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Sound reflects from hard surfaces. An echo is when a sound is heard again after reflecting and travelling back. Smooth, flat surfaces help to produce echoes. Irregular surfaces reflect sound in many directions, spreading it out. The particles of harder materials are held together by stronger forces so sound waves can’t make them vibrate much. Soft surfaces absorb a lot of sound because sound waves transfer energy by making the particles in them vibrate. Noise (unwanted sound) can be reduced by using soft materials to absorb it or hard materials to reflect it.