Before we took our measurements, each ultrasound transducer was independently calibrated and tested (Tesco, South Windsor, CT) to within ±15%, according to the manufacturer's guidelines, using a wattmeter (UPM-DT-10; Ohmic Instruments, Easton, MD). The test tank was filled with room-temperature degassed water. The transducer face was placed parallel to the center of the cone, 0.6 cm below the water surface. Before calibration, the surface of each transducer was checked to ensure that no air pockets or bubbles remained.