To identify the higher frequency mode of vibration you will have to use an audio spectrum analyzer. One way of making this measurement is to use a microphone/preamp/frequency counter, 3 strike the glass gently with a rubber bung, and record the average value of several measurements. Back in the Prep Room, the resonance frequency of each glass must be determined to an accuracy of ± 1/2 Hz. Thus the glasses should be carefully selected in the store (take a 512 Hz tuning fork and ignore the puzzled and anxious looks of the salespeople as you strike their glasses). The lower frequency glasses tend to flex more dramatically (because of thinner walls) and ring longer. 2 Although they all look very similar, their resonance frequencies vary over roughly a 100 Hz range with an equally unpredictable Q. The wineglasses we have been using for the last decade or so are white wine stem glasses. 1 Setting it up:Ĭhoose a large wineglass whose sides don't curve in too much at the top. This is a fine opportunity to point out to the class that, unlike one-dimensional simple harmonic o scillators, the higher mode frequencies are not multiples (harmonics) of the fundamental frequency and indeed, the ratios of the resonance frequencies are different for every glass. The peak-to-peak amplitude of the excursions are much less than in the fundamental mode (2 or 3%). The strongest of these is a mode in which the rim of the glass vibrates with three-fold symmetry (six nodes). The wineglass can also be excited (and shattered) in one of its higher normal modes of vibration. Peak-to-peak amplitudes have sometimes been as much as 5 or 6 mm (8-10% of the rim's diameter). The fundamental mode of vibration is that in which the rim vibrates with two-fold symmetry and assumes a ellipsoidal-like shape, alternating in orthogonal directions (see figure 1 following the Comments section). This is done by slowly increasing the volume in steps so that theaudience has time to appreciate the incredible distortions the glass manages to survive before its elastic limit is exceeded. The wineglass can be "played with" for a while before increasing the volume to the point at which it shatters. Rather than "freezing" the motion, the strobe frequency is set slightly above or below the resonance frequency resulting in an apparent slow vibration at the beat frequency (difference between strobe and wineglass frequencies). Stroboscopic illumination is used to make the rapid oscillations (typically 500-700 Hz) visible. This is demonstrated first so that students aren't left with theimpression that any loud high note will break a glass. It should be emphasized to the class that the frequency of the sound is all important to exciting resonance vibrations shear volume (intensity) won't do a thing if it's off resonance. However, its sound should be demonstrated to the audience by either tapping the glass or (better) rubbing its rim with a moistened finger. The resonance frequency of the glass must first be determined before lecture (the method is described in Setting it up). a beautiful and dramatic example of resonance.Ī tunable audio-oscillator/power-amplifier/loudspeaker combination is used togenerate sound waves of approximately 120 dB (very loud) and within ± 1/2 Hz of the resonance frequency of the glass. This can be done in the fundamental or next higher normal mode of vibration. When the intensity of the sound is increased, the large undulations of the glass exceed its elastic limit and cause it to shatter. Stroboscopic illumination makes it possible to actually see the vibrations in apparent slow motion. Sound waves of the right frequency are used to excite a wineglass in one or two of its normal modes of vibration. That way, you'll continue the tradition of deliciously prepared glassware-cooked recipes.Large speaker with signal generator/amplifier destroys a wineglass stroboscopic illumination shows vibration mode. The added protection, along with mindful handling, is the best bet for a safe and break-free move. Leaving a bit of room at the top, further secure the items with any available linens, towels, or more balled-up pages of newspaper. Glass cookware is sturdiest when placed on their side, so pack each vertically, being careful not to over-stuff the box. Once each item is wrapped fully, fortify your boxes by adding extra tape to the bottoms. Secure the newspaper with packing tape because you may need to remove ink stains after you finish unpacking. This helps prevent scratch damage from stacking cookware, ideally using at least two layers of sheets. Next, you'll need lots of newspaper or similar material that can be wrapped around each glass item as a protective layer. Larger ones are too spacious and won't provide the necessary support. First thing first - find quality, sturdily-built small or medium-sized boxes.
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