Equalizers for the hearing impared

Hey Now, although not a specific Altec subject, I believe hearing is important to all of us in audio, so I have taken the time to consult a friend of mine who knows a bit about hearing and such about this subject, as follows:

Info re hearing loss and hearing aids follows:
Questions and concerns regarding hearing loss, hearing aids, and dynamic
range are most worthy of discussion, and this topic may be more complicated
that what a lot of readers realize.
While studying Audiology as a grad student (my stepping stone to becoming a
"hearing scientist"), I worked as an extern at a local VA Hospital. One
common questions was, "Why can't they make hearing aids sound better?"
Most wearers were more concerned about the tonal quality (in lieu of
dynamic range restrictions), but the answer addresses both concerns. In an
attempt to answer their question, I used an analogy of visual impairment
and eyeglasses. If an individual has visual loss resulting from sustained
exposure to bright light (e.g., the sun), the damage will most likely be to
the eye's retina, and even the thickest eyeglasses won't be of much help.
And shining more light into this individual's eye wouldn't do much to
increase visual acuity either; it would probably hinder it. Even if a lens
can focus a sharp image on the retina, the visual distortion is internal to
the person (and their brain), thus making visual processing difficult.
Similarly, about 95% of the individuals with hearing loss have some form of
damage to the cochlea (the organ for hearing), and the distortion they
experience is internal. About 5% of individuals have a "conductive"
hearing loss. Conductive hearing losses can result from disarticulation of
the ossicles (middle ear bones), rupture of the tympanic membrane
(eardrum), blockage or fluid in the middle ear (e.g. otitis media which is
common among young children), and other causes. For these individuals, a
hearing aid that simply makes sound "louder" would be highly effective
because they wouldn't experience internal distortion or dynamic range
limitations (other than normal limitations).
Let's return to the topic of sensorineural hearing loss because this is
what most individuals experience as a result of noise exposure (which can
be very insidious), age, disease, and other causes. We all realize that
soft sounds need to be made louder in order for hearing-impaired
individuals to hear them. What most folks don't realize is that loud
sounds can be a lot more irritating (or even painful) to persons with
hearing loss. The sudden and rapid increase in loudness (note that
loudness is a perception, not a physical measure) to hearing impaired
persons is known as recruitment of loudness. This presents a problem to
Audiologist (or anyone qualified to fit hearing aids) because soft sounds
need to be increased in intensity (or "volume"), while loud sounds need to
be reduced in intensity. This, of course, requires compression circuitry
within the hearing aid. Peak limiting circuitry is also added to put a
ceiling on the maximum sound pressure level (SPL) that a hearing aid (HA)
wearer would experience.
A reduction in dynamic range probably isn't the culprit that makes music
listening less enjoyable for HA users. Years ago I had a lot of
involvement in radio transmitter design for an FM station. To my
knowledge, most radio broadcasts have very limited dynamic range (2 dB?)
plus extremely effective peak limiting circuitry (this is needed to protect
the transmitter). Despite the reduced dynamic range and potential for
limiting-induced distortion, FM broadcasts can be enjoyable and of good
fidelity. A mistuned station, however, can be quite irritating at any
volume level. I can't state that this is how hearing impaired individuals
perceive sound, but it is known that much of the distortion they perceive
is generated at the cochlear or "nerve" level. Even the most advanced
hearing aids (using digital signal processing) cannot correct for this
internal distortion. Hearing aids have improved vastly over the years,
but the advances aren't a result of "higher fidelity" transducers or
circuitry. Advances will come about as a result of re-shaping or modifying
the acoustical signal in ways that allow the wearer to hear and understand
speech (and enjoy music). Note that I used both "hear" and "understand" in
this last sentence: Just because a person hears a sound doesn't guarantee
that the sound makes sense. Central auditory processing is a topic of its
own, but its safe to say that auditory processing is compromised as a
result of sensorineural hearing loss. At present, nearly all hearing aids
are designed to improve communication and, consequently, are designed to
enhance speech but not music.
I know one musician who incurred sudden hearing loss as a result of
meningitis. This person is also an astrophysicist who does a lot of signal
processing and is knowledgeable in electronics. Because of the sudden
loss, he was able to accurately remember how sounds should sound compared
to his current hearing disability. During the past two years or so, this
person has worked on a better hearing device for musicians. Even with
high-fidelity earphones, signal sources, and a powerful signal processing
system (much larger than what can presently be fit into a hearing aid),
this person admits that re-creating "live" music with a hearing loss is a
difficult task.
It is very important to mention that hearing loss varies from person to
person, and that there's no single "best" hearing aid. Programmable
hearing aids allow adjustments (usually filter settings) to be
custom-tailored to each person. A lot of "fine-tuning" requires feedback
from the wearer; matching the high- or low-pass filter settings to match an
audiogram (the chart that shows the degree of hearing loss as a function of
frequency) doesn't always give the clearest sound. Adjusting compression
isn't easy either, as different situations require different settings.
We're usually happy with a new pair of glasses, as these don't take time to
adjust; this usually isn't the case with hearing aids.
Note: No references cited for the above info; wrote this off the top of my
head while at breakfast this morning. I believe the info to be accurate
(minus any typos).
Eric L. Carmichel
ELC Audio Engineering
www.elcaudio.com

I know a lot of info to take in but I do hope you all found it interesting, now excuse me while I go play with my Beryllium/Copper screwdrivers and diaphragms.

-Michael Edwards http://www.hostboard.com/forums/