Sound waves are classified as mechanical waves because they require a medium (substance) to travel through. Mechanical waves are waves that propagate through a material medium, transferring energy from one point to another by causing particles in the medium to oscillate or vibrate.
In the case of sound waves:
Medium Requirement: Sound waves cannot travel through a vacuum because they rely on the particles of a material medium to transmit energy. The medium can be a solid, liquid, or gas. In most everyday situations, air serves as the medium through which sound waves propagate.
Particle Motion: As sound waves travel through a medium, they cause particles of the medium to vibrate or oscillate. In a longitudinal wave like a sound wave, particles of the medium move parallel to the direction of wave propagation. In the case of sound in air, air particles are compressed and rarefied as the sound wave passes through.
Transmission of Energy: The energy of the sound source is transmitted to adjacent particles in the medium, causing a chain reaction of particle motion that propagates the wave. This transfer of energy through the mechanical motion of particles characterizes mechanical waves.
Contrastingly, electromagnetic waves, such as light waves or radio waves, do not require a material medium for propagation. They can travel through a vacuum because they are composed of oscillating electric and magnetic fields. Since sound waves rely on the mechanical motion of particles in a medium, they fall under the category of mechanical waves.
In summary, sound waves are called mechanical waves because they exhibit the characteristics of mechanical wave propagation—they require a medium, cause mechanical vibrations of particles in that medium, and transfer energy through the mechanical motion of particles.