Selection of Sound Silencers

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.

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)
A´wtg	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 A´wtg 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 A´wtd 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.