Basic Documentation

Radiated Sound Attenuation
Siemens Building Technologies, Inc. 65
Radiated sound is essentially the breakout component of the sound generated by the device
and has the potential of entering the room primarily on a direct path. Most often this
component of sound energy must penetrate a dropped ceiling or other architectural features
before entering a room.
For air terminal units located above a dropped ceiling, the radiated sound component is quite
often (and ideally) attenuated by the dropped ceiling and the plenum effect of the space
above the ceiling. Table 25 gives the attenuation effect of the combination of a typical
dropped ceiling and the “plenum” space above it. Use these values to determine the net
sound power level reduction in the radiated sound power level of devices located a few feet
above a dropped ceiling.
Table 25. Attenuation of a Dropped Ceiling and Plenum on Radiated Sound Ceiling.
Ceiling 63 125 250 500 1,000 2,000 4,000 8,000
Hz Hz Hz Hz Hz Hz Hz Hz
1/2-inch
Thick Tiles
Fiberglass
4 dB 7 dB 8 dB 9 dB 10 dB 11 dB 14 dB 18 dB
5/8-inch
Thick Tiles
Fiberglass
5 dB 8 dB 10 dB 12 dB 13 dB 14 dB 16 dB 19 dB
5/8-inch
Thick
Gypsum
Board
10 dB 15 dB 22 dB 26 dB 30 dB 28 dB 30 dB 30 dB
After the radiated sound attenuation has been deducted from the radiated sound power level
of a unit, the resulting sound power level value should be compared with the discharge sound
power level that will typically occur at the supply air diffuser(s) present in the room. Table 3 in
Chapter 2 indicates the resulting sound power level of two sound power levels.
Note that in some instances, a sound producing device may be within the occupied room
itself, or there may not be a dropped ceiling. In such cases, there is no attenuation as
indicated in Table 25, and the radiated sound power level must be directly compared with the
discharge sound power level in the room. In this type of situation, it is possible that the
combined sound power level is mainly due to the radiated sound pressure level.