Active attenuation of noise in ducts

Journal of Sound and Vibration (1978) 57(2), 308-309

ACTIVE ATTENUATION

OF NOISE IN DUCTS

Our paper [l] on the development of Swinbanks’ method of active attenuation of sound in ducts [2] described results obtained with a simple system. Further work to compensate for the phase and amplitude characteristics of certain of the system elements has led to a substantial improvement in the performance of the active absorber. In the present arrangement a spaced pair of capacitor microphones to sample the sound field is used, and also high fidelity loudspeakers, circuitry to compensate for the half-sine response of the microphone and loudspeaker pairs and a filter to correct the frequency response of the loudspeakers, so permitting operation in a frequency band which includes the loudspeaker mass-stiffness resonance. As the results show, the frequency range of operation has been significantly broadened with absorption being obtained from 30 Hz to above 200 Hz. Figure l(a) shows a typical absorption characteristic obtained with pure tones when the system was adjusted for optimum performance at 160 Hz. Also plotted are the points from I

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Figure 2. Attenuation of noise from a fan in 1/3rd octave bands for a system set at (a) 170 Hz, (b) 100 Hz. Air speed is 2.5 m/s. The solid lines represent the level with the absorber off, the broken lines are for the absorber on. 308 0022-460X/78/0322-0308 $01.00/O 8%1978 Academic Press Inc. (London) Limited

309

LETTERS TO THE EDIMR

Figure 9(a) of our earlier paper [l] to indicate the improvement. Figure l(b) shows the performance of the system set up at 100 Hz. Figures 2(a) and 2(b) give the performance of a modified version of the system when subjected to noise and airflow at 2.5 m/s from a centrifugal fan at the end of the duct. The solid lines represent the SPL in each +rd octave band with the absorber switched off, whilst the broken lines represent the level with the absorber in operation. For Figure 2(a) the system was adjusted for optimum performance at 170 Hz when the overall absorption for the given noise spectrum for bands 31.5 to 200 Hz inclusive was 15 dB. For Figure 2(b) the system was set up at 100 Hz and gave an overall absorption of 12 dB for the given spectrum over the same range. The performance with other noise spectra can be judged from the figures; in this connection note that it is possible to adjust the frequency of maximum absorption of the system to coincide with a spectrum peak. ACKNOWLEDGMENT This research project is being financed by the National Research Development poration.

Cor-

J. H. B. POOLE H. G. hVENTHALL

Department of Physics, Che(sea College, Pul ton Place, London S W6 SPR England (Received 30 November 1977)

REFBRENCES

1. J. H. B. POOLE and H. G. LEVENTHALL1976 Journal of Sound and Vibration 49, 257-266. An experimental study of Swinbanks’method of active attenuation of sound in ducts. 2. M. A. SWWBANKS 1973 Journal of Sound and Vibration 27,41 l-436. The active control of sound

propagation in long ducts.