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Hearing Aid Circuits

Not all hearing aids that look alike are alike.   Their performance and hearing may be significantly different.  Digital hearing aids process the sound and digital hearing aids may differ both in the type and speed of sound processing.  Speed of processing may vary from between 1 msec to 100 msec.

Because of these unseen differences in hearing aids, make sure you are able to return the hearing aid for a refund after trying the hearing aid for several weeks.  This is the only way to protect your purchase. 

The type of sound processing can be most crucial in patients with a sensorineural hearing loss.  This is the most common type of loss in adults and involves the hair cells or nerves of the inner ear.  This type of hearing loss is not just one of loss of loudness but usually has a degree of abnormal sound processing which may result in reduced discrimination or understanding of speech.  In addition, the degree of hearing loss may not be the same across all frequencies and there may be an intolerance to loud sounds, making simple amplification unacceptable to the patient.

The three main categories of hearing aid circuits are Sound Amplification, Noise Reduction and Special Features.

Hearing Aid Sound Amplification:  There are two important factors, the specific frequencies which are amplified and the amount or decibels that they are amplified. 

1)  Hearing Aid Compression Circuit:   One of the most annoying characteristics of some hearing losses is a reduced dynamic range of hearing.  In other words, a soft sound is not heard but a loud sound causes discomfort.  Medically, the patient is said to have recruitment.  Clinically, this presents as a patient who asks one to speak up and when the speaker does, the patient winces and asks the speaker not to yell.  Thus compression circuits amplify soft sounds more than loud sounds.  Amplification of some loud sounds may be reduced or not amplified at all.  The knee point is the exact point a hearing aid starts to reduce the amount of amplification of sound.  This process is called compression and prevents loud sounds from being amplified as much as soft sounds.

2)  Hearing Aid Matching the Shape of the Audiogram:   Similar to a home stereo system, hearing aids come with "equalizer" circuits to match the shape of the audiogram.   Stereo equalizers can be sold with a different number of bands, as little as 3 bands to over 30 bands.  The same is true for hearing aids.  Digital aids may vary from 3 to 12 bands of matching for the type of hearing loss.  A standard hearing test typically evaluates six to seven frequencies which can be matched during aid fitting.  Patients with a gentle sloping flat loss may not need as many matching points as a patient with normal hearing in some frequencies then a steep sloping hearing loss in other frequencies. 

Hearing Aid Noise Reduction Circuits:  The ability to understand speech is the prime function of a hearing aid.  Human speech centers on frequencies between 500 Hz to 2000 Hz.  Male speakers usually have lower pitched speech than female speakers.   By amplifying the speech frequencies and filtering out the non-speech frequencies which contain noise, speech is more understandable.  This is referred to as increasing the signal to noise ratio.  

1)  Hearing Aid Direct Audio Input:   Some hearing aids have the ability to directly input signals from a variety of sources which may include teachers, televisions and radios.   This very effectively reduces noise and makes sure the purest possible desired sound reaches the patient.   The source is usually a FM transmission device.

2)  Hearing Aid Noise Filtering:  The goal of noise filtration is to reduce the sound from frequencies which contain noise and augment frequencies which contain the desired sound.  The type of optimal noise filtration will vary in different settings.  Some aids have the ability to store and recall different noise filtration settings to be used for church, movies, concerts, male and female speakers.    

3)  Hearing Aid Directional Microphone   Some hearing aids have both an omnidirectional (detects sounds from all directions) and directional microphone (detects sounds from a single source).  The directional microphone will hone in on the speaking person and more effectively eliminate background noise.  The user can switch between the two different hearing modes.

Special Features:

1)  Hearing Aid Memory functions for different aid settings:  Some hearing aids can store and recall a number of different settings.   For example, omnidirectional versus unidirectional microphones, the amount of noise filtering and the degree of amplification of sound.   One would want to use different noise reduction settings when attending a lecture as opposed to a concert.

2)  Hearing Aid Telephone Function.  Hearing aids may have a "T" (Telephone) setting which uses an induction coil to detect sounds directly from a telephone.  All telephones which are produced today must be compatible with hearing aids.  When the "T" setting is used the hearing aid's microphone is turned off and environmental background noise is eliminated.  "T" settings may also detect FM transmission devices used to aid the hearing impaired.   (Note: The "M" setting refers to microphone.)

A conductive hearing loss (one involving sound amplification only) often occurs from disease of the eardrum or middle ear bones (ossicles).  Many times these types of losses can be corrected with surgery.  If not, a hearing aid should restore hearing to near normal.  Patients with a pure conductive loss usually have a relatively flat configuration of the audiogram and good understanding of speech.  In this case a basic hearing aid or even a RadioShack Amplified Stereo Listener may provide adequate heading..

Note:  RadioShack Amplified Stereo Listeners cost between $30 to $40 (price check June 2008) not including headphones.  The $40 unit has a three band equalizer.  -- See Product.   

Before buying a hearing aid be sure to obtain:

1)  References on the dealer
2)  Return Policy
3)  Warranty
4)  Sales Contract
5)  Features of the Hearing Aid

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