Aduiologists and UCL's

DZAPALA dzapala at aol.com
Fri Apr 25 01:07:58 EST 1997


John:

This is a difficult problem which is fortunately relatively rare.  This
doesn't help your child however.  Lets start by addressing this issue of
dynamic range, or the difference between the intensity level of the softed
sound audible and the loudest sound tollerable.  In the inner ear, there
are tiny cells called inner hair cells that have a dynamic range of about
20-40 dB.  To increase the dynamic range of the inner ear, some sets of
hair cells respond to less intense sounds (say 50 through 80 dB) while
others start to respond at more intense levels (say 60 - 90 dB).  So with
different sets of hair cells, an effective dynamic range of 55 dB (or
thereabouts) is accomplished.  

The inner ear also has outer hair cells. These cells physically amplify
the movement of the basilar memberane so that very slight sounds (0-45 dB)
cause the inner hair cells to fire.  With the combination of inner and
outer haircells,there is a dynamic range of about 90 dB.  
To increase the tollerance for sounds on the high end, muscles in the
middle ear contract to sounds above about 85 dB Hl.  These muscle
contractions add another 5 - 15 dB of dynamic range.  

In the case of hearing loss, all of these systems can fail.  without outer
hair cells, we would have a 50 dB hearing loss with little change in UCL, 
loose the acoustic reflex (middle ear muscles) and you loose another 15 dB
in dynamic range.  Loose inner haircells and the dynamic range can
collapse severely.  

In most of the cases I have seen where thresholds and UCLs were near
identical,  there is relatively better hearing in the low frequencies, say
below 750 Hz.  These cases, it is impotant to test UCLs at specific
frequencies and not 0only with speech stimuli so that a low tollerance
level in the low frequencies does not look like a very narrow dynamic
range in the high frequencies.  

In cases where the dynamic range is so reduced using hearing aids is
problematic.  Most individuals with severe hearing lss prefer peak
clipping as a method of limiting the output of a hearing aid so that
sounds do not excede tollerance levels.  Peak clipping produces distortion
and the hearing impaired listener can use this distortion to discriminate
between similar sounds.  However, that same distortion may reduce the
dynamic range further. 

I always try to fit hearing aids on children in a way that maximized
speech recognition.  However, when the dynamic range is reduced, this goal
might not be possible.  In these cases, teh hearing aid may help with
supra-segmental cues (voice and intonational information) as an adjunct to
speech reading, but not word recognition per se.

I think this problem will warrent a close working relationship between you
and your audiologist to fine tune the hearing aids so that they provide as
much auditory information possible while remaining comfortable to your
child.

I hope the best for you and your family.

God Bless

David Zapala, Ph.D.



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