Witt und Sohn

SOUND REDUCTION / SILENCERS


We offer a complete range of silencers and sound reduction equipment. The product programme is modular and can be adapted to suit most needs. It can be assembled quickly and to competitive prices. Dimension sheets and Insertion loss tables give an overview over the most often used silencer types.

For a more detailed description of the physical laws governing sound engineering please refer to our paper "Technical Guidelines for Fan and Sound Engineering".

The follow gives a brief description of our sound reduction/silencers product programme and the various insertion loss tables and dimension sheets.

SOUND REDUCTION EQUIPMENT TYPES

Sound reduction can become necessary in order to reduce the airborne noise from or through ducts. In those case round or rectangular silencers are often used close to noise source. To minimise airborne noise in enclosed spaces complete or partial sound enclosures are being used.

Many times a combination is necessary, since unless an enclosure is combined with inlet and outlet silencers, the sound will travel through the ducts.

To reduce structure borne noise vibration attenuator may be necessary which we also manufacture.

Cylindrical Silencers

As a standard we offer 4 types of cylindrical silencers.

Two types with a pod, CPA-1D and CPA-2D and two types without a pod, CA-1D and CA-2D.

The "D" denominates the nominal length and the internal diameter of the silencers (the 2D silencers have length of double the diameter). Other dimensions are manufactured can also be accommodated. The insertion losses for other dimensions can be found by interpolating the tables.

Rectangular Attenuators

As a standard we offer 7 different modules of rectangular attenuators. The width of a module is varied between 250 mm and 400 mm and the length from 600 to 2400 mm. The modules are varied by the relationship between the silencer wall and the channel to achieve a continuos range of insertion loss values. A complete attenuator consist of a number of the same modules combined to match the desired flow rate and duct size.

SOUND ENCLOSURES

The sound reduction properties of a sound enclosures is mainly determined by it's design and assembly. A major factor is the material selection and the relative size of the air gaps between the panels.

To cover most applications we have three different sound enclosure types:

1. Sound insulation

The fan casing is clad with sound absorbing material. In most cases inlet/outlet silencers or heavy gadge ducting is required to achieve the desired effect.

1. Sound enclosure type A

Self supporting sound panels are combined with 2 snaplocks???and can be assembled/disassembled at site. A multitude of designs are available such as sound enclosure with free inlet, ducted at inlet/outlet and or forced ventilation.

1. Sound enclosure type B

As is the case with type B many different designs can be accommodated. The main difference is a supporting frame to provide additional rigidity and heavier gadge material. This type is more suitable for larger installations or installation outside and with type A higher noise insulation is achieved than.

In addition for special applications we offer enclosures with higher insertion losses, for example double walled enclosures.

EXPECTED INSERTION LOSS VALUES ACC. TO VDI 2711:

I. Sound insolation
IIa−IIc Normal sound enclosure with reduced air gaps
IIIa-IIIb Double walled enclosures with very reduced air gaps

For axial flow fans and some other type of fans it is furthermore important to ensure that the fan never will operate in it's stall area.

Insertion losses

The insertion losses given for the various silencer types are approximations. Differing values can be found due to flow turbulence, higher air gaps etc. Depending on the application tolerances must be taken into account.

Dimension sheets

The dimensions of our standard silencers (cylindrical and rectangular silencers) can be found at the end. Principle diagrams of the type A and B sound enclosures are also shown.

Apart from the shown types and dimensions we also manufacture a large number of customised silencers. Please inquire of the need arises.

SELECTION

The selection of a silencer can best be demonstrated by using an example. The method can be used irrespective of whether a cylindrical silencer, rectangular silencer or an enclosure needs to be dimensioned.

Example

Specification

A discharge attenuator is required to reduce fan noise to 85 dB(A) at 1 metre.

The Oktavband values of the fan are*:

Oktavband Hz 64 125 250 500 1k 2k 4k 8k
Sound power level 108 108 109 115 106 105 100 95

Volume flow rate is 0,97 m3/s

Maximum pressure loss through attenuator to be 125 Pa (N/m2)

* From Sound calculation, e. g. from WITT&SOHN Fan Selection Programm

Step 1

Determine the attenuator insertion loss from SWL and target noise level

Oktavband Hz 63 125 250 500 1k 2k 4k 8k  
Fan sound pressure level dB 108 108 109 115 106 105 100 95 (1)
Awtg db -26 -16 -9 -3 0 +1 +1 -1 (2)
SPL @ 1m dB -8 -8 -8 -8 -8 -8 -8 -8 *(3)
Target 85 dB(a)** dB -80 -80 -80 -80 -80 -80 -80 -80 **(4)
Approx. attenuator insertion loss dB - 4 12 24 18 18 13 6 (1)+(2+3+4)

* Assumes free field conditions (no reflections from the wall) over a hemisphere

** As a guideline: 5 dB less than the target noise level in each oktavband

Step 2

Select attenuator to achieve the required insertion loss

From the attenuator insertion loss tables select an attenuator that has an insertion loss equal to or greater than the required insertion loss in each octave. In this example, the attenuator type 364 has an insertion loss that closely matches the required insertion losses.

Oktavband Hz 64 125 250 500 1k 2k 4k 8k
Required l. loss dB - 4 12 24 18 18 13 6
Insertion loss of type 364 db 5 8 16 24 29 26 18 16

Step 3

Checking that the attenuator insertion loss meets the target noise level

Oktavband Hz 64 125 250 500 1k 2k 4k 8k
Fan sound pressure level dB 108 108 109 115 106 105 100 95
Awtg dB -26 -16 -9 -3 0 +1 +1 -1
SPL @ 1m dB -8 -8 -8 -8 -8 -8 -8 -8
Insertion loss dB -5 -8 -16 -24 -29 -26 -18 -16
Iwtd SPL @ 1m dB 69 76 76 80 69 72 75 70

Total by normal DB addition = 84 dB(A)

Step 4

Determine attenuator cross section to meet the pressure loss requirement

A maximum of 125 Pa pressure loss was specified. The 'K' factor from attenuator insertion loss tabe − for 364 it is 5.9. From the rectangular attenuator pressure loss graph follow the diagonal 'K' factor line equivalent to 5.9 till it meets the 125 N/m2 (horizontal) pressure drop line. From this point go vertically down the graph to the attenuator face velocity in m/sec. In this example, 5.8 m/sec. An attenuator cross section can now be calculated from the required face velocity of 5.8 m/sec and the air volume at 0,97 m3/sec. The (minimum) cross-sectional area in this example is therfore 0,97/58%nbsp;m2 = 0,167 m2.

For a cylindrical attenuator this can be seen directely for the graphs for CPA-1/CPA-2D below.