Document ID: chunk:federal_register_of_legislation:F2016L00426:reg:10:p3
Version: federal_register_of_legislation:F2016L00426
Segment Type: reg
Provision Reference: reg 10 (pt 3/5)
Character Range: 92164–94973

buy self-adhesive steel sheet. The latter can simply be stuck on to existing components (inside or outside) covering about 80% of the flat surface area to give a 5 - 25 dB reduction in the noise radiated (use a thickness that is 40 per cent to 100 per cent of the thickness of the panel to be treated).
 Limitations: the efficiency falls off for thicker sheets. Above about 3mm sheet thickness it becomes increasingly difficult to achieve a substantial noise reduction.

  2.      FAN INSTALLATIONS

Typical applications - Axial flow or centrifugal fans.
Technique
Maximum fan efficiency coincides with minimum noise. Any fan installation feature that tends to reduce fan efficiency is therefore likely to increase noise. Two of the most common examples are bends close to the fan (intake side in particular) and dampers close to the fan intake or exhaust.
Ideally, for maximum fan efficiency and minimum noise, make sure there is at least  2 - 3 duct diameters of straight duct between any feature that may disturb the flow and the fan itself.
Noise reductions of 3 -12 dB are often possible.

  3.      DUCTWORK
Typical applications - Extraction, ventilation, cooling, openings in walls and enclosures.
Technique
Instead of fitting silencers, it is often possible to achieve a 10 - 20 dB reduction in airborne noise from a duct or opening by lining the last bend in the ductwork with acoustic absorbent (foam or rockwool / fibreglass). Alternatively, construct a simple absorbent lined right-angled bend to fit on the opening. Ideally, either side of the bend should be lined along a length equivalent to twice the duct diameter. Where flow velocities are high (> 3m/s), consider using cloth faced absorbent. Duct vibration can usually be treated by damping (as above).

  4.      FAN SPEED

Typical applications - Axial or centrifugal flow fans.
Technique
Fan noise is roughly proportional to the 5th power of fan speed. So in many cases it is possible to achieve a large noise reduction from a small drop in fan speed by changing control systems or pulley sizes and re-setting dampers. The following table provides a guide to the trade-off that can be expected.

FAN SPEED REDUCTION  NOISE REDUCTION

10%                  2 dB

20%                  5 dB

30%                  8 dB

40%                  11 dB

50%                  15 dB

  5.      PNEUMATIC EXHAUSTS

Technique
Almost invariably it is possible to reduce pneumatic exhaust noise permanently by 10 - 30 dB by fitting effective silencers. The following are the practical points that can make the difference between success and failure:-
  — back pressure : fit a larger coupling and silencer
  —  clogging : fit a straight-through silencer that cannot clog (and has no back pressure)
  —  multiple exhausts : manifold them into a