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From: davidl@teklds.UUCP (David Levine)
Newsgroups: net.micro.mac
Subject: Re: "Ultrasonic" hearing
Message-ID: <1517@teklds.UUCP>
Date: Thu, 16-Jan-86 13:22:16 EST
Article-I.D.: teklds.1517
Posted: Thu Jan 16 13:22:16 1986
Date-Received: Sun, 19-Jan-86 04:17:32 EST
References: <1280@brl-tgr.ARPA> <590@harvard.UUCP> <11370@ucbvax.BERKELEY.EDU> <251@birtch.UUCP> <1115@oddjob.UUCP>
Reply-To: davidl@teklds.UUCP (David Levine)
Distribution: net
Organization: Tektronix, Beaverton OR
Lines: 54
Fcc: macbox

In article <1115@oddjob.UUCP> paluzzi@oddjob.UUCP (Greg Hajek) writes:
>In article <251@birtch.UUCP> oleg@birtch.UUCP (Oleg Kiselev) writes:
>>>     I've never encountered a TV receiver or monitor that
>>>didn't emit an annoying high pitched squeal at at least a
>>>low audible level.
>>     ... Sometimes a sharp rasp on the side of the terminal would silence
>>it for an hour or so ( and many people got so irritated by the squeal they
>>did it routinely).
>
>I have had the same problem with my Mac, and also found that the "sharp rasp"
>(read: "swift kick") solution is usually effective.  Why is this?  I never
>had the problem before I blew my power supply, and had to replace the analog
>board.  Can anybody suggest a possible solution (or a reason why this problem
>would suddenly crop up, for that matter)?

I, too, can hear "ultrasonic" frequencies.  Those ultrasonic motion detectors
they use in some stores drive me just BATS.  Fortunately, my Mac doesn't make
those noises, for which I am grateful.

The reason TVs and monitors squeal is this:  A transformer is a stack of iron 
plates, separated from each other by a thin coat of lacquer or plastic, bound 
around with two or three wires.  Alternating current flows through one wire, 
causing magnetic resonances in the iron plates, which in turn causes current 
to flow in another wire or wires.  The ratio of the number of turns in the 
supplying wire to the number of turns in the receiving wire determines the 
ratio of output current to output voltage for a given input current and 
voltage.  That's how a transformer turns one flavor of electricity into 
another.

When the current flows, it causes magnetic stress in the iron plates.  They
attract and repel each other many times per second.  If the lacquer is flexible
or cracked, the plates actually move a tiny bit.  If they rub against each
other or any part of the case, this movement can cause a squeak or squeal.  
It's only obnoxious when the squeal is in audible frequencies.  As it happens, 
the flyback transformer of a television or monitor (which controls how often 
the scanning dot on the screen "flies back" to the left edge after scanning 
across a line) uses frequencies that are audible to some of us.  If you happen 
to have a transformer that squeals, and you can hear it, the device makes an 
annoying noise.  Otherwise, it doesn't.  

Some transformers always squeal, others never do, others sometimes do and 
sometimes don't.  If banging on the case helps, it's because it settles the
plates into a configuration where they don't rub against each other.  
However, eventually they work their way into a configuration where they 
squeal again.  There's no long-term solution except to replace the transformer 
with one that doesn't squeal (you might have to test a few at the frequency 
the Mac uses before you find one).  Alternatively, you could attend a few 
Quiet Riot or Twisted Sister concerts to eliminate your ability to hear high 
frequencies... :-)

Another modern Just-So Story from

David D. Levine (...{decvax,ihnp4,hplabs}!tektronix!teklds!davidl) [UUCP]
                (davidl%teklds%tektronix@csnet-relay.arpa)         [ARPA]