Handbook of air conditioning and refrigeration / Shan K

Example 19.1. A variable-air-volume box (terminal unit) receives conditioned air from an airhandling
unit through main and branch ducts. The VAV box is installed inside a ceiling plenum and
is connected to two slot diffusers by 10-in.- (250-mm-) diameter flexible ducts, each of which is 5 ft
(1.5 m) long. Information is available from manufacturers’ catalogs on the duct-borne fan noise at
the inlet of the VAV box, the radiated sound power level L w,rad, the discharge sound power level
L w,dis of the VAV box, and the sound power level of the slot diffuser L w,s. From measurements, the
transmission loss of the breakout noise through flexible duct wall and the insertion loss of the flexible
duct are known. These data are summarized as follows:
If the volume of the room is 5040 ft 3 (143 m 3 ), estimate the total sound pressure level in the occupied
zone with an average distance from the receiver of 8 ft (2.4 m). Ignore the fan noise transmitted
through the return duct system.
Solution
1. From Eq. (19.11), the room effect at the octave band center frequency of 125 Hz of this occupied
zone is
L pr � L wr ��5 log V � 3 log f � 10 log r � 25
Octave band center frequency, Hz
� �5 log 5040 � 3 log 125 � 10 log 8 � 25 ��8.8 dB
The room effect of other octave bands can be estimated similarly as follows:
SOUND CONTROL 19.27
125 250 500 1000 2000 4000
Fan noise at inlet, dB 65 63 61 52 46 45
Lw,rad, dB 60.5 55 50.5 46.5 41.5 37.5
Lw,dis, dB 63 60.5 47 47.5 54.5 52
Lw,s,dB 40 39 35 33 31 24
Flexible, TLout, dB � 8 � 11 � 14 � 17 � 20 � 25
Flexible, IL, dB/ft 0.50 0.85 1.48 2.2 2.04 1.64
Octave band center frequency, Hz 125 250 500 1000 2000 4000
Room effect, dB � 9 � 9.5 � 11 � 11.5 � 12.5 � 13.5
2. For path 1, after considering the influence of the environmental adjustment factor E f, one
finds that the radiated sound power levels of the VAV box L rad,r � L w,rad-E f, in dB, are as follows:
Octave band center frequency, Hz 125 250 500 1000 2000 4000
Reverberant field, dB 57.5 53 49.5 45.5 40.5 36.5
Because the transmission loss of the casing of the VAV box is rather high, the breakout of ductborne
fan noise through the casing is far smaller than L rad,r, and is ignored.
The sound pressure level in the occupied zone due to the radiated sound power level from the
VAV box can be calculated by subtracting the plenum ceiling effect from L rad,r and then converting
to the sound pressure level through the room effect.