Sound waves are caused by vibrating objects. The waves can pass quickly through solids, a bit more slowly through liquids, and more slowly again through gases. They can't travel through a vacuum. If a sound wave hits your ear and makes it vibrate, you hear the sound.
Sound waves can be reflected by hard, flat surfaces, and absorbed by soft ones. Reflected sound waves are echoes. They can also refract.
The loudness of a sound depends on the amplitude of the wave, and the pitch depends on the frequency.
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