Apparatus Competition

2006 AAPT Summer Meeting

Syracuse, NY

 

Glass Breaker / Resonance Box

 

David Kardelis

College of Eastern Utah

451 E 400 N

Price, UT  84501

 

435-472-8824

david.kardelis ‘at’ ceu ‘dot’ edu

 

Abstract:

A replacement for the demonstration using sound to break a wine glass is shown.  The wine-glass demonstration is dramatic but I found that it was hard to do.  The idea to use sound to break plate glass was mentioned on Tap-l about 6 years ago. The basic box is similar to what was discussed, but the support system for the glass plate in this version makes it much easier to use.

Construction of Apparatus: 

The wine-glass demonstration is dramatic but I found that it was hard to do.  It required a high quality driver and a loud volume that makes the demo uncomfortable to do.  Finding the resonance on a glass was difficult, often requiring an oscilloscope and microphone to find. The use of the oscilloscope made the students think that resonance was not an easily detectable phenomena.

The idea was to break plate glass was mentioned on Tap-l about 6 years. The basic box is similar to what was discussed back then.  I immediately went about making a version of the box as described back then. The support for the glass was small corks glued to the top of the box.  I found that I could not get the glass to break because the corks were too wide causing the plate to bounce around since I was not at the  Nodal point exactly.  The desisgn did not allow for easy adjustment due to different lengths of glass. The demonstration sat under a desk for 4 years. It took me about 4 years of reworking the basic design for supporting the glass. I kept coming up with overly complicated adjustable supports in my head before I had an “A-HA” moment about 10 minutes before a class. I quickly made the  new mount and broke my first piece of glass on the box before class started.  I had to show it to my students in that class even though we were no where near discussing resonance. The response was terrific and the ease incredible.  The demo is almost silent so I no longer disturb the entire floor with the high pitched sound needed to break a wine glass.

Actually everything in this project was scrap from something else. The speakers were left over from student speaker building projects, as was the gasket material, the particle board, glue etc.  The cost to build this is about $30 even if you have to buy everything.  About $10 worth of particle board local hardware store and the woofers were about $10 each (MCM electronics or Parts Express).  A small length of wire and a handful of drywall screws or finish nails are handy.

The outside dimension on my box are 24” x 11.5” x 11.5”.   I used ¾” particle board.

                        Cut List    4 pieces 10” x 10”
                                         2 pieces 10” x 24”
                                         2 pieces  11.5” x 24”

In two of the 10”x10” pieces rout/cut out an approximately 7” hole to hold the speaker. Adjust the size to fit your speaker.

In one of the 11.5” x 24” pieces drill three 2” holes, one in the center of the board, the other two 3” from each end.    

Build a box using the remaining pieces, I glued mine together using Liquid Nails Construction adhesive but would probably use regular wood glue now.  (I had an open tube of Liquid Nails, the students used on their speaker project).  This will leave you an open topped box.  Using two  6” wide boards as a spacer pushed up against the outside face, glue one of the 10”x 10” boards with your speaker mounted on it into the box, speaker cone facing toward the outside of the box. (This configuration puts the electrical connections in the same compartment) Remove the spacer boards and repeat on the other end. I connected the speaker in series since they were 4 Ohm speakers, connected so they push in phase. Run the wires out a hole in the side of the box or mount a connector to the box, I used banana plugs.

Screw or glue the 12x24” board with the holes drilled in it to the top sealing the box.  I used foam gasket material to seal the box and a touch of caulk where the gasket material butted up together.  See the picture.  This allows access to the inside in the future. I also caulked all the interior seams but do not really think this is necessary. 

To support the glass, I cut two approx 3” x 4” pieces of sheet magnet, this can usually be found for free since refrigerator magnets are often given away as advertisements. These magnets were then glued to the top just inside the outer holes.  I then took two L shaped steel brackets to support the glass. The thin steel makes a knife edge to hold the glass at the node (.227 L from the ends of the glass).  I had tried small rubber corks as originally suggested but they were too wide and the glass rocked off them. Also corks did not allow for adjustment in placement of the supports.  The magnets allow for easy repositioning of the supports for different length pieces of glass.

In my first test run, I took a pane out of a broken window from my house and using a glass cutter, cut it into 3” strips about 27.25” long (69.4 cm).  3” made the glass wider than the holes and the 27.25” was how big the glass pane was.  I originally designed the box for a 24” length of glass.   I tried just using the Pasco 9587C function generator.  It did not have quite enough power to break the glass, but the amplitude did get quite large.   So I took my adapter cable and fed the Pasco output into the  input of a 100W Radio Shack PA  amplifier (garage sale find) and then connected that to the woofers in the box.  I only had to go about 1/2 of the way to full volume for the glass to break at about 35Hz.  I did it twice and the two broke within .3 Hz of each other and since this glass has some crud on the ends from being in the window I thought this was pretty good. 

The air movement through the holes will easily blow out a lit match. 

Use of Apparatus: 

The apparatus can be used to show forced vibration of the glass plate.  It can also be used to show resonance of the plate.  At higher levels, the corrected nodal points of .227L from the ends versus the “ideal” nodes at 0.25l from the ends can be shown.  The highly increased amplitude at resonance can visible be seen and with an increase of the amplitude from the amplifier, the glass can be vibrated past the elastic limit and shattered.  All of this at a barely audible level. 

  Possible uses beyond a cool demo to show resonance:
              Mapping the response of the glass to frequency to get the Q-value.
              Lab to find the speed of vibration in the glass.
              Show higher harmonics of the standing wave.

Once the box is built a small amplifier ~30W+ and a stable frequency generator are needed.  I use a Radio Shack PA amplifier and a Pasco 9587C function generator. 

I hook the output of the function generator to the amplifier and then the amplifier to the speaker connections in the box. 

I take the length of cut glass, I am still using the broken window pane from my house. I mark the middle of the glass with a permanent marker, I also place a mark at .227L from the ends.  I place the glass on the supports lining the center mark on the glass with a center mark on the box.  I then adjust the two metal supports so that they line up with the 0.227L marks. The function generator and amplifier are turned on and the frequency adjusted till I get a large amplitude (resonance).  At this point the amplifier volume is turned up until the glass breaks.