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A megaphone, speaking-trumpet, bullhorn, blowhorn, or loudhailer is usually a portable or 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. A 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Since the 1960s the voice-powered acoustic megaphone described above has been replaced by the electric megaphone, which uses electric power and a folded horn to amplify the voice.

An electric megaphone is a handheld public address system, an electronic device that amplifies the human voice like an acoustic megaphone, using electric power. It consists of a microphone to convert sound waves into an electrical audio signal, an amplifier powered by a battery to increase the power of the audio signal, and a loudspeaker to convert the audio signal to sound waves again. Although slightly heavier than acoustic megaphones, electric megaphones can amplify the voice to a higher level, to over 90 dB. They have replaced acoustic megaphones in most applications, and are generally used to address congregations of people wherever stationary public address systems are not available; at outdoor sporting events, movie sets, political rallies, and street demonstrations.

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