Hearing Aids

Basic Functions

Hearing aids consist of a microphone(s) that converts the incoming acoustic signal to an electrical signal that is a duplication of the acoustic spectrum of the incoming signal. This conversion is followed by an amplifier that magnifies the signal received from the microphone. This stage is typically followed by a multichannel digital signal processor (DSP) that manipulates the amplified signal to match the magnitude and configuration of the hearing loss of the aided ear(s). Finally, the digitally processed amplified signal is forwarded to a receiver that reconverts the amplified electric digitally processed signal back to an acoustical signal and sends it to the ear in a variety of ways.

Technological Advances

In the past decade, significant advances have been introduced to hearing aids, and these advances have accelerated so much that manufacturers introduce major changes every 3 to 6 months whereas in the past major changes occurred every couple of years. Current advances include multichannel signal processing. That is, the processing chip within the hearing aid divides the incoming acoustic signal into as few as 2 channels (low and high frequency) or as many as 20 channels (independent narrow frequency ranges). When divided in this manner, the incoming signal can be manipulated independently in each channel for improved control of amplification, noise reduction, and the management of feedback. Examples of the use of multichannel signal processing include the following: (a) digital signal processing, (b) feedback management to eliminate or dramatically reduce the presence of feedback and allow for greater available amplification, (c) noise reduction for improved listening comfort in noisy environments, and (d) automatic adaptive directional microphones for improved recognition of speech in noisy environments.

This type of microphone design contains two microphones where one is forward facing and the second faces the rear. In this design, the signal processor within the hearing aid will automatically activate the forward facing omnidirectional microphone when there is a single talker in a quiet listening situation. Then the signal processor within the hearing aid will automatically activate the second rear-facing microphone when the processor detects a second signal arriving from the side or back of the listener and when the level of the rear signal is greater than the level of the signal arriving from the front. The action of the rear facing microphone is to attenuate the sound from the side or rear so that the signal arriving from the front is amplified to improve the signal to noise ratio. Advances in directional microphones have greatly improved user satisfaction and benefit with hearing aids in noisy listening environments, and this technology is improving rapidly.

Other advances include expansion to reduce the amplification of annoying low-frequency energy [Page 482](circuit noise of the microphone and low level environmental noise) for patients with normal hearing in the low frequencies; data logging that allows audiologists to read from the fitting software how the hearing aid is being used and store information about the typical listening environments of the patient; multiple memories that allow the audiologist to place into the hearing aid different frequency/gain responses for specific listening situations such as watching television or communicating on the telephone; wireless Bluetooth technology for connecting the hearing aids to other communication devices (cell phone, FM listening devices, MP3 devices, etc.) or allow for “cross communication” between the right and left hearing aids so a change (i.e., manipulating the volume control, changing programs, etc.) in one hearing aid automatically occurs in the other hearing aid; and slim tube open fits that are small behind-the-ear hearing aids using a narrow plastic tube or wire and inserted into an open earmold to send the amplified sound to the ear canal. The use of an open earmold eliminates the occlusion effect (sensation of the head in a barrel) and provides excellent high frequency amplification.

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