I
decided to set up a speaker to try and shatter a wineglass
with sound waves. Opening up the cupboard, I found an
old wineglass with thin walls. Perfect.
To obtain
the natural frequency of the wineglass I set up an amplifier
and microphone, with the output of the amp connected
to an oscilloscope. Placing the microphone next to the
wineglass, I 'pinged' the wineglass and captured the
oscilloscope waveform. The resulting waveform 'rings'
at the natural frequency - just what we want!
The
natural frequency for this particular wineglass was
found to be ~1170 Hz. I entered this value into a frequency
synthesizer program and connected a horn speaker to
the output of an old 6W amplifier. Crude!
The
speaker, glass, and a few other components were enclosed
within an air-tight plastic box as obvious protective
measures.
Initially
setting the volume quite low, I used the same microphone-oscilloscope
setup on the other side of the wineglass to check for
resonance. At 1170 Hz, the wineglass was vibrating,
but not enough. Trailing values from 1160 to 1180 Hz
showed that maximum vibration occurred at 1168 Hz. I
had found the exact natural frequency.
I plugged
this value into the synthesizer and spun all intermediate
volume control sliders to max. A video camera was set
up outside the 'blast box' to capture some of the action.
Don't
forget to turn up the sound! Otherwise you probably
won't even know when it breaks...(The noise reduced
version enables you to hear the glass shattering more
easily, but the raw capture gives an idea of how loud
the 1168 Hz tone is).
And
here are the results... a poor old shattered wineglass.
If you watched the video, you would have seen the little
paper bits that jumped off the wineglass before it shattered.
Those paper bits indicate magnitude of vibration, and
I noticed the glass shattered soon after the vibrations
were strong enough to kick the paper off.
Unfortunately,
as soon as the glass fractures, it no longer resonates
at the same natural frequency and doesn't 'ping' anymore,
so there is no further destruction other than the possibility
of smashing it on the ground.
A better
view of the internal fractures in the glass. The main
fracture actually extends almost right around the goblet.
It appears
that shattering a wineglass is not such hardwork after
all! A quick lashup was able to do the trick. But what
if a bigger glass is used with a bigger amp?
