Megaphone

A megaphone, speaking-trumpet, bullhorn, or loud hailer is a portable, usually hand-held, cone-shaped acoustic horn used to amplify a person’s voice or other sounds and direct it in a given direction. The sound is introduced into the narrow end of the megaphone, by holding it up to the face and speaking into it, and the sound waves radiate out the wide end. The megaphone increases the volume of sound by increasing the acoustic impedance seen by the vocal cords, matching the impedance of the vocal cords to the air, so that more sound power is radiated. It also serves to direct the sound waves in the direction the horn is pointing. It somewhat distorts the sound of the voice because the frequency response of the megaphone is greater at higher sound frequencies.

Since the 1960s the voice-powered acoustic megaphone described above has been replaced by the electric megaphone, which uses electric power to amplify the voice.

The initial inventor of the speaking trumpet is a subject of historical controversy; both Samuel Morland and Athanasius Kircher invented megaphones around the same time in the 17th century. Morland, in a work published in 1655, wrote about his experimentation with different horns. His largest megaphone consisted of over 20 feet of copper tube and could reportedly project a person's voice a mile and a half.

Twenty years earlier, Kircher described a device that could be used as both a megaphone and for "overhearing" people speaking outside a house. His coiled horn would be mounted into the side of a building, with a narrow end inside that could be either spoken into or listened to, and the wide mouth projecting through the outside wall.

Morland favored a straight, tube-shaped speaking device. Kircher’s horn, on the other hand, utilized a “cochleate” design, where the horn was twisted and coiled to make it more compact.

A later, papier-mache trumpet of special design was the Sengerphone.

Additionally, in ruins of Tiwanaku are stones around the central place with holes shaped in megaphone's profile. Their purpose is today unknown, but as local guards can show, it is possible to amplify human voice as it is loud enough to hear it across large area.

...
Wikipedia