[Concerning hearing aids]

Ear trumpet copy musical instruments as: horn, trump, trombone, banjo and specially bell. All of them utter sound but their shape is used by ear trumpet to collect sound. Sea shells were used as trump by fishermen for many centuries, the same shells became an hearing device during the nineteenth century by inversion of his function. For creators a same shape has necessarily two opposite functions: if an instrument is able to utter sound, the same design is also able to collect sound. Megaphone and ear trumpet are entirely similar but their function is just the opposite.