|Abstract or Summary
- The noise problem in air conditioning systems is receiving ever
increasing attention because of the importance of noise on a comfortable
human environment. Some work has been done on the solutions
to the noise problem by an investigation of a few system elements.
The acourstical characteristics of dampers become more important
with the present tendency to use high air velocity in the air distribution
system, but this element has not been investigated.
The experimental study presented here concerns the attenuation
characteristics of the duct section and damper, and noise generation
characteristics of a conventional single -blade damper and two proposed
airfoil dampers with solid and perforated surfaces respectively.
Air velocity in the duct, degree of damping, size, and damper configuration
were chosen as variables for this study.
During the performance of this experiment, two important preliminary
studies were done on the characteristics of an acoustic plenum
chamber and microphone windscreens. Because of the limited
fan capacity available, the construction of an acoustic plenum chamber
with acceptable performance was necessary. Because of the importance
of the wind noise effect on noise measurement, the construction
of a satisfactory windscreen was needed.
In the study of noise attenuation without air flow, perforated
airfoil dampers were found to be most effective in general. But, as
the flow damping increases, the effectiveness decreases at high
frequencies. At high frequencies, the single -blade dampers were
found most effective. Since high frequency noise control is much
easier and more inexpensive than low frequency noise control, the
airfoil damper is favorable as far as the attenuation without air flow
The attenuation spectrum did not change much at the middle frequency
range regardless of the type of damper, and the degree of flow
damping was found to be the most significant variable in changing
the attenuation spectrum. The rate of change of attenuation with respect
to damper position was not a linear function for any damper.
The attenuation for the perforated airfoil damper was not sensitive to
changes in damper position or configuration.
The results on noise generation showed that increasing the flow damping causes high noise generation. This result was most evident
for single -blade dampers. The increase in flow damping made the
spectra smoother for all dampers tested. There existed a frequency
range where the noise generation was most sensitive to the damper
setting. The sound level distribution of the generated noise level
was high at low frequency and low at high frequency, so the noise
generation results in a problem of low frequency noise control. An
increase in air velocity resulted in higher levels of noise generation,
but this did not influence noise generation as strongly as the degree
A result of this work showed that it might be possible to improve
the acoustical characteristics of dampers by combining desirable
property of both solid and perforated airfoil dampers. An airfoil
damper with perforated surface on the upstream side and solid surface
on the downstream side could show better acoustical properties.
Further research on the acoustical properties of other types of
dampers and other system elements should be done in the near future.
The design criteria necessary to improve the aerodynamic air flow
patterns should be a parallel study.
Much research will be necessary to provide the engineering profession
with the additional information needed to achieve a more comfortable
human environment through better air conditioning design.