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Singing Wineglass Trick OPTOMIZED!
2015 W. Beaty

Try rubbing your dry finger rapidly across glass.   There is little 
adhesion, *no stick and slip* motion.   Instead it slides like thick 
grease, as if static friction was too low relative to dynamic friction.  

Human skin is oily.   DOH!

Only if we rub a dry fingertip very, very slowly (unlike the higher speed 
needed to 'sing' a glass,) only then will we note any stick and slip at 
all.   And it doesn't do this fast, more like crunk-crunk-crunk.  We need 
to be able to "squeal a finger" akin to a malfunctioning windshield wiper.  
Dry fingers won't do this.

Now try rubbing a wetted finger across glass.  (Just wet then rub once.  
Don't rub several times, just observe the first event.)   Still almost no 
stick and slip motion, eh?   More like wet, pasty grease.

Finally, wet your finger and repeatedly abrade it upon the glass, or even 
better, wet and then stroke your finger across some clean cloth, repeat.  
Or far better, *wet with wine*, scrub against a napkin, repeat this a few 
times.  Or if no alcohol available, scrub your fingertip with saliva 
across the sharp edge of lower teeth.   Clean off the edge of the glass as 
well; don't re-deposit old finger grease back onto your skin.

In other words, remove all the normal skin oils.

With a wet, alcohol-cleaned finger we observe a very large stick-and-slip 
effect.  The skin adheres to the glass; static friction is now very high 
compared to dynamic friction.  And we'll be able to 'sing' the glass even 
with a very gentle touch.

Super powers: in a restaurant with borderline-lossy wineglasses, you'll be 
able to sing your glass while nobody else can.  (First surreptitiously 
scrub your fingertip on a wine-wetted napkin.)

If a wineglass just won't start up, just realize that the whole system 
constitutes a parametric amplifier, and small transients may take 
near-infinite time to grow to audibility.  So, thwack it hard with a 
fingernail, then while it's already ringing, lower your moving finger and 
start circling.  That way the effect doesn't have to exponentially ramp-up 
all the way from thermal noise fluctuations.

While hand-washing dishes, after your fingertips have been scrubbed 
extremely clean, try rubbing the rim of various glasses in clean or 
slightly-soapy water.   In these best-case conditions you can locate which 
of your own glasses are impossible to sing.   Also, the ones which sing 
best can be used in the "exploding wine-glass" demonstration using a 
100-watt amplifier, microphone, and the horn-driver from an outdoor PA 
loudspeaker.

.

---

Also note: with a half-full glass, the tiny ripples on the liquid surface 
will form a broad cross-shape, and the cross pattern is rotating in synch 
with your finger motion.  These are Faraday Surface Waves.  They're very 
weird: they rely on a nonlinear effect, their frequency is half that of 
the drive frequency, not a harmonic.  They're a classic example of the 
existence of SUB-harmonics, and also one example of a soliton.

Listen carefully to the singing.   It's not smooth, but instead varies 
rapidly like wow wow wow wow.    The "antenna pattern" of acoustic wave 
emission is also cross-shaped with four nodes.  It's like a rotating 
tuning-fork emission, but with four nodes rather than two.   It rotates 
with your finger, so you'll hear four peaks, four "wow" variations, for 
each complete circle made by your finger.   More detail: the pattern is a 
standing wave, but the nodes are moving, therefore the standing wave is 
composed of waves of slightly different frequencies moving in opposite 
directions.   The beat-note is in space as well as time.  Cool, eh?   And 
this reveals another optimization hint:  if you suddenly change the speed 
of your finger, it will move onto an antinode and damp out the ringing.   
So, if the speed of your circling motion is extremely uniform, the ringing 
sound will build to intense volume level.  Monitor your own speed 
variations by listening for the repeating nodes and peaks.  If your speed 
is fluctuating, your finger will absorb much of the vibrations, and 
instead the sound will weakly appear and vanish.

Another hint:  tap the edge of a wine glass hard with a fast-moving 
fingernail.   Does it ring for awhile, or does it just go "thunk?"   If it 
doesn't ring, then you'll find it nearly impossible to sing that glass 
with a fingertip.    The singing-wineglass effect requires a low-loss 
(high-Q) resonator.   However, if using all of the hints discussed above, 
sometimes you can 'sing' a glass when nobody else can.

Here's an unsucessful demonstration (so far!)  Use your glass-tubing 
cutter tool or carbide "poker chip keychain knife sharpener" and put a 
tiny scratch near the rim of the glass.   Now make the glass ring 
intensely loud.   Shatters?!

1. First test the wineglass for low-loss resonance
2. Well scrub the fingertip and glass rim, remove all oily contamination
3. Alcohol seems to work better than non-alcohol
4. Tap the glass to get it ringing before your moving finger makes contact
5. Rub with utterly constant speed, listening for an unvarying wow-wow-wow 
sound


 






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