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Treating low-frequency compressor noise

The problem

Air compressor with combined reactive and absorptive silencer

Large reciprocating air compressors producing high noise levels at low frequencies can be found in a wide range of manufacturing plants. In one company, two compressors operating at 370 rpm (revs per minute) drew air from the roof where the intake filters and other items of equipment needed regular attention. The A-weighted noise level near the air intakes was found to be 88 dB, determined almost entirely by energy in the 63 and 125 Hz frequency bands.

Absorptive silencing was discounted as inefficient at these frequencies. It would also have been impractical as the size of silencer necessary would have meant a need for additional roof support.

The solution

The solution was to install an intake silencer that could combine adequate low-frequency attenuation with low resistance to the air flow.

A combined reactive and absorptive silencer was selected for each compressor. The silencers consisted of an all-welded outer tube with a cross-sectional area at least 15 times greater than that of the system side pipework. This change in cross-sectional area resulted in a loss of sound energy due to wave reflection and cancellation. An acoustically absorptive lining within the enlarged silencer casing further reduced the noise level.

The cost

About £2500 each. (1995)

The result

The attenuation achieved in this example with a 200 mm diameter intake pipe and an 800 mm outside diameter silencer casing with an overall length of 3000 mm was sufficient to render the compressor noise effectively inaudible above other sources in the working area. The residual noise of adjacent equipment masked the full performance of the attenuator at the higher frequencies.

The sound pressure levels are shown in the table, along with the actual octave band sound reduction levels. Overall there was a 17 dB reduction and an attenuation of 35 dB at 31.5 Hz.

Sound pressure level at 1 m from the compressor intake:

A-weighted Octave band centre frequency (Hz)
31.5 63 125 250 500 1000 2000 4000
Before treatment 88 115 111 98 77 77 73 69 66
After treatment 71 80 80 76 66 68 65 58 56
Attenuation 17 35 31 22 11 9 8 11 10
Updated 2010-04-02