At 3:25 AM +0100 3/5/04, Listar wrote:
>Date: Thu, 04 Mar 2004 15:54:50 -0500
>From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxx>
>Subject: [OM] Re: OT Speaker and hi-fi bragging
>
>Then we have to face the fact that the hearing range of we feeble humans
>begins to deteriorate at age 19. Too bad that we old farts can no
>longer hear the fine quality of the units we worked so long and hard to
>be able to afford. At age 18 I could hear just fine up to 21KHz (except
>we didn't know what a KHz was then) But not any more. The rest of you
>probably can't either except that you just don't know it.
In my youth, I could hear at least to 26 KHz, but I sensed it as a pressure
rather than hearing it as a tone. So could at least one girlfriend. We were
all in our twenties.
This started when said gf came back from the local chain drugstore complaining
that they left the ultrasonic burglar alarm on, and it was driving her right
out of the store. Now we all "know" that 21 KHz is the limit, but then she
never complained about any other store. So I went to drugstore, and sure
enough I could sense it as well. So I asked the store manager if the burglar
alarm was on, and she said (with a mixture of surprise and suspicion) that it
was on standby. I told her that both I and gf could hear it, despite the
textbooks, and suggested that she was losing customers for it. They would be
uncomfortable, and would flee, never knowing exactly why they didn't like the
store. Well, there is no reason for the alarm to be on (versus in standby) in
the day, so she turned the alarm off. Pressure disappeared.
How do I know that it was 26 KHz? In those days (the 1970s), standard
ultrasonic transducers for use in the air come in just two frequencies, 26 KHz
and 40 KHz, and 40 KHz is far too high for any of this to happen. Even today,
most alarms use one of these two frequencies, mainly because the transducers
are dirt cheap. (Underwater transducers come in a wider range of frequencies.)
Why was I digging into ultrasonic transducers? Because I was building
instruments to track rats. These instruments were used for pharmacological
research. There was a problem with this ultrasonic doppler approach -- rats
can vocalize at 50 KHZ and presumably can hear at least to 50KHz, probably far
higher, and the 40 KHz transducers are *very* loud.
Joe Gwinn
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