APPLICATION: Silencing of noise in the delivery of constant-speed compressors, blowers, and vacuum pumps.

Reciprocating compressors, rotary positive blowers, and vacuum pumps can deliver very high noise and pulsation levels. The predominant source of noise in reciprocating compressors is impulsive pressure fluctuations associated with gas induction and expulsion. Reciprocating compressor intake noise can be radiated directly into the atmosphere. Pipework connected to blowers or vacuum pumps contains pressure fluctuations which cause the pipe walls to flex and radiate noise. The less massive the piping, the greater will be the radiation. (Compressor casing is of relatively heavy construction, and consequently emits less noise than the pipework.)

Most of the noise is associated with the following fundamental frequency and its harmonics, inherent in the design of the compressor, blower, or vacuum pump:

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Fundamental frequency = RPMx no. of pulses per revolution    Hz


Octave band analysis tends to show that most of the noise is associated either with this fundamental frequency or with its second harmonic. Evaluation of compressor noise should use octave band or narrower band analysis. (Overall noise level measurements in dBA are unlikely to give adequate assessment.)

Silencing principles

The most effective attenuation is gained by fitting a silencer as close as possible to the noise source. On a Roots Blower, the silencer ideally is fitted directly to the inlet/outlet of the machine, without any intervening pipework which could radiate noise. The following points should be observed:

(a)   Flexible connectors can be used to isolate pipework from mechanical vibrations of the compressor/silencer combination. However, flexible connectors between compressor and silencer should be avoided.

(b)   The Compressor and pipework should be isolated from the support structure with rubber or other mountings, to avoid transmission of vibrations.

Compressor silencers

These silencers are reactive in type, but incorporate an absorptive packing. Reactive silencing is based on the principal that a discontinuity in flow section will reflect noise back towards its source. Reactive silencers therefore consist of a number of chambers interconnected with a choke tube, designed according to the required noise attenuation.

In addition to the reactive silencing, absorbant material is incorporated to reduce acoustic energy over a broad band of high frequencies. For smaller machines up to 42 m3/min, the standard silencers are available in light gauge construction. Larger silencers are specially designed to suit individual applications.

Separator silencers

In vacuum pump atmospheric exhaust applications, there are two basic problems:

  • (i) noise
  • (ii) entrained fluid in the evacuated gas

Noise and Pulsation Control engineers can provide a compact unit to combine noise attenuation with separation of entrained fluid.


(Primary separation - gravity stage)

Entrainment-laden gas enters through a choke tube, and heavier entrainments either drop by gravity or impinge against a head plate (whence they are thrown to the wall).

(Secondary separation-centrifugal stage)

The gas stream enters a separating element through vanes which impart a rotational flow, causing centrifugal separation.


The minimum thickness for shell and heads is 5mm. Carbon steel construction is used for standard duty. In pressure applications, silencers can be designed to pressure vessel codes such as BS5500 and ASME VIII. Construction can also be of stainless steel or other specified material.


Noise and Pulsation Control Ltd. model CRS silencers are guaranteed (subject to following our installation recommendations) to give agreed noise reductions at fundamental frequency up to 25 dB, and for integrity for a period of one and a half years after initial operation or two and a half years after shipment.

Application data

For a specific recommendation on silencer, a customer should provide the following information:

  • Gas
  • Specific gravity and ratio of specific heats
  • Flow rate (state the pressure and temperature for which flow rate is specified)
  • Operating pressure
  • Operating temperature
  • Allowable pressure drop
  • Connection size and type
  • (Roots Blower)                                      revolutions per minute
  •                                                            number of lobes/rotor

  • (Vane pump)                                         revolutions per minute
  •                                                             number of vanes

  • (reciprocating compressor)                     revolutions per minute
  •                                                             number of cylinders
  •                                                             number of stages
  •                                                             whether single-acting or double-acting
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