SOUND FIELD IN A ROOM AND ITS ACTIVE NOISE-CONTROL

The classical normal-mode solution of the steady-state sound field in a room is revised by the addition of a direct wave term to the summation of normal functions, which represents only the reverberant sound field. It is shown that the latter as a whole may be reduced by using an active noise control system with both the pick-up microphone and the cancelling loud-speaker in the corner region of the room, well within a quarter wavelength to the three adjoining surfaces. The corner ANC system appears as the best solution of low-frequency noise reduction in the room, notwithstanding that the reduction is limited by the direct wave radiation of the secondary source. A general formula of possible noise reduction of such a system is proposed and the means of its realization discussed Low frequency reduction of 8 dB has been realized for a band noise, centring at 100 Hz. The performance of the corner ANC system is independent of the shape, size and content of the room, provided that it is reverberant. Further development of this idea is discussed.