Baffle chamber and industrial silencers

01

Choose the silencer type for the noise spectrum

The OSHA Technical Manual describes silencers as devices inserted in the path of a flowing medium, such as a pipeline or duct, to reduce the downstream sound level. It names four types: dissipative (absorptive), reactive (reflective), a combination of the two, and pneumatic or compressed air silencers. It says the type required depends on the frequency spectrum of the noise source and its operating conditions. [1]

A workplace noise limit can set the target noise reduction. Under US rule 29 CFR 1910.95, when sound levels exceed the permissible exposures in its Table G-16, such as 90 dBA for 8 hours a day, 95 dBA for 4 hours or 100 dBA for 2 hours, feasible administrative or engineering controls must be used. Personal protective equipment is required only where those controls fail to bring levels within the table. A hearing conservation programme is required when 8-hour time-weighted exposure reaches 85 dBA. A silencer on the noise source is one such engineering control. [3] [1]

Dissipative silencers surround the airflow passage with sound-absorbing material. OSHA lists very good attenuation from medium (500–2,000 Hz) to high (above 2,000 Hz) frequencies with low-to-medium pressure loss, but poor attenuation below 500 Hz and sensitivity to moisture and particulates. Reactive silencers use sound reflections and large changes in area. They give good low-frequency attenuation, can be designed to minimise pure tones and can be used at high temperature and in corrosive environments. Their drawbacks include a relatively narrow attenuation range, high-to-medium pressure loss, sensitivity to particulate and moisture contamination, and cost when built of corrosion-resistant materials or to a custom design. A combination silencer adds sound absorption to a reactive design for high-frequency attenuation. [1]

OSHA's list of silencer applications includes high-pressure gas regulators, air vents and blowdowns, internal combustion engines, reciprocating and centrifugal compressors, rotary positive displacement blowers, rotary vacuum pumps, industrial fans, HVAC systems, totally enclosed fan-cooled motors and gas turbines. For devices that exhaust compressed air to atmosphere, it says diffuser silencers can provide 15–30 dB of noise reduction but can cause unacceptable back pressure, and that pneumatic silencers need routine inspection because they plug with debris. It advises working with the manufacturer to identify all operational and physical constraints. [1]

Give the supplier the noise source and its octave-band spectrum, the gas, flow rate, temperature and pressure, the allowable pressure drop, the target noise reduction, the connection sizes and space available, moisture, particulate and corrosive content, and the codes that apply.

02

Check insertion loss, flow noise and pressure drop data

ASTM E477 covers laboratory tests of duct liner materials, integral ducts and in-duct absorptive straight and elbow silencers for building ventilation systems. It measures acoustical insertion loss, airflow-generated noise and pressure drop as a function of airflow. Reactive mufflers and silencers for uses other than ventilation are excluded from its scope. [2]

ASTM notes that a silencer's insertion loss varies with frequency and with the direction and speed of airflow. Because silencers obstruct the air path, pressure drop and airflow-generated noise must also be quantified, and both increase with air speed, so data are needed at more than one airflow. Laboratory results describe performance under controlled conditions, and installed performance may differ. [2]

03

Ask for octave-band attenuation at your source's tonal frequencies

The UK Health and Safety Executive (HSE) describes screw compressors supplying air to aeration tanks at a sewage treatment works that exposed maintenance engineers to A-weighted noise levels of about 100 dB. Noise generated inside the blowers escaped through the intake pipework, with a tonal peak in the 63 Hz octave band. Because conventional absorptive silencers were inefficient at these frequencies and disproportionately large, a twin-chamber reactive silencer was fitted in each air intake pipe: the first chamber was designed to resonate for maximum attenuation at the fundamental frequency, the second to reduce energy at the principal harmonic frequencies. [4]

Measured at 1 m from the compressor intake, HSE reports an overall reduction of 20 dB, with 37 dB in the 63 Hz band and 30 dB in the 125 Hz band, but 4 dB or less from 2,000 Hz upward. [4]

Editorial note: this matches OSHA's description of reactive silencers as good at low frequencies with a relatively narrow attenuation range. [1] Give the supplier an octave-band spectrum of your source, ask for predicted or measured attenuation in each band at your flow, and ask what pressure loss the silencer adds.

04

Casing, baffle chambers, lining and diffusers

Which part does the attenuating depends on the type: reflections and area changes in the chambers of reactive designs, absorption around the airflow passage in dissipative ones. [1]

05

What to settle with a silencer maker before pricing

The frequency-range row is the only one with a published figure, OSHA's 500 Hz boundary for dissipative designs; attenuation by octave band and pressure drop at your flow have to come from the supplier. [1]

06

Send the source, flow and limit data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Source: [equipment; noise spectrum]
Gas: [type; flow; temperature; pressure]
Target: [noise reduction; limit]
Pressure drop: [allowable]
Connections: [size; flanges; space]
Contaminants: [moisture; particulates; corrosives]
Codes: [applicable]

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