Hearing aid
Overview
A hearing aid is an electroacoustic device which typically fits in or behind the wearer's ear
Ear
The ear is the organ that detects sound. It not only receives sound, but also aids in balance and body position. The ear is part of the auditory system....

, and is designed to amplify and modulate
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...

 sound for the wearer. Earlier devices, known as "ear trumpet
Ear trumpet
Ear trumpets are tubular or funnel-shaped devices which collect sound waves and lead them into the ear. This results in a strengthening of the sound energy impact to the eardrum and thus a better hearing for a reduced or decreased hearing individual....

s" or "ear horns
Horn (acoustic)
A horn is a tapered sound guide designed to provide an acoustic impedance match between a sound source and free air. This has the effect of maximizing the efficiency with which sound waves from the particular source are transferred to the air...

", were passive
Passivity (engineering)
Passivity is a property of engineering systems, used in a variety of engineering disciplines, but most commonly found in analog electronics and control systems...

 funnel-like amplification
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...

 cones designed to gather sound energy and direct it into the ear canal. Similar devices include the bone anchored hearing aid
Bone Anchored Hearing Aid
A Bone-anchored hearing aid is a type of hearing aid based on bone conduction. It is primarily suited to people who have conductive hearing losses, unilateral hearing loss and people with mixed hearing losses who cannot otherwise wear 'in the ear' or 'behind the ear' hearing aids...

, and cochlear implant
Cochlear implant
A cochlear implant is a surgically implanted electronic device that provides a sense of sound to a person who is profoundly deaf or severely hard of hearing...

.
There are many types of hearing aids (also known as hearing instruments), which vary in size, power
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...

 and circuitry.
Among the different sizes and models are:

This was the first type of hearing aid invented by Harvey Fletcher
Harvey Fletcher
Harvey Fletcher was an American physicist. Known as the "father of stereophonic sound" he is credited with the invention of the audiometer and hearing aid...

 while working at Bell Laboratories.
 
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