Phased array transducer for emitting 40-kHz air-coupled ultrasound without grating lobes

We introduce an ultrasonic one-dimensional (1D) air-coupled phased array transducer, operating at 40 kHz without any grating lobes. It is well known, that this achievement is only possible when each transducer element is small enough for an element pitch that is equal to or less than half of the wavelength, i.e. ≤ 4.3 mm for 40 kHz operation frequency. As far as we know, conventionally low frequency array designs for air-coupled transducer applications were not possible at 40 kHz due to this inter element spacing requirement. The main idea is to separate the acoustic aperture from the actual radiating aperture of the ultrasonic transducer. We use acoustic waveguides, forming a smart packaging layer, to fulfill the half-wavelength criteria for the pitch and to benefit from concentrating the acoustic energy from several single ultrasonic transducers into a smaller effective aperture. A proof-of-concept prototype array was built and characterized. An impressive 130 ± 1 dB sound pressure level (SPL) is observed at a distance of 1 m. The opening angle of the array is 110° in total, without any grating lobes. For future work, we plan extending the approach to build an air-coupled fully populated 2D phased array transducer as well.