Figure 7-2. Plan View of Supply Duct for Example

TM 5-805-4/AFJMAN 32-1090

Figure 7-2. Plan View of Supply Duct for Example.

Octave Band Center Frequencies

sufficient to lower the fan noise significantly.

Therefore the diffuser noise should be equal for all

63 1 2 5 250 500 1k 2k

4k

four outlets. Thus, in the center of the room it is

Room Factor

found that the required IL must be increased by

(sq/ft.)

450 600 450 500 600 600 600

the factor of 10log(4), or 6 dB. In this case the

sound pressure level in the room for all four

"Rel SPL"

-9 -11 -9 -10 -11 -11 -11

diffusers would be:

Total Lw (d.

Octave Band Center Frequencies

above)

60 58 44 35 40 40 23

63 125 250 500 1k 2k 4k

Lp-Octave band

Sound level

51 47 35 25 29 29 12

Lp (4 diffusers)

57 53 41 31 35 35

18

f. Step (6). In this step the Lp from e. above is

NC 30 criteria

57 48 41 35 31 29

28

compared with the NC 30 criteria.

Required

Octave Band Center Frequencies

Reduction

(Considering

63 125 250 500 1k 2k

4k

Four

Outlets)

0

5

0

4

6

0

Lp (one diffuser)

51

47

35

25

29

12

To provide the additional IL required for the fan

NC 30 criteria

57

48

41

35

31

29

28

noise in the 125 Hz octave band a 5 ft. long

Required

standard pressure drop muffler could be installed

Reduction

0

in the 60x24 duct in the fan room, or in the 12x24

duct leading to the classroom. The location of

This analysis shows that the sound due to the

choice would depend on the need for sound attenu-

operation of the fan just meets the selected goal in

ation in other portions of the duct system. The

the 125 Hz octave band. In addition the diffuser

sound in the 1,000 and 2,000 Hz octave bands are

sound, from one diffuser, would just meet the

due to the diffusers. Mufflers in the duct will not

criteria in the octave band centered at 2000 Hz.

attenuate this sound. For the diffuser noise one

However, for this classroom one should also con-

solution would be to increase the diffuser size, and

sider the total sound from all four diffusers. As the

this would require changing the diameter of the

two diffusers closest to the fan are at identical

diffuser drop from a 10 in. to 12 in. diameter

duct distances from the fan the sound of each

yielding lower sound power levels by the order of 8

diffuser can be assumed to be identical. Also the

to 10 dB.

added duct length to the next two diffusers is not

7-12