Measurements of ultrasonic velocity and attenuation require ’’diffraction corrections’’ for the finite size of the source and receiver. These diffraction corrections depend on the area of the source which is usually taken as the physical area of the piezoelectric element of the the transducer. Measurements are reported which indicate that the effective area of a disk‐shaped element may vary with the exciting frequency and may differ by 40% from the physical area of the disk. The method reported may be of value in the selection of transducers, in accurate diffraction correction calculations, and in the field of ultrasonic exposimetry.