Abstract: Methods, systems, and apparatus, including computer programs encoded on computer storage media, for an ultrasonic sensor. One of the systems includes a first ultrasonic assembly that includes a first waveguide formed by a first monolithic body, and a first ultrasonic transducer mounted to the first waveguide, wherein the first monolithic body includes a first mounting surface adapted to couple to the first ultrasonic transducer and a first coupling surface adapted to couple to an outer surface of a pipe; and a second ultrasonic assembly that includes a second waveguide formed by a second monolithic body, and a second ultrasonic transducer mounted to the second waveguide, wherein the second monolithic body includes a second mounting surface adapted to couple to the second ultrasonic transducer and a second coupling surface adapted to couple to the outer surface of the pipe.