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Re: [APD] Fw: DIY reflector?

Well, I meant that it was not visually perceptible. But it would not likely be audible either. If a child can hear as high as 20kHz, that's not too rare. If a middle-aged male hears well at freqs above 15kHz, that's not too common and is probably a sign of a sound-sheltered life ;-)
. The high frequency one can sometimes hear from CRT displays, like old TVs, was usually from the flyback transformer transferring vibrations to other parts of the the chassis, although it can be from other things too. I recall one TV set that my father and I rebuilt when I much younger than now. It made an awful squeal that tested out somewhere around 18 kHz. Such a racket. But it wasn't a racket to him and I know it would not be one to me now.
Anybody that can hear 30kHz or higher, call Guiness! They have a place waiting for you. Even if a coil in an electronic ballast is operating at, say, 45kHz, other parts of hte circuit can introduce noise on the power lines, and emi radiating form the lines, that can cause audible static on electronic devices.

----- Original Message ----
From: Donald Hellen <donhellen at horizonview_net>
To: S. Hieber <shieber at yahoo_com>; aquatic plants digest <aquatic-plants at actwin_com>
Sent: Thursday, July 20, 2006 9:11:07 PM
Subject: Re: [APD] Fw: DIY reflector?

On Thu, 20 Jul 2006 12:22:12 -0700 (PDT), "S. Hieber"
<shieber at yahoo_com> wrote:

>An advantage of an electronic ballast operating at 15k-80kHz is that the on/off cycling is in no way perceptible. Also, any man over 40 would be lucky to be able to hear a hum in that frequency range.  ;-)

Unfortunately, I hear the high frequencies too well.
Odd, but true. With hearing aids, though, I wouldn't
notice something that high since I would hear the lower
frequencies much louder. 

Television picture tube scanning guns (somewhere in the
15 khz range) used to drive me nuts. I could hear them
through windows and tell that a TV was on even if the
sound was muted. Now I'd have to be a lot closer to the
TV to hear it. 

With those electronic ballasts, it would be very easy
to stabilize them mechanically, which is the problem
with the 60 cycle ballasts. If the laminations of the
ballast transformer get loose from the iron core, they
buzz like crazy. You can sometimes fix this, but it's
not worth the trouble in most cases.

Imagination is more important than knowledge.
   -Albert Einstein
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