Handbook of air conditioning and refrigeration / Shan K

Duct Rumble
Transmission Loss
sound power level in the ceiling plenum or the occupied zone, and is transmitted to the receiver.
Noise that radiates through the duct wall and causes the duct wall to vibrate, caused either by internal
sound waves or by airflow turbulence, is called breakout. Noise can also be transmitted into a
duct and then travel along the duct-borne path, either discharging into a space through the duct
opening or breaking out into ambient air, where it is transmitted to the receiver. The transmission of
external noise into a duct section through the duct wall is called break-in.
Breakout fan noise from the discharge duct directly under a rooftop packaged unit into the ceiling
plenum may cause a serious acoustic problem in the underlying conditioned space.
Duct rumble is the result of air pressure fluctuations caused by variations in the speed of the fan,
fan motor, or fan belt or due to the airflow instabilities transmitted to the fan housing or nearby
ductwork. When the air pressure fluctuations are exerted on large, flat, and unreenforced duct surfaces
that have resonance frequencies near or equal to the fluctuating frequencies, the duct surfaces
vibrate. Poor fan outlet connections often generate duct rumbles. Duct rumble can produce sound
pressure levels of 65 to 95 dB at frequencies from 10 to 100 Hz.
Changing the fan speed as well as the frequencies of the air pressure fluctuations is one method
to reduce duct rumble. Another method is to increase the rigidity of the duct wall and thus change
the resonance frequencies. Low-frequency duct rumble noise is very difficult to attenuate by duct
liners.
Transmission loss (TL) is the reduction of sound power when sound is transmitted through a wall, a
partition, or a barrier. The relationship between transmission loss and mass of the material indicates
that the sound transmission loss of a homogeneous solid partition is a function of its mass and the
frequency of sound f transmitted through it. This relationship can be expressed in the following
form:
where m s � surface density of homogeneous partition, lb/ft 2 (kg/m 2 ). Concrete walls and galvanized
sheet ducts show that for each doubling of the mass partition, TL increases about 2 to 3 dB
for low-frequency noise (less than 800 Hz), and TL increases about 5 to 6 dB for high-frequency
sound (800 Hz and over).
(19.7)
The TL-mass relationship is valid only when sound is incident on the surface of the partition in
a normal direction and is an approximation. Actual measured data may show considerable deviation
from the predicted values. Factors that cause deviations are nonhomogeneity, cracks, stiffness, and
resonance. Cracks and gaps around doors, windows, duct and piping sleeves, or other openings
on the partition or wall may considerably reduce TL. The TL of a well-sealed partition of 100 ft 2
(9.3 m 2 ) surface area may drop from 40 to 20 dB because of a total of 1 ft 2 (0.093 m 2 ) of openings
on that partition.
Breakout and Break-in Sound Power Level
TL � F(log m s, log f )
According to ASHRAE Handbook 1999, HVAC Applications, the breakout sound power level radiated
from the outer surface of duct walls L w,out, in dB, can be calculated as
L w,out � L w,in � 10 log
S
A i
� TL out
SOUND CONTROL 19.19
(19.8)