Handbook of air conditioning and refrigeration / Shan K

Then the sound pressure level in the occupied zone due to the slot diffuser can be estimated by subtracting
the room effect from L s,r.
6. The resultant estimated sound pressure level in the occupied space L p,t, in dB re 20 �Pa, is the
sum of the contributions of all sound paths and can be calculated as follows:
These resultant sound pressure levels at the center frequencies of various octave bands are all equal
to or below the NC 40 curve.
19.9 ROOFTOP PACKAGED UNITS
Basics
Octave band center frequency, Hz
125 250 500 1000 2000 4000
Ls,r, dB 37 37 34 32 30 23
Room effect, dB � 9 � 9.5 � 11 � 11.5 � 12.5 � 13.5
Octave band center frequency, Hz
125 250 500 1000 2000 4000
Path 1 43.5 34.5 28.5 22 14 8
Path 2 44.5 35 26.5 13 8 0
Path 3 44.5 35 26.5 13 8 0
Path 4 52 48 40.5 28 29 27.5
Path 5 52 48 40.5 28 29 27.5
Path 6 28 27.5 23 20.5 17.5 9.5
Path 7 28 27.5 23 20.5 17.5 9.5
Lp,t, dB 56 51.5 44 32.5 32.5 31
SOUND CONTROL 19.29
Sound pressure level, dB 28 27.5 23 20.5 17.5 9.5
Rooftop packaged units are widely used in low-rise commercial buildings. Because of the use of
large rooftop packaged units, many rooftop packaged units are curb-mounted on lightweight roof
deck construction. The distances between the fan and ceiling plenum, and between the fan and conditioned
space, are sometimes shorter than the distance between the fan in the AHU and space.
They may cause unique noise control problems. Objectionable noise from a rooftop packaged unit
usually includes direct transmission of fan and compressor noise through the building roof and ceiling,
structure-borne vibration, duct-borne fan noise, duct breakout noise, and duct rumble.
Locating the rooftop packaged unit at least 25 ft (7.6 m) horizontally away from areas with critical
sound control requirements, such as conference rooms and executive offices whose NC curve
should be 35 dB or less, is the most effective and economical method of noise control. A rooftop
packaged unit can be located over buffer spaces, low-use storage areas, high-noise loading areas,
and acoustically nonsensitive spaces such as rest rooms and stairwell buffer spaces.
Using a number of small packaged units instead of one or two large units divides the impact
over a greater roof area and is sometimes beneficial. A rooftop packaged unit with a lower sound
power level is often one size larger than the smallest unit for that capacity. The fan in such a unit is
properly sized, with less restricted inlet and outlet conditions inside the unit.