Part I - Effects of Diameter on Active Noise Control in Rectangular and Round Ducts

Abstract

Abstract : Active noise control (ANC) is particularly useful in hard-walled ducts where plane waves propagate. Higher order mode waves are much more difficult to control. Basic acoustic principles dictate that tile cut-on frequency at which higher order modes will first begin to eclipse simple plane waves in a duct will be determined by the cross-sectional diameter of the duct. The lowest frequency for higher order modes will increase as duct diameter decreases. Therefore, the range of frequencies where plane waves dominate will be greater and effective control using ANC better as duct diameter decreases. The result is that somewhat higher frequencies can be controlled with ANC for smaller diameters. Below the first higher order mode cut-on frequency for the largest size studied, there should be little difference in ANC effectiveness between the duct sizes. To test those suppositions, a commercially-available ANC system was used to reduce random noise in rectangular and round ducts having different diameters. Results showed that insertion loss (IL) ranged from S to 30 dB in frequencies ranging from 40-1000 Hz, and varied inversely with size as expected. There was no difference in IL below 280 Hz (p=0.7751) between the different diameter ducts. There was a significant difference between duct diameters above 280 Hz (p<0.0001). The same tests were conducted on a rectangular duct with one cross-sectional dimension fixed and one varied at seven different sizes. Results showed similar IL from 5 to 30 dB that varied inversely with size. There was no difference in IL below 280 Hz (p=0.3348) between the different duct dimensions. There was a significant difference between duct dimensions above 280 Hz (p=0.0220).