At 05:05 PM 11/5/2010  -0500, mcneely4@cox.net wrote:
>I have used (and have seen a professional use) a long piece of round
>steel 9/16 inch stock.  Just put one end on the place suspected of
>producing the noise, and the other end against the mastoid bone (the
>convex mass behind the ear).  Move the sensor end about a bit, and
>pinpointing the noise is pretty easy.  Actually, anything that will
>transmit sound readily and that can fit between your ear and the
>suspect part will work.

Yep.  That's how you do it with a screwdriver.  But I bet if you try
a decent stethoscope you won't want to go back.  It removes the
limitation of having a straight line between you and the DUT (device
under test) and if something whacks or grabs the end it won't get
punched through your mastoid bone (which is spongy enough that I
suspect it is fairly easily punched through) or otherwise rearrange
your anatomy.

Having seen a lathe with a piece of stock sticking out the back of
the spindle that got just that tiny bit too far out of true and
suddenly turned into a whirling scythe, I have a lot of respect for that.

It also amounts to a mechanical amplifier, as a diaphragm an inch or
so across moves air through a 3/16" tube,.  Gently scratching the tip
across a surface can be almost painfully loud.  And although I doubt
if it makes much difference in this application, it's a much
higher-impedance device, easier to drive.  But to get the idea, think
of using your 9/16 bar as a phonograph needle.

Yours,
David