I was having a discussion with someone about wether or not the actual sound wave and not the flexing of the roof or surrounding attachments can shatter a windshield.
I came across this article on why its so hard to shatter a square foot of glass and thought it was worth sharing.
"Dear Ralph, A fine question, indeed. At first it was not obvious that you absolutely cannot shatter a square piece of glass using resonance. But I will attempt to shed some light on the subject. The resonant frequency of glass depends on a number of factors, including shape, size, and chemical composition, but also on the physical stresses that are present in the glass crystal itself. All these will contribute to defining the natural frequency of a particular glass sample. The shape of the glass really relates to the boundary conditions of the glass. When a glass piece is vibrating at it's natural frequency, it means that there exist a number of preferred complex vibrational modes that are active on the surface. In order to use the resonant frequency like an antenna to transfer acoustic energy into the crystal structure of the glass, a large subset of those resonant modes needs to be free to vibrate. A slight diversion: consider a radio antenna. An optimized antenna is a whole fraction of a wavelength, preferably 1, 1/2, or 1/4. The smaller the antenna gets, the fewer vibrational modes are availble for the incoming radio wave to excite in the antenna, so the weaker the signal. Also, the harmonic waves (less than 1 wavelength) amplitudes decay rapidly with decreasing wavelength, so not only are there fewer modes, but the permitted modes are the weaker ones. In solving the 3D wave equation, the solution for a circular boundary (such as a round drum) are Bessel functions, while the solution for a square boundary is limited to sines and cosines. This limitation of the square drum head to vibrating in sines and cosines may be why breaking a square piece of glass is more difficult (but probably not impossible given enough input acoustic energy). Read on: Consider the case where a wine glass is gently struck and allowed to ring. Then, if you place your finger on the glass somewhere and strike it again, the resonant pitch is the same, but the amplitude is reduced. This is because you are forcing a node (point at which no surface motion is allowed) by placing your finger on the glass. This will prohibit certain vibration modes that require an antinide at the point where your finger is on the glass, so you are restricting the available vibrational modes of the surface, and reducing the overall energy in the surface vibrations making the amplitude of the sound less. This is much like the square boundary condition limiting the vibrational modes to sines and cosines- there are a large number of other modes that are not possible given the boundary condition. Back to coupling acoustic energy into the glass. If there are fewer permitted vibrational modes, then the ammount of availble acoustic energy that will couple into the glass will also be reduced. In order to couple enough energy into the crystal structure to break it, you would need to increase the input energy accordingly. There comes a point, however, where the input energy is so great that it may no longer be resonance that is causing the shattering, but just the compression of the airwaves against the surface (i.e., the sledge-hammer approach). So it _may_ be possible to break square glass given the right conditions, but these will change from sample to sample, and is much more difficult than for other glass shapes such as a round piece or a bell-shape (wine glass). This sounds plausible to me, but I don't have any direct experience"
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