Handbook for Sound Engineers

Acoustical Noise Control 91

4.4.4 Attenuation of Noise in Ducts

Metal ducts with no linings attenuate fan noise to a
certain extent. As the duct branches, part of the fan noise
energy is guided into each branch. Duct wall vibration
absorbs some of the energy, and any discontinuity (such
as a bend) reflects some energy back toward the source.

A very large discontinuity, such as the outlet of the duct

flush with the wall, reflects substantial energy back

toward the source. This results in attenuation of noise

entering the room, as shown in Fig. 4-45. Unlike many

other systems in acoustics this is one attenuation that is

greater at low frequencies than at the highs.

Lining a duct increases attenuation primarily in the

higher audio frequency range. Fig. 4-46 shows

measured duct attenuation with 1 inch duct lining on all

four sides. The dimensions shown are for the free area

inside the duct. This wall effect attenuation is greatest

Figure 4-43. Typical paths by which HVAC noise can reach sound-sensitive rooms.

Return

supply

"B"

HVAC unit

1 5 3 6 8 9

10

11

12

2

4

7

"A"

Figure 4-44. Noise from HVAC equipment may be
reduced by isolation mounts, or it may actually be ampli-
fied. (After ASHRAE, Reference 18.)

12

6

3

0



2

3

6

9

Transmitted force–dB

1/4 1/3 1/2 1 2 3 4 5 6

Disturbing frequency fd

Natural frequency of isolator fn

No isolation

Force

amplified

Isolation

Figure 4-45. The effect of duct cross-sectional area on the

attenuation of HVAC noise. (After ASHRAE, Reference 18.)

20

15

10

5

0

63 125 250 500 1k 2k

Reduction loss–dB

Octave band center frequency–Hz

Duct cross-sectional area

25 in

2

100 in

2

400 in

1600 in 2

2

4k