The two procedures mainly used for determining ultrasonic power—the sound radiation force method and the planar scanning technique—are complicated, and require a relatively expensive measuring setup. These procedures are therefore unsuitable for the routine inspection of ultrasonic equipment used in medical and technical applications. In contrast to these procedures, the thermoacoustic sensor is well-suited for this purpose. It is of simple design, small in dimension, and its application requires only relatively inexpensive equipment. The ultrasonic power is determined from the heat produced by the ultrasonic signal in a suitable absorbing material. A disadvantage of this first sensor type is that the power of the ultrasonic signal can be determined only if its amplitude spectrum is known. The new thermoacoustic sensor allows the power of ultrasonic signals to be determined even if their amplitude spectra are unknown. When this sensor type is used, the ultrasonic signal first travels a short distance in a weakly absorbing propagation medium before it enters the absorber, where its energy is completely transformed into heat. In this case, the ultrasonic power is determined from the frequency-independent temperature gradient originating in the weakly absorbing propagation medium immediately in front of the absorber.