Sound waves do not technically "disappear," but rather they diminish or become imperceptible as they propagate through a medium. There are several mechanisms through which sound waves can decrease in intensity or become indistinguishable:
Absorption: When sound waves encounter a material, some of the sound energy is absorbed by the material. The energy is converted into other forms, such as heat, causing the sound wave to lose intensity. Different materials have varying absorption properties, with soft and porous materials generally absorbing more sound than hard and smooth surfaces.
Scattering: Sound waves can scatter when they encounter irregularities or obstacles in their path. This scattering can cause the sound to change direction and disperse, leading to a decrease in its intensity over distance.
Reflection: When sound waves encounter a boundary between two media, such as air and a solid surface, a portion of the sound energy can be reflected back. The reflected sound wave can interfere with the original sound wave, resulting in cancellation or reduction of the overall sound.
Diffraction: Sound waves can diffract or bend around obstacles or through openings. As they diffract, the waves spread out, resulting in a decrease in sound intensity in certain directions.
Distance and spreading: Sound waves naturally spread out as they propagate, leading to a decrease in their intensity with increasing distance from the source. This spreading occurs due to the geometric spreading of the wavefront, causing the same amount of energy to be distributed over a larger area.
It's important to note that while sound waves may become imperceptible, they still exist in the form of tiny fluctuations in air pressure. They can potentially be detected by sensitive instruments, even if they are no longer audible to the human ear.