Hearing Loss

Hearing loss is typically described as being conductive, sensorineural, or mixed.

Conductive hearing loss refers to an impairment of one's ability to conduct airborne sound through the middle ear to the inner ear. Scar tissue or otosclerosis, the abnormal growth of bone within the middle ear, can lead to restricted movement of the ossicles. Recently it has been shown that there can also be conductive problems with the basilar membrane of the inner ear that reduce the efficiency of energy transfer to the hair cells (Holt).

Sensorineural hearing loss refers to impairment of the sensory unit consisting of the auditory nerve and the hair cells that excite it.

Sometimes the distinction between these two types of hearing loss can be made with a simple tuning fork test. If the tuning fork cannot be heard when sounded in air, then the base of the tuning fork is placed against the hard bone behind the ear. If the person can now hear it by conduction through the bone, then conductive hearing loss is indicated. It in cannot be heard by either air or bone conduction, then sensorineural loss is indicated.

Hearing Loss
0 to -15 dB Normal range
-16 to -25 dB Minimal loss
-26 to -40 dB Mild loss
-41 to -55 dB Moderate loss
-56 to -70 dB Moderate/severe loss
-71 to -90 dB Severe loss
> -91 dB Profound loss
American Speech and Hearing Association

The "power of ten" or logarithmic nature of hearing response is evident in the fact that a loss in sensitivity by a factor of 10,000, or -40 decibels, is still at the edge of "minimal loss". By the admittedly simplistic "rule of thumb" for loudness, this -40dB sound would still be 1/16 as loud as the 0 dB reference. 0 dB in this table represents the normal hearing threshold, or 0 dB Hearing Level. The categories of hearing loss are based on measurements at 500, 1000 and 2000 Hz.

Assessment of hearing lossHearing Aids

Hearing concepts

Dangers of Loud Sounds


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Hearing Aids

Sometimes a satisfactory level of hearing can be restored by a hearing aid - a combination of a microphone to sense ambient sound, an amplifier, and a tiny speaker that projects the amplified sound into the ear canal. A typical modern hearing aid would employ an electret condenser microphone - small and rugged with a high signal-to-noise ratio. The frequency range of application is typically 100-10,000 Hz. While some assistance may be rendered by bone conduction, this discussion will be limited to hearing aids that operate by sounds produced in the air.

Wearing Styles
ITE In-the-ear
BTE Behind-the-ear
ITC In-the-canal
CIC Completely in-the-canal
Body Worn on body (profound loss)

A basic hearing aid may be called a linear circuit aid, implying that it simply amplifies any ambient sound that reaches it. It is important for such a hearing aid to contour the amplification to the nature of the hearing loss of the individual, which typically means amplifying high frequencies more than low frequencies. Presbycusis, the progressive loss of high frequency hearing with age, often calls for amplification of high frequencies with little or no bass boost. A basic hearing aid may have three frequency bands to permit the amplification to be adjusted based on the audiogram.

The next step up in sophistication of the hearing aid would be to employ some kind of audio "compression". Compression implies the adjustment of the "gain" or degree of amplification based on the input level, it being a practical fact that louder sounds wouldn't need as much amplification. This compression would reduce the amplification for loud sounds either at the microphone end or at the speaker end. Some types of compression are called "adaptive compression" in that some logic is used to compress some kinds of sounds more than others.

For those hearing aids that use adaptive compression, but not digital logic, some are classified under the headings ASP and K-AMP circuits. The ASP units monitor incoming sounds and automatically change the gain, output and frequency response. The K-AMP approach detects and amplifies only quiet sounds while leaving louder ones unaltered.

Currently under very active development are the digital programmable hearing aids that use a digital signal processor (dsp). They can be programmed to more nearly fit the detailed needs of an individual user and open the door to more sophisticated approaches to assisting the user. Since the understanding of human speech is often the highest priority, and since speech has identifiable characteristics like vocal formants, some steps can be made to program the hearing aid to amplify speech sounds more than some distinctly different other types of sounds. A friend with a digital hearing aid told me something like "I leaned over an expressway bridge and listened to the traffic noise. After a short time there was a kind of burbling sound like the hearing aid was trying to make voices out of this sound." An intriguing idea, that we might get enough sophistication into hearing aids to recognize and selectively amplify the sounds of meaningful human communication.

Another approach to hearing assistance is the cochlear implant. Currently very expensive and in the experimental stage, it is one of the future possibilities.

(Tal Berkowitz is acknowledged for investigative work on this topic.)

Assessment of hearing loss

Hearing concepts

Dangers of Loud Sounds


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