Investigation of a mutual radiation impedance anomaly between sound projectors mounted in an array

Measurements of the electrical admittance of acoustically small, low loss, sound projectors near resonance show that the admittance diagram is a circle, indicating a constant radiation impedance, only when the measured projector is part of an array in which the sound field conditions are the same for each projector. In other arrays the individual projectors may suffer large changes in admittance as the driving frequency is varied, including the generation of negative conductance values near resonance.

The formation of this anomaly has been studied by investigating the behaviour of five A.D.P.-Glass projectors in a cruciform array in conjunction with computations based on an accurate knowledge of both the equivalent circuit of the projectors and the relevant radiation impedance data. This has shown that the anomaly is due to the high mutual impedance values involved and the fact that these values are dependent on the position of the projectors in the array. Near resonance, where the mechanical impedance of a high Q projector is approaching zero, this leads to large changes in the phase of the radiating face velocities of neighbouring projectors. This in turn produces excessive changes in the total radiation impedance and mechanical displacement of projectors working near resonance. In a practical system the behaviour may cause failure of the projectors or driving amplifiers.

Various palliatives or remedies are suggested to modify or overcome the anomaly.