A party horn is a horn formed from a paper tube, often one that is flattened and rolled into a coil, and which unrolls when blown into, producing a horn-like noise. The item is not known consistently by any term in English, also being known by a number of local variations, neologisms, and individual terms, often containing variants and synonyms of blowing (puffing, blow-out etc.) and noise (whistle, squeak etc.)
Modern variations have a plastic mouthpiece, which prevents the swift degradation of the device from exposure to the moisture of the mouth. Often the paper tube contains a coiled up metal or plastic strip that rapidly retracts the horn when you stop blowing on it. Others have a brightly colored feather attached to the end which vibrates in the outgoing air flow as the horn is blown.
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Some people distinguish between different types and are quite specific with the various type:
party horn, tweeter, squeaker, noisemaker - shaped like a horn, makes some sort of a tooting noise
paper tongue, party proboscis, blow-out, blow tickler - does not make a tooting noise, unrolls when one blows into it, often used to try to hit fellow party-goers in the face
noisemaker - usually made out of tin, a small box with a handle on it, when spun around it makes a clicking noise
clapper - has two parts on it, usually the same shape, when waved back and forth, makes a clapping noise