The low-frequency noise generated by the vibration of the compressor in the machinery room of refrigerators is considered as annoying sound. Active noise control is used to reduce this noise without any change in the design of the compressor in the machinery room. In configuring the
control system, various signals are measured and analyzed to select the reference signal that best represents the compressor noise. As the space inside the machinery room is small, the size of a speaker is limited, and the magnitude of the controller transfer function is designed to be small
at low frequencies, the controller uses FIR filter structure converged by the FxLMS algorithm using the pre-measured time signal. To manage the convergence speed for each frequency, the frequency-weighting function is applied to FxLMS algorithm. A series of measurements are performed to design
the controller and to evaluate the control performance. After the control, the sound power transmitted by the refrigerator is reduced by 9 dB at the first dominant frequency (408 Hz in this case) and 3 dB at the second dominant frequency (459 Hz here), and the overall sound power decreases
by 2.6 dB. Through this study, an active control system for the noise generated by refrigerator compressors is established.
The mechanisms of feedforward and feedback methods were analyzed for active vibration control. A feedforward controller was designed in the frequency domain using optimal control theory. The feedback control uses the direct velocity feedback method. The two control methods were applied
to a plate, and the mechanisms were analyzed by examining the structural intensity map. In the case of the feedback system, the disturbance acting on the structure serves as a source, and the control force acts as a sink to reduce the vibration energy of the structure. On the other hand, in
the feedforward system, the energy is reduced by the destructive interference of the intensity generated by the disturbance and control force. In this case, when analyzing the vibration intensity of the structure, component by component, the intensity generated by the control force is interfered
with mainly the mutual power terms. They are the product of the force due to disturbance and the velocity due to control force, vice versa. Based on this analysis under the source-sink relationship of the feedback system, we confirmed that a higher control performance can be obtained by the
control force at a point where the structural intensity is in a more easily flow position.
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