Abstract: The method of measuring focused ultrasonic field based on infrared thermal imaging technique has the advantages of high scanning speed, high spatial resolution and wide frequency range. But, due to the limitation of infrared penetration ability, an air layer is needed between infrared camera and ultrasonic absorbers, which makes the internal sound field of ultrasonic absorber more complex. Currently there are three models to describe the internal sound field, and the corresponding experimental verification is obtained. In this paper, the internal sound field distribution and the sound intensity estimation method of the ultrasonic absorber based on the three models are analyzed, which provides a theoretical reference for further research on this technology.