Extending the receive performance of phased ultrasonic transducer arrays in air down to 40 kHz and below

We present a versatile ultrasonic one-dimensional (1D) air-coupled phased array transducer with a focus on its receive performance. In addition to its beam steering capability in transmit mode, it can be used to listen to ultrasonic waves at 40 kHz and below. We fulfill the half-wavelength criteria for the element spacing by introducing an additional layer, consisting of many tapered sound tubes (waveguides). These sound tubes separate the effective acoustic aperture from the actual receiving aperture of several ultrasonic transducers. This ensures that the array is not affected by grating lobes over a wide range of steering angles, and, thus, the array is capable of receiving directed ultrasound with only one primary receive direction for an adjustable angle (receive-beam forming). Based on this approach, we built and characterized a proof-of-concept prototype device. Our results prove that the array is able to receive ultrasonic waves over a wide range of 110° in total, with a receive sensitivity of -55.9 dB (0 dB equal 1 V/Pa). Thus, the device has huge potential for new sensory applications and the approach is compatible with micromachined ultrasonic transducers as well.