Final answer:
A vibration is a source of sound, creating sound waves that are mechanical waves combining kinetic and potential energy. As sound moves between media with different propagation speeds, its wavelength changes, but its frequency remains constant.
Step-by-step explanation:
A source of sound is a vibration. When an object vibrates, it causes the nearby particles in the medium (such as air, water, or solids) to vibrate as well, creating a sound wave that is a type of mechanical wave. Mechanical waves combine kinetic energy, due to particle motion, and potential energy, due to the elasticity of the medium. This is different from thermal energy, which is related to the random motion of particles, and is not organized into waves.
When sound passes from one medium to another where its propagation speed is different, its wavelength changes, but its frequency remains the same because the frequency is determined by the source of the sound, not the medium through which it travels.