Air duct design & noise reduction
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Type of room
Noise level
Commercial
Service
Grade I
Grade II
Grade III
Grade IV
45
45
55
55
Vestibule
Seasons hall
Grade I
Grade II
Grade III
45
45
45
Hairdressing/Barber’s room
45
Recreation facilities
45
Meeting room
Office
Grade I
Grade II
Grade III
Grade IV
（
30
）
40
45
55
Ballroom, non-operating time
Operating time
40
—
Note: In short, air duct/port design/type selection shall follow the 3 steps below:
1. Roughly calculate air duct cross sectional area and air port air delivery/return areas
according to rated unit air flow and suitable air speed;
2. According to “National common ventilating pipeline calculation table”, preliminarily
determine air duct size and dimensions; preliminarily determine air port size by referencing
relevant manufacturer product samples;
3. Calculate air resistance in air delivery/return system; further verify air duct by referencing unit
air exhaust static pressure; select correct type of air ports. When necessary, repeat these 3 steps.
Note: For higher user requirement on noise, silencers shall be installed on air delivery/return
ducts;
a. Resistive silencer. By means of sound absorbing material stuck on inner wall of air duct,
sound energy propagating along air duct is partly converted to thermal energy to reduce noise.
This type of silencer has good performance for medium and high frequency noise and
relatively poor effect on low frequency noise.
b. Reactive silencer. By means of sudden expansion, contraction and bypassing (resonance
cavity) of air duct cross section, noise propagating along air duct at specific frequencies and
frequency bands is reflected at points of sudden change. The type of silencers has good
performance for low and medium frequency noise.
c. Combined resistive and reactive silencer (wide frequency band silencer). This type of
silencer integrates advantages of above types and has good performance at low, medium and
high frequencies. Reduction of noise at low frequencies and some medium frequencies makes
use of reactive silencing by sudden change of pipeline cross sectional area and resonance
sound absorption by cavity. Reduction of noise at high frequencies and most medium
frequencies makes use of sound absorbing materials.
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