP Machined Products
Reference Price	Price on request

Hot Customizable for Working Conditions 11 Technical Parameters

Product Overview

The square duct muffler is a noise reduction device installed in rectangular anti-corrosion ventilation ducts, designed to reduce fan and airflow noise. It has a rectangular box shape with an outer shell made of flame-retardant PP sheets folded and welded. Internally, multiple sound-absorbing panels filled with acoustic materials are parallelly arranged. As airflow passes through the channels between these panels, sound energy is absorbed and converted by the acoustic materials, thereby reducing the fan noise propagating along the duct. The rectangular cross-section matches that of the square duct, with square flanges at both ends for connection.

In anti-corrosion exhaust systems, noise generated by fan impeller rotation and airflow pulsation travels along the duct to the workshop and exhaust stack outlet, affecting the operating environment and compliance with factory boundary noise standards. Resistive mufflers are highly effective in reducing mid-to-high frequency noise and are the most commonly used form for fan noise control. The greater the number of sound-absorbing panels, the narrower the inter-panel channel, and the longer the muffler, the higher the noise reduction performance, but the corresponding pressure drop also increases. A balance must be struck between noise reduction and energy consumption. The PPs outer shell is acid and alkali-resistant and self-extinguishing, making it suitable for corrosive exhaust systems.

Xicheng Environmental can design and manufacture square pleated mufflers based on fan airflow, noise spectrum, and rectangular duct dimensions. The channel velocity, panel spacing, and muffler length are determined to achieve the target noise reduction. The sound-absorbing panels are protected by anti-corrosion perforated panels or corrosion-resistant facing materials to prevent fibers from being carried away by airflow. Square flanges and maintenance structures are provided at both ends, and the impedance compound form can also be combined to broaden the noise reduction frequency band.

Working Principle

The square muffler primarily utilizes the principle of resistive absorption. The internal sound-absorbing panels are filled with porous acoustic materials and have a perforated protective panel surface. As airflow carrying noise travels through the channels between the panels, sound waves enter the pores of the acoustic material, causing the air and fibers inside to vibrate. Sound energy is converted into minor heat energy through friction and viscous effects and dissipated, weakening the reflected sound waves. With each section of the sound-absorbing channel, sound energy is continuously attenuated, and the longer the muffler, the greater the noise reduction.

Resistive mufflers are particularly effective against mid-to-high frequency noise because high-frequency sound waves have short wavelengths and are easily absorbed by porous materials. They are relatively less effective against low-frequency noise. When low-frequency components are prominent, the thickness of the sound-absorbing panels can be increased, or an impedance compound structure (adding reactive expansion chambers or resonant chambers to the resistive channels) can be used to broaden the noise reduction frequency band. The PPs outer shell and corrosion-resistant facing ensure that the sound-absorbing material is not wetted or hardened by corrosive gases. The perforated facing panels prevent fiber dispersion, maintaining the stability of the sound-absorbing panel structure and preventing rapid degradation of sound absorption performance in corrosive environments.

Structural Components

The square muffler consists of a rectangular outer shell, sound-absorbing panels, protective facing structure, end flanges, and maintenance parts. The outer shell is formed by folding and welding PP sheets into a rectangular box with reinforcing ribs and is equipped with square flanges at both ends for connection to the rectangular duct. The sound-absorbing panels are long rectangular strips installed parallelly, filled with acoustic cotton or materials, and covered with a corrosion-resistant perforated protective panel and protective fabric to prevent the sound-absorbing material from being carried away by airflow.

The ends of the sound-absorbing panels can be designed as wedges or streamlined flow guides to reduce local resistance and regenerative noise as airflow enters the inter-panel channels. Several equally wide airflow channels are formed between the panels, and the panel spacing is determined based on the controlled velocity. The outer shell is equipped with inspection doors or removable end plates for easy inspection and replacement of the sound-absorbing material. For wide-frequency noise reduction, an impedance compound muffler can be formed by combining reactive chambers or micro-perforated structures with the resistive panel structure.

Specification Model Table

The following table shows the influence patterns of the main design parameters of the muffler, with specific calculations based on noise reduction requirements.

Parameter	Increase Impact	Design Principle
Noise Reduction Length	Noise reduction increases	Based on target noise reduction
Panel Spacing	Narrow spacing improves noise reduction	Balance flow velocity and pressure drop
Channel Velocity	High velocity causes regenerative noise	Control economical velocity

Product Features

The square duct muffler combines resistive and absorptive noise reduction, anti-corrosion and flame-retardant properties, and is designed for compatibility with rectangular ducts, making it a common device for fan noise control. Its main features are as follows.

Resistive panel structure, effective in reducing mid-to-high frequency fan noise
PPs board outer shell, acid and alkali-resistant and self-extinguishing
Rectangular cross-section with square flanges, directly compatible with rectangular ducts
Anti-corrosion and anti-flight protective facing for sound-absorbing panels, ensuring stable performance
Streamlined panel tips reduce resistance and airflow regenerative noise
Panel spacing and length are custom-designed based on airflow and noise
Can be configured as an impedance compound structure to broaden low-frequency noise reduction

Outer housing material	PPs Flame Retardant Boards
Section Shape	Rectangle
Sound Attenuation Principle	Resistance absorption as the main feature
Internal Structure	Parallel Silencer Sheet
Sound Absorbing Material	Porous acoustic absorption material
Face Protection Form	Anti-corrosion Perforated Protective Panel
Connection Method	Square Flange
Effective Bandwidth	Mainly medium and high frequency
Corrosion resistance performance	Acid and Alkali Salt Resistance
Flame Retardancy	Self-extinguishing
Spacing between chips	Based on noise spectrum design

Application Industries

laboratory rectangular exhaust fan muffler
electroplating workshop anti-corrosion exhaust fan inlet/outlet muffler
semiconductor plant rectangular exhaust duct fan noise reduction
chemical workshop indoor anti-corrosion fan noise control
ceiling-mounted rectangular exhaust dry pipe pleated muffler
factory boundary exhaust stack rectangular pipe muffler section
impedance composite muffler applications requiring low-frequency performance

Typical Installation Locations

Rectangular mufflers are installed between the rectangular ductwork and the fan or at the fan outlet pipe section. Common configurations involve placing an inlet muffler after the fan's flexible connection and an outlet muffler at the fan's exit, maximizing noise blockage along the ductwork in both directions. For workshop noise reduction, they are installed on the fan's inlet side, while for factory boundary exhaust stack noise compliance, they are placed near the exhaust stack on the outlet side. Flexible connections are used to isolate the muffler from the fan, preventing vibration transfer that could cause shell regenerated noise.

During installation, the muffler's orientation follows airflow indicators. Square flanges must be centered for airtight sealing, and the muffler's shell and ductwork should be independently supported. The muffler's weight must not rest on the fan or flexible connection. The flow velocity inside the muffler must be controlled within the recommended range, as excessive velocity can generate airflow regenerated noise, counteracting muffling effectiveness. Inspection doors should face the operational side. Regular checks are required to ensure acoustic panels are free of dust accumulation, moisture, or damaged protective surfaces. Dust-laden gases should be filtered or washed before entering the muffler to prevent acoustic hole blockage, which would reduce muffling performance.

Depends on the primary noise reduction target. Fan noise propagates simultaneously to the inlet and outlet ducts; install the muffler on the fan inlet duct section to reduce noise at the workshop and collection points; install it near the exhaust stack at the fan outlet to reduce exhaust stack outlet and factory boundary noise; install at both inlet and outlet for high requirements. In corrosion-resistant exhaust systems, the fan outlet is often connected to an exhaust stack, making the outlet muffler most common. Installation position should be as close to the fan as possible to minimize sound transmission through unsuppressed pipe sections and pipeline Recycled noise.

Resistive mufflers perform well on mid-to-high frequency noise but have limited effectiveness on low-frequency noise due to the long wavelength of low-frequency sound waves, which are not easily absorbed by conventional thickness acoustic absorption materials. For fan noise dominated by low frequency or high noise reduction requirements, thickening the muffler plates, increasing the bulk density of the acoustic absorption material, extending the muffler length, or adopting impedance composite mufflers utilizing the reactive principles of expansion chambers and resonance chambers for low-frequency noise reduction can be considered. It is recommended to conduct a noise spectrum test on the fan first and then design the muffler based on the spectrum to avoid selecting based solely on experience.

Attenuation level increases with length within a certain range, but not infinitely linearly, and length increase also raises resistance, occupies space, and increases cost. The attenuation level of resistive mufflers is approximately proportional to effective length, but high-frequency sound may be attenuated completely within the channel, rendering excessively long ends ineffective; there is also an upper limit failure frequency issue in inter-plate channels. In engineering, the required length is calculated based on target attenuation and octave bands, with segmented design generally being more economical and reasonable than single excessively long sections, determined by acoustic design.

Acoustic absorption materials exposed directly to acidic or alkaline mist will absorb moisture, harden, or even corrode, leading to pore blockage and reduced sound absorption capacity. Corrosion-resistant mufflers feature corrosion-resistant PP shells and perforated facings. The acoustic absorption materials are covered with a protective fabric to isolate droplets and dust, and moisture-resistant and corrosion-resistant acoustic materials are selected when necessary. Misted and dusty gases should be pre-treated with spray washing and de-misting before entering the muffler to reduce moisture and dust accumulation on the acoustic absorption materials. During maintenance, if the facings are damaged or the materials have hardened, they should be replaced promptly, otherwise the muffling performance will significantly degrade.

The pleated muffler exhibits both frictional resistance along the flow path and local resistance at the pleat ends. The magnitude of the resistance is influenced by the channel flow velocity, pleat spacing, pleat head shape, and length. Properly designed mufflers (controlling flow velocity, adopting streamlined pleat heads, ensuring the total cross-sectional flow area between pleats is not less than the duct cross-section) typically have resistance within acceptable ranges. If the pleat spacing is too small, the flow velocity is excessively high, or the muffler is overly long, the resistance will significantly increase, reducing system airflow, increasing fan energy consumption, and generating flow Recycled noise. During design, both noise reduction capacity and pressure drop loss should be calculated.

Please provide the fan model, airflow and pressure, rectangular duct cross-sectional dimensions, fan noise spectrum or sound power level (at least total noise value and speed), target noise reduction or factory boundary and workshop noise standards, installation location and available length, exhaust gas composition temperature and dust/fog conditions, flange standards and material requirements. Noise spectrum data is required to economically determine the silencer sheet structure and length; only providing duct dimensions can only estimate based on experience, with noise reduction difficult to guarantee. If installation space is limited, it should be stated in advance to adjust the number of sheets and layout.

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