ABSTRACT
This chapter begins with the underlying physical properties of ultrasound speed, attenuation in air, and scattering from targets. Ultrasonic sensors work with acoustic frequencies above 20 kHz, which is at the upper limit of audible human hearing. Ultrasonic sensing in robotics is popular due to the ability to directly achieve range sensing cheaply, simply, unobtrusively, and with low power consumption. Ultrasonic sensing is sometimes called sonar derived from sound NAvigation and Ranging. The speed of sound varies significantly with atmospheric conditions, such as temperature, humidity, pressure, and altitude above sea level. Due to geometric spreading of the wavefront from a point source, the power of an ultrasonic pulse reduces with the square of the distance it travels in a lossless open medium. Acoustic wave propagation is disturbed by changes in the acoustic impedance of the medium, which is defined as the product of medium density and speed of sound.
