There is simply a lack of research works based on large-scale samples, measured in water pools. As a result, considerable studies on underwater absorption are only limited to theoretical analyses and numerical calculations ( 13– 22), whose idealized assumptions may not hold in practical scenarios, while almost all the existing experimental works ( 23– 30), measured in the water impedance tube, are based on small samples, which may not reflect the true performance in complex environments ( 31). One reason for this state of affairs is the difficulty of experimental measurements, owing to the large wavelength involved and the required large impedance mismatch of the solid material for a water impedance tube ( 11, 12). The latter represents the higher-frequency branch of the waterborne acoustic waves that has found widespread use in medical applications. Despite its apparent importance, however, this topic is nowhere as intensely pursued as either airborne audible sound ( 4– 7) or ultrasound ( 8– 10).
![underwater sound reference division ultrasound underwater sound reference division ultrasound](https://venturebeat.com/wp-content/uploads/2018/11/IMG_20181124_231607.jpg)
The efficient absorption of low-frequency underwater acoustic waves has especially attracted strong interest, notably for the applications in underwater sensing and stealth technologies ( 1– 3). Underwater acoustics represents an area of study that is important for the subsurface exploration and object imaging in rivers and oceans that occupy the majority of Earth’s surface.