hearing aid, device used in some forms of deafness to amplify sound before it reaches the auditory organs. Modern hearing aids are electronic. They contain a tiny receiver and a transistor amplifier, and are usually battery powered. Some are small enough to fit into an arm of a pair of eyeglasses, or into the outer ear. The bone-conduction hearing aid, placed behind the ear, channels sound waves to the adjacent bony part of the skull, which then transmits the vibrations to the auditory nerve of the cochlea. The air-conduction hearing aid amplifies sounds and directs them into the ear toward the tympanic membrane. In recent years, a number of advancements have been made to hearing aids, improving the comfort, sensitivity, and aesthetic quality of the devices. Today, many hearing aids are customized to amplify only those noises (e.g., high frequency) that the user has difficulty hearing. Cochlear implants have been developed for use by certain totally deaf people. They consist of mechanical replacements for ineffective hair cells in the inner ear, which transform sound vibrations into electronic impulses that stimulate the auditory nerve.

hearing: see ear.

Partial or total inability to hear. In conduction deafness, the passage of sound vibrations through the ear is interrupted. The obstacle may be earwax, a ruptured eardrum, or stapes fixation, which prevents the stapes bone from transmitting sound vibrations to the inner ear. In sensorineural deafness, a defect in the sensory cells of the inner ear (e.g., injury by excessive noise) or in the vestibulocochlear or eighth cranial nerves prevents the transmission of sound impulses to the auditory centre in the brain. Some deaf people are helped by hearing aids or cochlear implants; others can learn to communicate with sign language and/or lip reading.

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or Seeing Eye dog

Dog professionally trained to guide and protect its blind master. They have also been used to assist persons with hearing impairments and restricted mobility. Systematic training of guide dogs originated in Germany during World War I to aid blinded veterans. At the age of approximately one year, the dog is trained for three or four months. Retrievers and German shepherds are the most widely used breeds.

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Device that increases the loudness of sounds in the user's ear. Its principal components are a microphone, an amplifier, and an earphone. Hearing aids are increasingly smaller and less conspicuous, fitting behind the earlobe or within the ear canal. They have widely differing characteristics, amplifying different components of speech sounds for maximum comprehension by each wearer. Hearing aids with automatic volume control vary the amplification automatically with the input.

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In law, a trial, or more specifically the formal examination of a cause before a judge according to the laws of the land. In popular usage the term often refers to a formal proceeding before a magistrate prior to the inception of a case, and in particular to a preliminary hearing, where a magistrate or judge determines whether the evidence justifies proceeding with the case.

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