92 ChaDter 4

for the smaller ducts. For midband frequencies, a 10 ft
length of ducting can account for 40 dB or 50 dB atten-
uation for ducts 12 inches x 24 inches or smaller. There
is a trade-off, however, as decreasing the cross section
of the duct increases the velocity of the air moving
through it. Higher air velocities produce greater turbu-
lence noise at the grille/diffusor. Great stress is
commonly placed on attenuation contributed by
right-angle bends that are lined with duct liner. Fig.
4-47 evaluates attenuation of sound in lined bends. Only
lining on the sides is effective, which is the way the
elbows of Fig. 4-47 are lined. Here again, attenuation is
greater at higher audio frequencies. The indicated duct
widths are clear measurements inside the lining. The
lining thickness is 10% of the width of the duct and
extends two duct widths ahead and two duct widths
after the bend. It is apparent that the lining contributes
much to attenuation of noise coming down the duct, but
less so at lower frequencies. Here too, there is a
trade-off. Every bend, lined or not, increases the turbu-
lence and therefore the noise.

-1

q 0 A-

24" X 36"

5

"63 125 250 500 1 k 2 k 4k 8k
Octave band center frequency-Hz
Figure 4-46. Measured noise attenuation in rectangular
ducts. (After ASHRAE, Reference 18, which attributes
Owens-Corning Fiberglas Corp. Lab Report 32433 and
Kodaras Acoustical Laboratories Report KAL-1422-1
submitted to Thermal Insulation Manufacturer's
Association.)

4.4.5 Tuned Stub Noise Attenuators

Fan blades can produce line spectra or tonal noise at a
blade frequency of

(4-5)

RPMx Number of blades

60 Hz

Blade frequency =

Usually this noise is kept to a minimum when the
HVAC engineer selects the right fan. If such tones
continue to be a problem, an effective treatment is to
install a tuned stub filter someplace along the duct.
These can be very effective in reducing fan tones. A
typical stub and its attenuation characteristic are shown

in Fig. 4-48A. The comparable characteristic of a reac-

tive muffler is also shown in Fig. 4-48B.

0 I I I I I I

Octave band center frequency-Hz

63 125 250 500 1 k 2k 4k 8k

Figure 4-47. Noise attenuation in HVAC square-duct

elbows without turning vanes. (After ASHRAE Reference

18.)

n

Duct

Frequency +

A. The tuned stub offers attenuation in a narrow band and

is useful in reducing tonal noise from HVAC equipment.

m 20r

42 2

Frequency +

B. The reactive muffler offers a series of attenuation peaks

down through the spectrum. These are useful in reduction

of specific noise components.

Figure 4-48. The tuned stub and reactive muffler used to

attenuate tonal components of noise.

4.4.6 Plenum Noise Attenuators

As previously stated, a most effective procedure in

noise reduction is to reduce the noise at, or very close

to, the source. If a system produces a noise level that is

too high at the sound room end, one possibility is to

install a plenum in the supply and another in the return

line. Such a plenum is simply a large cavity lined with

absorbing material, as shown in Fig. 4-49. Sometimes a

nearby room or attic space can be made into a

noise-attenuating plenum, usually at the source. The

attenuation realized from a plenum can be estimated

from the following expression: 19