Beam-steering acoustic warning system of electric vehicles by using pipe-connected acoustic array

Abstract

In slow-moving speed under 30 km/h, electric vehicles (EVs) are very quiet. Due to its low detectability, it induces safety issues for other road users, especially for the visually impaired communities. Moreover, numerous global regulations have recommended the minimal noise of EVs. Hence, a warning sound is urgent to be installed. A steerable directive warning sound system can focus a sound beam to the targeted pedestrian while minimizing the radiation to other directions. By using a pipe-connected acoustic array installed in the front part of a car, beamforming is realized. The intended working frequency ranges from 1.25 kHz to 5 kHz. A simple point source is realized by having a pipe-connected loudspeaker system. By having a small inner diameter, i.e. 8 mm, the directivity factor can be neglected thus the far-field radiation is approximated as omnidirectional. The piping system can be modeled via an equivalent acoustical circuit. However, a peak difference of 50 Hz is observed between the prediction and the measured value. Broadside array alignment is suitable for the practical use of vehicle warning sound. Such alignment allows control of the sound beam projection in a 2D plane. An optimal beamforming filter optimizes the directivity index of the beam by minimizing the point source power maintaining the sound power at the steering angle. The filter can be simply calculated because the pipe-connected loudspeaker system, which is the element of the array, is omnidirectional. The array is designed for having seven elements with an adjacent separation distance of 70 mm each. By calculation, within the intended working frequency and steering angle ranging from -45$^\circ$ to 45$^\circ$, the directivity index above 6 dB is achievable. The 3-dB bandwidth is expected to be less than 25$^\circ$. The array is evaluated in 1/3 octave band analysis for four certain steering angles: 0$^\circ$, 15$^\circ$, 30$^\circ$, 45$^\circ$. The main lobe of the sound beam is projected at the correct steering angle with an error of 0$\pm$2.5$^\circ$. Meanwhile, the sidelobe pattern deviates from the predicted pattern when the wavelength is comparable to the adjacent distance of virtual sources. This is due to the additional time delay of scattering that affects the sidelobe. The main lobe, which is projected forward, is not affected by scattering. The directivity index is measured within 8 dB – 14 dB. The 3-dB bandwidth ranges from 15$^\circ$ to 20$^\circ$ with a maximum deviation of 5$\pm$2.5$^\circ$ relative to the prediction. A set of signals is prepared to steer the sound beam from 0$^\circ$ to 45$^\circ$ with a steering resolution of 5$^\circ$. The fastest steering speed is evaluated at 15$^\circ$/s. The total sound pressure level is maintained over different steering angles. Moreover, it shows that the steering resolution is enough to project a constant sound characteristic within the range of steering angle.