When studying sound waves, we typically consider only compressible fluids because sound is a mechanical wave that requires the propagation of compressions and rarefactions through a medium. Compressible fluids, such as gases like air, fit this requirement and are suitable for the transmission of sound.
In a compressible fluid, the molecules are not tightly packed together, allowing them to move more freely and easily. When a sound wave passes through a compressible fluid like air, the wave causes small variations in pressure, resulting in compressions (regions of higher pressure) and rarefactions (regions of lower pressure). These pressure variations are responsible for transmitting the energy of the sound wave.
On the other hand, incompressible fluids, such as liquids, do not undergo significant changes in volume or density when subjected to pressure variations. In liquids, the molecules are closely packed, and their intermolecular forces prevent large-scale compression and expansion. As a result, sound waves do not propagate efficiently through incompressible fluids like they do in compressible fluids. Instead, the energy of sound waves in liquids is typically dissipated quickly as heat.
Therefore, when studying sound waves, we focus on compressible fluids like gases, particularly air, as they allow for the transmission and propagation of sound waves. The behavior of sound waves in compressible fluids, such as their speed, frequency, and other characteristics, can be accurately described and studied within the framework of compressible fluid dynamics and acoustics.