From: dstine@cisco.com (David Stine)
Newsgroups: rec.pyrotechnics
Subject: Re: ringadingding
Date: 22 Dec 1992 03:03:55 GMT
Organization: cisco Systems, Menlo Park, California, USA
NNTP-Posting-Host: regal.cisco.com

In article <8052@tekig7.PEN.TEK.COM> drchambe@tekig5.pen.tek.com writes:
>In article <FXc3VB5w165w@tsoft.net> bbs.byron@tsoft.net (Byron Reynolds) writes:
>>I was behind an earthen bunker of sorts about 20 feet away, and I was 
>>situated in such a way that the danger of being hit with flying debris 
>>was not an issue.  However, I did not think about the "bang" enough.
>> 
>>Well, I got my "bang", and lost my hearing for about 3 days.  It started
>>with a shrill "sqeeeeeeeeeeeeeee" sort of sound that began building up
>>shortly after the blast, until all I was left with was pure "sqeeee".
>> 

>In hundreds of rounds,[of 5-inch gun firing] no one ever reported a
>noticeable hearing loss. After reading of this and the problems suffered by
> an earlier poster from 1/8 tsp of NHI3, it would seem that the impulse shape
> has a lot more to do with hearing effects than the power of the explosion.
>

From what I learned in college, I might be able to add a second voice to this.
There was a course offered in "Human Factors Psychology", which on the surface
sounds like it has nothing to do with the topic at hand. It was taught by Dr.
Tom Ayres, who at the time held the world's record for the most sensitive
ears; he was recorded as being able to discern noise at a level 12dB below the
standard for '0 dB'. Needless to say, Dr. Tom was a specialist in hearing; he
would attend on-campus concerts with a sound-pressure meter, an exposure meter
and some very expensive shooting muffs and later tell us how much hearing
could could expect on average to have lost from attending the ZZ Top concert
w/o hearing protection.  ;-)

From what I recall from college:
- most damage is done by very sharp impulses; it isn't so much the magnitude
  of the impulse as the rise time slope. The less time your ear has to 
  compensate for the amplitude shift, the more signifigant the "level shift"
  in your hearing. Your ears are truly marvelous; they operate in millisecond
  time. (otherwise, you couldn't tell which direction a noise is coming 
  from)
- total hearing loss is rarely due to a single trauma; Dr. Tom said that what
  in fact happens are several "level shifts", which is a incremental increase
  in the lowest level of sound you can detect or hear. After a particularly
  loud exposure to an explosion (impulse) or a sustained loud level (say
  a thrash-metal band in a small club or front-seat tickets at a Who
  concert), your hearing base level shifts up for some period of time due
  to the trauma and swelling in the ear. For most people, the level will
  return to normal with no further exposure to noise in a few days; the
  amount of time to return to normal varies from person to person.
  He referred to this as a "temporary level shift."
- recurring exposure to loud impulse noise or sustained loud volume noise
  would repeat the "temporary level shift" and the ear would in effect 
  "scar" (that's not quite the biological mechanism involved, but Dr. Tom said
  that it was the easiest way to think of it),  and the level shift would
  start becoming permanent, little by little.
- one more thing that he stressed on us; people subjected to impulse noises,
  be they gunshot, hammering, riveting, jackhammers or drumming, would
  experience a "notch" in our hearing. This was a long-term effect that
  was still under some research in the early 80's, but they were finding
  that people who were losing their hearing due to impulse noise would
  loose a notch at around 4kHz (I don't remember the exact frequency) first;
  this notch would grow wider and deeper with time. He _did_ note that people
  who go deaf due to impulse noise do so faster than most people exposed
  to sustained loud noise.

Please note that all of this is based on early 1980's instruction and
research.

summary: always use eye and ear protection.

dsa



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