PURPOSE: To break a glass beaker with a sound wave.
DESCRIPTION: An audio oscillator and a 40 Watt power amplifier are used to drive
a heavy-duty horn
driver. The driver is mounted behind a shatter proof enclosure. The beaker is
positioned 1 or 2 cm from the speaker, resting on a foam pedestal with either the node or
the antinode aimed at the speaker.
The signal is provided by a Pasco function generator. The signal is split by a T and is sent to the oscilloscope as the channel 1 input and synch, and it is also sent to the input of a Bogen Challenger amplifier. The amp output is sent to a model SD-370 horn driver made by Atlas/Soundolier.
The very precise determination of the resonant frequency is important. The resonant frequency is found with the sound at an intermediate level. This is accomplished by changing the frequencies of the oscillator very slowly above and below the resonant frequency of the beaker. A large increase in the amplitude and an abrupt 90 degree change in phase are observed at resonance. The resonant frequency of a typical 700 ml, 900 ml or 1000 ml pyrex beaker is usually somewhere between 800 and 980 Hz. The resonance of any particular beaker can be sampled by tapping the rim of the beaker. The oscilloscope is used to find and display the resonance of the beaker from the microphone. The O-scope is triggered by the function generator.
After the resonant frequency is found and the vibration of the beaker has been observed,
the amp can be turned up slowly and the increased oscillations will eventually exceed the
elastic limit of the beaker and it will shatter. When the beaker is vibrating it can be
seen by the eye using a strobe light. The motion may be displayed for a large group
using the video camera mounted directly above the beaker. A pyrex beaker has four nodes
and antinodes. The pour spout of a beaker forces a node at that point, so the
positions of all the other nodes and antinodes are determined by the location of the
spout. Very similar to a coffee cup, whose nodes and antinodes are determined by the
location of the handle. A microphone is mounted 1 cm from the beaker, at 90 degrees
from the speaker. Black felt is used as a backdrop for the strobe light.
EQUIPMENT: Beaker on a foam pad in a shatter proof box, horn driver, amplifier, connectors and adapters, oscilloscope, function generator, microphone, stroboscope.
SETUP TIME: One hour.
Below are some notes from previous discussions:
---------------------------------------------------------------------------------------------------------------------------
Date sent: Thu, 31 Aug 1995 12:05:44 -0400
I have found out that what you set them on makes a big difference. Use styrofoam about an inch tall.
The shield that I use is the sameone that Pasco uses for their Chladni Plates. Set it up between the glass and the students. When you hit resonance, if you touch the beaker and you will see it dampen on the oscilloscope. Also, turn up the volume slightly and turn it down fast and you will see the ringing from the beaker damp away.
The mechanical universe has it at the end of Resonance.Doug
Date sent: Tue, 29 Aug 1995 19:10:16 -0400
The amp is a 40-Watt Bogen Challenger, model C-100. The explosion proof horn is made by Atlas Sound of Parsippany, NJ. It has a die cast metal enclosure. We first tried the horn model PD-5VH. It is rated at 40 Watts but blew because the driver, also rated at 40 Watts continuous, gave a peak power of more than 100 Watts at full crank during the demo. So we upgraded to a horn that can handle 100-Watt square waves. It is the model SD-370 made by Atlas/Soundolier
Atlas Sound
phone (201) 887-7800
10 Pomeroy Rd.
Parsippany, NJ. 07054
Wineglasses work great, BUT you have to use expensive leaded crystal with a high Q. I have a faculty member here who keeps blowing my driver diaphragms out. Jerry could you give me details onyour driver/tweeter. I had a supply of good wineglasses cheap now I can't convincethis person High Q is what break glasses, he keeps bringing me glasseswhich is "the right shape" with no Q to speak of. If they don't ring fora long time they won't break. Zig
Date sent: Mon., 28 Aug 1995 15:26:40 -0400
There are some tricks to doing this. You must know what to look for at resonance and how the beaker responds. If you’re using a beaker and not a wineglass then it is important to know where the nodes and antinodes are located. With the wineglass it is not a problem because it does not have a lip pour spout like the beaker does so the nodes and antinodes will occur wherever you point the driver. The output of the generator is split through a T connector and input simultaneously to drive a 40 watt amplifier and also trigger the scope. The mic signal is input to the scope channel two. The speaker and mic are placed very close to the beaker, almost touching. The driver should point to a node and the mic should point to an antinode, but there is some argument about choosing a node or an antinode for the driver to point at. I point the driver at a node.
Turn on the signal generator and sync the scope. View this sync signal. Turn on the amplifier and drive the speaker with this same signal but carefully keep the volume at a moderate level so as not to break the beaker. Chop the scope inputs to view both the output of the microphone and also the sync on channel. The trickiest part to this demo is to determine resonance. At resonance the beaker will cause a large amplitude and phase shift in the mic signal. But it isn't that easy because there are often large beats near resonance, depending on what king of glass or beaker you choose. Your frequency generator must be stable and must have a good fine tune knob. The hardest part about learning to do this demo is to learn how to identify the resonance frequency very precisely.
When you’re close you’ll see a large amplitude gain. But be careful. It is EXACTLY at the peak amplitude that resonance occurs. You must be dead on. As little as a fraction of a hertz drift away from resonance and it will vibrate the beaker but it will NOT break it. When I give this demo set up to faculty they often fail because they are not careful enough at finding resonance. You should not have trouble if you have the equipment and set it up to find resonance response correctly. And you have the right kind of beaker.
We use Pyrex 900-ml beakers. They often come in near 880 Hz although every one is different. On some the glass wall is thicker and they come in lower, about 790 Hz and are harder to break. Some have a thinner wall and come in higher, about 900 Hz and these break the best.We use explosion proof tweeter speakers that can handle 100 Watt square waves. I wear ear muffs with a 29 decibel reduction and the audience is protected from glass fragments by a shield.
To enhance the view of the mode of vibration of the beaker use a strobe light adjusted by eye to get a submultiple of the harmonicswhere the beaker oscillates visibly.
