The common technique for suppressing grating lobes in an ultrasonic array consists in transforming a full array in a sparse array. This technique does not consider it the shape of the elements, or the distribution of a fixed number of elements around a defined space. In this work a new criteria for optimal array design is proposed, which is validated with the help of a simple array fabrication process based in ferroelectret technology. As well, the potential of ferroelectrets as active material for air-coupled ultrasonic transducer design is demonstrated.