A concave reflector can be used to concentrate a beam of plane ultrasonic waves in the focal region, where the intensityIfis much larger than the intensityIiin the plane wave. When the sound wave‐length is small compared to the dimensions of the beam and reflector, one can use the well‐known Fraunhofer diffraction formulas to calculate the intensity gain, i.e.,If/Ii. Expressions are derived for the maximum and average intensity gain in the zero‐order image when the ultrasonic beam is circular or rectangular, together with formulas giving the total intensity falling upon circular or rectangular areas of arbitrary dimensions in the focal region.