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A
Straight Dope Classic from Cecil's storehouse of human knowledge11-May-1990
Dear Cecil:
Can operatic sopranos really break glasses with their high notes?
What note does the trick? How come they don't break windows and
eyeglasses and whatnot at the same time? Can women do this better
than men? Can I learn how? Or have I been the victim of an
elaborate hoax? --Vox Clamantis, Chicago
Dear Vox:
I dunno--you ever buy whole-life insurance? Now _there_ was a hoax.
Shattering glasses, on the other hand, is entirely legit. Enrico
Caruso and Italian opera singer Beniamino Gigli are said to have
managed it, and I seem to remember Ella Fitzgerald doing it once in
a Memorex commercial.
The technique is simple. First you find somebody with perfect pitch
and leather lungs. Then get a crystal glass and tap it with a spoon
to determine its natural frequency of vibration (this varies with
the glass). Next have the singer let loose with precisely the same
note. When he or she is dead-on pitchwise, the glass will commence
to resonate, i.e., vibrate. Then turn up the V. Bingo, instant
ground glass.
What we have here is a graphic demonstration of forced oscillation
resonance. If something has a natural rate of vibration, pump in
more energy of the same rate and with luck the thing will vibrate
so bad it'll self-destruct. It's like giving somebody on a swing a
good shove at the top of every arc--soon they'll reach escape
velocity and soon after that they'll be picking vertebrae out of
their sinuses.
Breaking glasses, however, is strictly light entertainment. For
real forced oscillation action you want a suspension bridge. In
1831 troops crossing a suspension bridge near Manchester, England,
supposedly marched in time to the bridge's sway. Boy, did they get
a surprise. Ever since soldiers have been told to break step when
crossing bridges. The same fate befell the Tacoma Narrows
suspension bridge in Washington State on November 7, 1940, only it
wasn't soldiers that caused it to collapse, it was the wind.
But back to the home front. Crystal is more vulnerable than
ordinary glass because it has more internal structure, which allows
waves to propagate. (Take my word for it.) But you can annihilate
damn near anything given enough volume. One physicist, obviously
one of your classic Roommates From Hell, claims he inadvertently
shattered a glass lamp shade while playing the clarinet.
Think of the possibilities. Most of us don't have the pipes to
break glasses by sheer voice power, but we all have clarinets,
don't we? Unfortunately, none of the standard physics cookbooks
gives a detailed glass-bustin' recipe. Too bad. A fascinating
classroom demonstration like this would surely convince many young
people to give up MTV and devote their lives to science.
SHATTERING MYTHS
Dear Cecil:
In the matter of glass-shattering vocalism, Cecil seems to have
been led astray by Gunter Grass's fictional tin drummer, Oskar. In
fact, there is no authentic record of glass being broken by the
unamplified human voice. Dorothy Caruso categorically denied rumors
that her late husband had accomplished the feat; a fortiori it was
beyond Gigli's comparatively feeble instrument. Practically
speaking, there are reasons to believe the thing impossible, and
without going into technical detail, the following are among them:
(1) Glass is simply much too strong. Try shattering a wine glass in
your (gloved) fingers. Not easy. Now imagine doing the same with
the puny little bands of your vocal cords. (2) Coupling acoustic
energy from larynx-to-air-to-glass is highly inefficient due to
large impedance mismatches; by contrast, marching troops couple
very efficiently to bridge platforms. (3) In glass shattering
attempts, resonance or no resonance, the glass structure finds
other ways to dissipate energy short of fracturing. Remember the
playground swing in which successive small but well-timed swings
sent your sister sailing higher and higher? And the tales of going
"over the top" when the process went critical? Alas! it never
happened, because other dynamic processes supervened ("Gee, Mom, we
were just playing") before the longed-for loop could occur.
--Timon, Dallas
Dear Timon:
A fortiori? Supervened? Boy, I see I wasn't the only one to get a
Word-A-Day calendar for Christmas. As for glasses, let's clarify
one thing: it is certainly possible to shatter glasses with the
amplified human voice. The folks at the Memtek company in Fort
Worth, Texas, which makes Memorex recording tape, do it all the
time for sales demonstrations and whatnot. (You'll remember that
Memorex used to run those TV commercials showing Ella Fitzgerald
and others breaking glasses with their voices.)
What's more, they do it pretty much the way I described: they go
out and get a drinking glass with high lead content, tunk it with
a rubber mallet to make it ring, then read the frequency on an
analyzer. Then they get a singer to sing the same note (typically
F above middle C), amplify it to maybe 92, 94 decibels, and with
luck you get glass shrapnellini. Memorex technicians using a strobe
have found that prior to the break the sound causes the rim of the
glass to deflect as much as a quarter inch. (I get this from Rick
Needham, engineering manager, lest you think I am making this up.)
Your beef is that I suggested this could be done with the
unamplified human voice. I'll grant I haven't been able to turn up
a documented instance of this, but it seems subsidiary to my main
point, which is that you can shatter glasses with sound, and
furthermore that the human voice, which can generate a relatively
pure tone, is well suited to this purpose. Furthermore, none of the
technical people I spoke to about this seemed to think doing it by
voice alone was completely impossible. Admittedly 90-plus decibels
is pretty damn loud, but one of the reasons the Memtek folks crank
it up that much is that they're using an inexpensive ($7) glass
rather than fine crystal, which is more fragile. So let's not be so
negative, Timsy. It's the can-do attitude that has made this
country great.
BRIDGE CRASH NEWS FLASH!
In his recent treatise on whether singers can break glasses with
their voices, Cecil mentioned "forced oscillation resonance," in
which an external force amplifies the natural vibration of an
object, sometimes with destructive results. As an example he cited
the 1940 collapse of the Tacoma Narrows bridge. The usual
explanation for this disaster is that the wind gusted (to be
precise, "generated a train of vortices") in perfect synch with the
bridge's natural rate of bounce, causing its demise.
Reader Wilbur Pan has alerted us to a recent report in Science News
heaping abuse on this widely held view. Mathematicians Joseph
McKenna and Alan Lazer doubt that a storm could produce the
perfectly timed winds required. They're working on a "non-linear"
model of bridge behavior they hope will provide a better
explanation. The main problem apparently is that when the roadway
of a lightly constructed suspension bridge flexes, the cables
supporting it go slack, introducing an element of unpredictability
in which little causes (i.e., the wind) produce big results (i.e.,
a collapsing bridge). They hope to have the mathematical model
describing this effect finished in five years--not the most
aggressive schedule in the world, but apparently this is government
work. You'll read about it here first.
--CECIL ADAMS
The Straight Dope / Questions or
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