From: Dave Arnett <davea@cv.hp.com>
Newsgroups: comp.sys.hp48
Subject: Re: Noise from new HP48GX?
Date: 18 Sep 1996 00:59:12 GMT
Organization: Hewlett-Packard, MCD

Robert George <robert@erc.msstate.edu> wrote:

Hiya, Robert.

<snip>
>... there is a
>faint, but discernable, high-pitched whine comming from the LCD panel.
>
>This is undoubtably the static inverter for the AC panel.  Has anyone
>else noticed this noise?

This is a common question, my friend.  The best answer I have is below.
It is intended to be general, for Andre Schoorl's use, so I may write
at a more basic level than you need.  (Andre - this is the third time
I have responded to this question over the years, and it's a very 
reasonable question.  Could this be a candidate for the FAQ?  DA.)

I will point out at the start that I am no longer on the HP48/HP38
design team, as the entire product family was moved to our sister
division in Singapore.  I am not up to date on any changes that may 
have been made to the product since the design responsibility moved.
However, your description of the sound seems quite familiar

The guts of the HP48 keyboard are assembled from several layers of 
mylar and similar material.  Some of the layers have conductive ink
to create switch contacts.  Some are insulators, with appropraitely 
located holes through which the switch contacts meet.  Some are 
embossed with spherically domed areas, to provide the snap-spring
feel when you press the plastic keys.  The entire multi-layer
sandwich is assembled onto plastic pegs on the backside of the
HP48 top case, along with the metal chassis, and then the pegs are 
'heat staked,' positively capturing and holding the matrix together.

These parallel planes can act like a capacitive speaker.  Most
loudspeakers in home electronics are inductive: a sheet or cone moves
because current passes through an attached coil positioned in a 
magnetic field.  A capacitive speaker creates sound by moving a sheet 
or diaphragm due to electric charge forces, whether attracting or 
repelling.  The HP48 keyboard has the necesary characteristics to be
a very inefficient (therefore quiet) capacitive speaker.

When the HP48 'scans' the keyboard, charges are placed on the parallel
sheets of mylar.  They tend to move as a result of the charge, and 
create a faint squeal or whine.  The loudness will depend on a number 
of factors, and can vary significantly from HP48 to HP48.  Some folks
report that, on their HP48, the sound is stronger on the back than on
the front; others report the opposite.  

Good Day!
Dave.

------
I don't speak for HP when I post here.
