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Sound reduction
Sound reduction can basically be achieved in
two ways: by absorption and by reflection.
There are various stages of sound reduction in a ventilation plant,
including:
* Sound reduction in straight ducts
* Sound reduction in ducts with internal sound insulation
* Sound reduction in bends and branch ducts
* Sound reduction in connection with exhaust into the chamber
* Sound reduction through changes in cross section
* Sound reduction through end reflection. At the point where a
duct enters a room, part of the
sound is reflected back to the duct. This acts as a sound reduction
for the room.
*Sound reduction through room absorption.
The difference between LW - LP can be considered as a sound reduction
due to room absorption.
* Sound reduction through sound absorbers. If the natural attenuation
in a system is not sufficient, sound absorbers must be added.
The degree of sound reduction as a result of the aforementioned
stages can usually be derived from tables, diagrams and the manufacturer’s
data. The following will examine sound reduction in ducts with
internal insulation since this a simple method of reducing noise.
Sound reduction can roughly be estimated as follows:
D = K . O/A . a^1.4 [db/m]
K = correction factor, approx. 0.7 at 250 Hz
O = absorbing circumference (m) A = free duct area
a= absorption coefficient |
Q/A will vary with the shape of the valve’s
cross section, and sound reduction can be
enhanced if the cross section is divided into
several ducts. This phenomenon serves as a
basis for inserting so-called bafflers in sound
absorbers to enhance sound reduction. |
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Sound calculations
Sound calculations to determine the required sound reduction in
the main sound absorber installed after a fan or, for example,
after a
damper. As a basis, use the room assumed to be most exposed. This
is usually the room with the shortest main duct stretch from the
fan to
the room. The following sound calculations must then be performed
item-by-item as shown in the example below for an injection system.
| 1. Use the room’s permissible
sound pressure
level from the injection system as a basis.
2. First, check that the selected valves meet
the requirement of item 1. If this is in order,
proceed to the next item.
3. The permissible sound pressure level from
the duct system can now be determined by
subtracting the value of item 2 logarithmically
from the value of item 1. As previously shown,
a diagram is used for this purpose.
4. Add sound reduction due to room
absorption to the value of item 3. (Standard
addition)
5. Add sound reduction due to end reflection to
the value of item 4. (Standard subtraction)
Having now proceeded from the room , past
the valve and into the duct system, the value
of item 5 corresponds to the permissible sound
power level in the ducts right after the valve
(since room absorption has been included).
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6. Sound reduction in the
duct, bends,
branches, etc. until the sound source is
reached.
The sum of the values in 5 and 6 (standard
addition) yields the permissible sound power
level just after the sound source.
7. Find the sound source’s sound power level
in the manufacturer’s catalogue.
8. The difference between the values in item 7
and item 6 yields the sound reduction required
in the sound absorber.
A calculation as shown above must be
performed for each individual octave band.
Sound is normally most critical at frequencies
of 250 Hz and 500 Hz. Accordingly, it usually
suffices to perform calculations for these
frequencies.
There is user-friendly software program
available for these types of calculations,
which greatly facilitates the work. |
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