Anti-Disinformation

There were a variety of classroom demonstrations (some of which are fun and some of which are corny) that illustrate the idea of natural frequencies and their modification. A pop bottle can be partly filled with water, leaving a volume of air inside that is capable of vibrating. When a person blows over the top of the bottle, the air inside is set into vibrational motion; turbulence above the lip of the bottle creates disturbances within the bottle. These vibrations result in a sound wave that is audible to students. Of course, the frequency can be modified by altering the volume of the air column (adding or removing water), which changes the wavelength and in turn the frequency. The principle is similar to the frequency-wavelength relation of air columns; a smaller volume of air inside the bottle means a shorter wavelength and a higher frequency.

A toilet paper roll orchestra can be created from different lengths of toilet paper rolls (or wrapping paper rolls). The rolls will vibrate with different frequencies when struck against a student's head. A properly selected set of rolls will result in the production of sounds that are capable of a Tony Award rendition of "Mary Had a Little Lamb."

Maybe you are familiar with the popular water goblet prom trick that is often demonstrated in a Physics class. Obtain a water goblet and clean your fingers. Then gently slide your finger over the rim of the water goblet. If you are fortunate enough, you might be able to set the goblet into vibration by means of slip-stick friction. (It is not necessary to use a crystal goblet. It is often said that crystal goblets work better; but the trick is just as easily performed with clean fingers and an inexpensive goblet.) Like a violin bowstring being pulled across a violin string, the finger sticks to the glass molecules, pulling them apart at a given point until the tension becomes so great. The finger then slips off the glass and subsequently finds another microscopic surface to stick to; the finger pulls the molecules at that surface, slips and then sticks at another location. This process of stick-slip friction occurring at a high frequency is sufficient to set the molecules of the glass into vibration at its natural frequency. The result is enough to impress your dinner guests. Try it at home!!

Perhaps you have seen a pendulum bob vibrating back and forth about its equilibrium position. While a pendulum does not produce a sound when it oscillates, it does illustrate an important principle. A pendulum consisting of a longer string vibrates with a longer period and thus a lower frequency. Once more, there is an inverse relationship between the length of the vibrating object and the natural frequency at which the object vibrates. This very relationship carries over to any vibrating instrument - whether it is a guitar string, a xylophone, a pop bottle instrument, or a kettledrum.

To conclude, all objects have a natural frequency or set of frequencies at which they vibrate when struck, plucked, strummed or somehow disturbed. The actual frequency is dependent upon the properties of the material the object is made of (this affects the speed of the wave) and the length of the material (this affects the wavelength of the wave). It is the goal of musicians to find instruments that possess the ability to vibrate with sets of frequencies that are musically sounding (i.e., mathematically related by simple whole number ratios) and to vary the lengths and (if possible) properties to create the desired sounds.

224 viewsShane St. Pierre, 01:59