Investigation into the feasibility of using a parametric array control source in an active noise control system

Conventional Active Noise Control (ANC) systems that minimise pressure at a point in space have typically used loudspeakers as control sources, which are virtually omnidirectional within the low frequency range of interest. One obvious disadvantage of this is that locations remote from the desired control point may actually experience an increase in sound pressure level. The parametric array is capable of producing a highly directional beam of low frequency sound via the nonlinear interaction of emitted ultrasonic waves with air. Although significant research and development of the parametric array for use in audio systems has been undertaken, the feasibility of using a parametric array as the control source in an ANC system has not yet been fully investigated. Within this paper, the theory governing the operation of the parametric array and the resulting restrictions upon the production of low frequency sound are discussed. Experimental testing of a commercially available parametric array indicates that although highly directive low frequency noise of sufficient amplitude for some ANC applications can be produced, there are a number of practical concerns. The noise floor of the parametric array is high at low frequencies, resulting in a poor signal to noise ratio. The nonlinear nature of the sound production process means that the precise control in amplitude and phase required for ANC cannot, as yet, be achieved. In addition, there are concerns regarding the safety of the high amplitudes of ultrasound emitted by the parametric array. These limitations would all need to be overcome before the parametric array could be successfully implemented as a control source in an ANC system.