Physics

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Sound waves are longitudinal waves that must pass through a medium, such as air. Echoes are reflections of sounds. The frequency of ultrasound is above 20 kHz. Ultrasound is useful for cleaning and also used for checking if a baby is healthy regards of weight,height and nutrition. Properties of sound waves

When an object or substance vibrates, it produces sound. These sound waves can only travel through a solid, liquid or gas. They cannot travel through empty space.

Longitudinal waves

Sound waves are longitudinal waves - the vibrations are in the same direction as the direction of travel.