HVAC Application
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HVAC Application |
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The low frequency noise
generated by the fan is attenuated by the active duct silencer. The
silencer contains only a single center baffle so there is minimal
restriction to flow. The system components were located on the main trunk
of the supply duct. The controller is mounted on the side of the duct.
Microcomputer hardware and advanced adaptive signal processing software are
used to generate inverse sound waves which destructively interfere with the
propagating noise. Each silencing system consists of a controller, input
and error microphones and one loudspeaker. The figure below shows how the
active duct silencer cancels unwanted noise. Upon initial start-up the controller generates a self-calibration noise in order to form a model or electronic representation of the duct. As the sound passes the input microphones the acoustic energy is converted to analog electrical signals which are transmitted to the controller. The electronics within the controller convert the signal to digital format. The controller model processes this digital signal to compensate for delay and amplitude changes from the input microphone to the loudspeaker. This signal is further processed to compensate for the frequency response of the loudspeakers, inverted to create a mirror image of incoming noise, converted back to analog and sent to the internal power amplifier. The power amplifier output is sent to the speaker module located on the duct. The loudspeakers convert the electrical signal to acoustical energy which cancels the ductborne sound. Sound that is not cancelled is detected by the error microphones which are downstream of the speaker modules. This error signal is transmitted back to the controller, such that by continuous adaptation, the cancellation of the ductborne noise is optimized. The system is adaptive. The controller compensates for source and environmental changes. The controller maintains optimal cancellation at all times by constantly adjusting for changes in the speed of sound and input and output transducer characteristics and sensitivity. The active duct silencer has been installed and monitored by qualified personnel to ensure optimal performance.
Current technology and cost
limit the application to primarily control the duct noise of VAV
(variable-air-volume) class of HVAC air handlers. Typical plant lengths
measure approximately 2 to 3 ft. at the intake section and 4 to 5 ft. at the
discharge section. Since each control is single input single output in
structure, each VAV fan required two controllers - one for the intake, one
for the discharge.
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Copyright 2005, Applied Signal Processing, Inc. |
