audiology student project

HearWHAC HearWHAC at
Sat Jan 13 15:38:37 EST 1996

Jeff Sirianni, sirianni at UTS.CC.UTEXAS.EDU posted a request from a teacher:
>>One of my students, Nathan, who is deaf in both ears, is working 
>>on a freshman research project on hearing aids. He would like to figure 
>>out someway to reduce the problems of moisture in hearing aids. <snip> He 
>>would like to 
>>figure out how to coat a hearing aid with a material (perhaps a polymer) 
>>in such a way that the moisture could be reduced. He knows about the 
>>Goretex covered aids. He also has to figure out how to cut down on 
>>moisture from condensation inside the hearing aid.

In the 1940's and 1950's hearing aids used crystal microphones. Crystal, 
like salt, will collect moisture. Thus, back in those days, they had far 
greater problems with moisture and hearing aids than we have today. (Hard to 
believe, isn't it!) Of course, hearing aids were also very bulky at that 

My father used to collect one gallon glass jars (they are quite large) and 
place some calcium chloride in the bottom. He would place a plate or 
something over the calcium chloride so the hearing aid would not touch it. 
At night the person could place their hearing aid in the jar and by morning 
it was completely dried out.

We used to get calcium chloride at lumber yards or the farm machinery place 
(it is also used as an anti-freeze for the water in tractor tires) in 100 
pound sacks and it was my job to divide it up into small amounts and place 
it in the jars and seal them very tight. If it is left unsealed, it will 
quickly turn to liquid.) Calcium chloride sucks in a large amount of 
moisture before it turns to liquid itself. We used to put one kernel of 
calcium chloride on a table and watch it slowly collect moisture and turn 
into liquid.

If you ever use it, be sure to wear gloves or something so as not to touch 
it. I would never want my hearing aid to touch it. It is corrosive.

Paul Woodard :-)

PS It was quite an improvement when crystal microphones were replaced with 
magnetic mikes. In 1970 or so there was another great improvement, the 
electret condenser mike. They were much less susceptible to breakage when 
dropped. Except the first ones amplified the bass too much. Progress is not 
always better!

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