1. modeorders. curves for the cylinder with four m-1-4 groups of modes highlighted. There appears to be much more activity of the n=2 circumferential mode order than would be expected from theory5but the classic cylinder dispersion curves canclearlybe seen.
2. The first twotestswereperformed to obtain abase line for thebarecylinder andwhen lined with the foam. A comparison of the average vibration levels on the cylinder walls with and without the foam is shown in Figure 11. It can be seen that a substantial vibration reduction on the order of 10to 15dB occurs above 200 Hz, and reductions of 5-10 dB down to 100 Hz. It should be noted, however, that compared to the size of the cylinder, the thickness of the foam and the extent of coverageturned outto be excessive. The foam
3. Frequency (Hz) Figure 15:Noise reduction duetoDVAs the results shown in figure 16 the actuators on the end caps and 4 (two pairs) on the cylinder wall were used (four channels). Eight microphones were used as error sensors, and four accelerometers were used for the Referencesignals. The control system forthistest used 255 taps ineach of the 16 control filters, 200 taps for each of the 32 plant model filters and 150 taps for each of the 16 feedback removal filters. The sample rate was 600Hz and the controlbandwidthwas 40-200Hz. The results show that substantial reduction (4dB broadband and up to 9dB at resonances) in the interior sound level can be achieved using a feedforward active control system with accelerometer Referencesignals and structural actuators. The flanking problem is currently being addressed and these tests will be repeated without actuators ontheend caps.