In the case of longitudinal waves, the concept of wavefronts can still be applied, although it might be a bit different from transverse waves. In longitudinal waves, the particles of the medium vibrate back and forth parallel to the direction of wave propagation, causing compressions and rarefactions.
The wavefront of a longitudinal wave represents the surfaces that connect points in space that are in the same phase of the wave at a given instant. While transverse waves have crests and troughs that can be used to define wavefronts, longitudinal waves do not have these distinct features.
Instead, wavefronts in longitudinal waves are typically represented by regions of the wave where the particles of the medium are at their maximum compression or rarefaction. These regions can be visualized as surfaces that connect points where the particles are in the same phase of oscillation.
For example, in a sound wave traveling through air, the wavefronts can be visualized as concentric spheres expanding outward from the source of the sound. Each sphere represents the position of the compressed or rarefied region of the wave at a given instant.
It's important to note that the concept of wavefronts is a useful tool to describe the propagation of both transverse and longitudinal waves, but the specific visual representation and characteristics of wavefronts may vary depending on the type of wave.