How were degrees of hearing loss first measured and determined?

Hearing regarding what is normal, etc. has ABSOLUTELY NOTHING to do with standard deviations.  The normal category was NEVER subjected to testing thousands of people with and without hearing loss and identifying a MEAN hearing value for normal and one standard deviation for the cut-off of normal.
During World War II, many veterans came home complaining of hearing loss.  It was decided to determine a NORMAL level of hearing.  Based on hearing science, it was identified two things:  One was that the dB scale is logarithmic and not linear, so means and SD cannot be used.  Second, it was measured that human beings identified a DOUBLING of loudness every 10dB (based on research done in the 1930s duplicated many times.  Whispered speech- the softest level of speech an AVERAGE person with normal hearing could detect and understand was at 30dBHL.  Thus, it was originally decided that normal hearing would be the first step BELOW a whisper meaning a person with normal hearing CAN hear a whisper and a person with the first level called a MILD hearing loss could barely detect a whisper.  Thus, the original CUT-OFF of normal was 25dBHL.
ASHA modified this YEARS  ago maybe in the 1960s.  It was decided to use half the loudness of a whisper rather than one step below a whisper.  A whisper is 30dBHL so half the loudness is 20dBHL.  Thus, it was decided that normal hearing would be UP TO 20dBHL.  The other decisions of mild, moderate, etc. were developed by a combined group of audiologists (more in the hearing sciences) and sound scientists and they identified the various categories with were then set by the American National Standards Institute or ANSI in the 1960s with the agreement that 20dBHL would be the cut-off of normal and the other categories as is.  ANSI and ASHA working with ANSI also identified that it would be best NOT to test hearing above 120dBHL since the thresholds of PAIN from hearing is 130dBHL so 120dBHL is half the loudness of pain.
Dr. Jay R. Lucker, Ed.D., CCC-A/SLP, FAAA

Certified/Licensed Audiologist & Speech-Language Pathologist
Specializing in Auditory Processing Disorders &
Language Processing Disorders
Dept. of Communication Sciences & Disorders
Howard University
Washington, DC
contact: 301-254-8583 or via this email address