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A styrophone is an acoustic device made from expanded polystyrene foam (often referred to by the genericized trademark styrofoam). There are at least two varieties:
The musical instrument consists of a foam box that is forced to vibrate by a variety of means, such as friction against a wooden stick. Technically, that instrument is a type of friction idiophone. Styrophones tend to last for only one performance, as the friction destroys the material. 
The loudspeaker is a cheap home-made variety that consists of a crystal earpiece attached to a cone made from a foam cup or sections of packing material.