Yes, in the case of an air-water interface, water acts as a free boundary for sound waves. When a sound wave encounters an air-water interface, it experiences a change in the medium it propagates through, transitioning from air to water or vice versa.
At this boundary, certain properties of the sound wave change, such as its speed and direction of propagation. The change in medium results in some of the sound energy being transmitted across the boundary, and some of it being reflected back.
Key points about sound waves at an air-water interface:
Reflection: When a sound wave traveling through air encounters the water surface, a portion of the wave's energy is reflected back into the air, following the law of reflection. This reflection is why you can hear echoes when you shout across a large body of water, like a lake or an ocean.
Transmission: Another portion of the sound wave's energy is transmitted through the water surface, causing the sound wave to continue its propagation through the water. The transmission of sound across an air-water interface is also influenced by the angle of incidence and the acoustic properties of both media.
Refraction: Sound waves also experience refraction at the air-water interface. Refraction is the bending of the sound wave as it changes speed and direction while crossing the boundary. The amount of refraction depends on the angle of incidence and the speed of sound in both the air and water.
Absorption: Some of the sound energy will be absorbed by the water at the interface and further into the water medium. This absorption leads to a decrease in the intensity of the sound wave as it travels deeper into the water.
The air-water interface acts as a free boundary because sound waves can interact with it without significant impedance or constraint. The transmission, reflection, and refraction of sound waves at this interface play essential roles in various natural phenomena, such as underwater acoustics and how sound is perceived near bodies of water.