audibility & linear hearing instruments

AUDIBILITY & LINEAR HEARING INSTRUMENTS

•Lenses for Eyes vs. HIs for EarsFitting hearing instruments for presbycusis is very different from fitting lenses for presbyopia. The general public is well aware that eyeglasses work better for vision than hearing instruments do for hearing.


•Lenses for Eyes vs. HIs for EarsThe visually challenged consumer is also aware that most typical vision problems result from an improper focus of light on the retina.Simply, the retina does for vision what hair cells do for hearing. (the retina changes light into electricity—the language our brains understand)


•Lenses for Eyes vs. HIs for EarsThe refocusing of light on the retina with properly fitted lenses is similar to fitting a person with conductive hearing loss with hearing aids.


•Lenses for Eyes vs. HIs for EarsFor poor vision, incoming light must be properly focused upon the retina to be properly conducted or refocused.For conductive hearing loss, sound must simply be made louder so that it can get through the middle ear and reach the normal cochlea (no damaged hair cells).


•Lenses for Eyes vs. HIs for EarsIf optometrists had the same job as hearing instrument specialists, they would most often be seeing patients with scratched or damaged retinas. However, this is more of an exception than the rule for that profession.


•Lenses for Eyes vs. HIs for EarsFor the most part, hearing instruments are fit where the hair cells of the cochlea have been damaged—similar to the distortion of vision experienced through macular degeneration.The hair cells of the cochlea are the “retina” of the ear.


•Lenses for Eyes vs. HIs for EarsThe benefits of a hearing instrument are not always as immediately obvious as the benefits of a vision lens.If the anticipated hearing instrument(s) benefits are not clear to the patient/client or if the hearing instrument(s) is not physically comfortable, they will not wear them.


•Audibility and Speech in NoiseFor most people with hearing loss who wear hearing instruments, increasing audibility is not enough and the importance of understanding speech in noise cannot be overestimated.


•Audibility and Speech in NoiseWhy do those who wear hearing instruments bitterly complain regarding their ability to hear in noise?Damaged/missing hair cells result in reduced clarity and certainly manifest their reduced numbers when attempting to transmit complex information to the brain.


•Audibility and Speech in NoisePatient/clients with speech in noise challenges must have the opportunity for a better signal-to-noise ratio.While normal hearing individuals can understand speech fifty percent of the time when the speech and noise are equally loud, pathologic ears are unable to do so—the speech must be several decibels louder than the noise.


•Audibility and Speech in NoiseResearch has found that with each one decibel of speech to noise improvement, a pathologic ear may have a ten percent better opportunity to understand that speech signal.


•Audibility and Speech in NoiseThese one decibel improvements can result into a fifty percent greater advantage of understanding speech in noise—if the patient/client has enough residual hearing ability to perform.


•Audibility and Speech in NoiseSince there are now digital hearing instruments utilizing directional microphones, a five to six decibel of speech to noise ratio can be electro-acoustically achieved. Let’s review the history of hearing instrument technology.


•Carbon microphones (developed around 1900)

When coupled with early electronic amplifiers would produce more gain; however, they also produced significant amounts of distortion


•Vacuum tubes (developed around 1920)

Were more compact and reliable than the carbon era. Also, created more amplification and less distortion.


•Transistor (developed in the mid-1940s)

Now, not only could hearing instruments be worn on the body, they could now be worn on the head i.e. ear level. More amplification with even less distortion became a reality.


•Integrated Circuits (IC) (developed 1970s)

Would utilize thousands of transistors in the design of the amplifier chipset.

Was the beginning of tremendous miniaturization of hearing instruments i.e. from ear level to in-the-ear transducer/amplifier location.


•Digital age (mid 1990s)

The integrated circuits were miniaturized further to include millions of transistors.

They became “digitized” through the development of analog to digital (A/D) converters.


• Body hearing aids

Were found in the carbon, vacuum tube, and transistor phases of hearing instrument development


•Eyeglass hearing aids (used transistors & ICs )

Were introduced to “hide” the hearing aid.

Also to “hide” the wiring required for a CROS fitting.


•Behind-the-ear hearing aids (use transistors and IC)

Early versions were ear level however, bigger than some ears!

Today’s versions are very small and discreet.


•In-the-ear hearing aids

May be either custom or modular in design.

Have an additional acoustic advantage with microphone placement inside of the ear.


•Completely-in-the-canal hearing aids

Very discreet—translates into consumer appeal.

Its deep insertion can create added acoustic amplification.


Fitting of hearing instruments is not an exact science. Unlike a lens for vision, the “retina” of the ear requires a sharpening of the traveling wave and the separation of speech from noise which a damaged cochlea is unable to perform.


•Hearing Instrument Fitting Methodology

Hearing instrument fitting formulae are used to determine how much amplification is required to achieve audibility for certain types of hearing loss.


• Hearing Instrument Fitting Methodology

There are two basic fitting formulae. One is based upon the threshold measurements of hearing loss—known as linear fitting formulae. The other is based upon the supra-threshold measurements of residual hearing ability—known as compression fitting formulae.


We will discuss more regarding various hearing instrument fitting formulae next week.

audibility & linear hearing instruments

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