Glass shatter
For the glass we use about eight times bigger. However, the wavelength of sound in air is much greater than the size of the glass. The loudspeaker needs to distort the glass so that opposite sides of the glass are moving in opposite directions as shown by the arrows. The diagram below shows the vibration mode used in breaking for the glass viewed from above (you can also see this type of deformation happening in the video as well). The problem with using a normal loudspeaker is that you excite the glass in a very inefficient way. What loudspeaker should you use?Ĭonventional loudspeakers don’t work. The glass should also have a very simple shape and not be embossed. We need a strong resonance, because then it is possible to force the glass to vibrate with a big enough displacement to break. Damping provides a force that acts to stop objects vibrating, so we don’t want that. The best way to find the right glass is to go to a shop with lots of glasses and go around gently tapping them! This indicates a glass with little damping. When you ping it with a finger it should ring easily and for many seconds. The glass must have a very strong resonance. We use large red or white wine glasses with thin walls. If the force from the sound wave making the glass vibrate is big enough, the size of the vibration will become so large that the glass breaks. Therefore the glass needs to be moved by the sound wave at that frequency. How is the glass broken?Ī glass has a natural resonance, a frequency at which the glass will vibrate easily. We accept no responsibility for people trying this experiment, it should be left to the professionals. Typically the loudspeaker produces about 100+ dB for someone close to the glass, therefore hearing protection is needed for those close-by. The glass doesn’t normally travel far when breaking, but sometimes it is surprising how far the glass will go. People near to the glass and loudspeaker need to wear safety goggles and ear protection (ear defenders or ear plugs). If you watch the high speed footage above, you’ll see glass goes in all directions. This is dangerous and is liable to lead to shattered glass going into the mouth. There several videos on Youtube showing people doing this acoustically by placing glass right close to the mouth. This is great for building tension in the audience! Safety It is quite nerve racking doing this live on stage, because it sometimes takes a bit of fiddling and tuning to work. To do it reliably for a science demonstration requires practice and careful choice of the glass and loudspeaker. This may have appeared in the movies, but is it really possible to shatter glass with sound? Yes, but this is harder than you might think. (c) Acoustical and Audio Engineering, University of Salford A classic demo of resonanceĪn opera singer hits a high note and this shatters a glass in the drawing room.