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Question

Why does a piece of chalk produce a squeal if you hold it incorrectly. What determines the pitch you hear?
Asked by: dr. grey

Answer

The squeal is produced as a result of the piece of chalk skipping along the blackboard.

As the piece of chalk is positioned in the angle in which the noise is produced, given an equal and constant amount of pressure applied by the hand at certain points along the chalk, the chalk 'vibrates' thereby causing the chalk piece to bounce along the blackboard. Taking into consideration the facts that the angle, the pressure and the application points remain constant, the squeal is constant. It this could be practiced, the noise would be constant and fluid. Rather, the noise is not constant and fluid because the angle, the pressure, or the application point (specifically around a Z-axis) changes as the chalk is dragged along the board. This is a normal motion considering that the application is the human hand that must change to conform the the comfort level of the user rather than the comfort level of the chalk.

The pitch of the squeal changes as a result of a change in the angle the chalk is dragged, the amount of pressure that is applied, and/or the point(s) at which the pressure is applied. This can be proven by the fact that if the chalk were place at a perpendicular (90 degree) angle to the blackboard, common to the Z-axis, and given (for the sake of argument) 5 PSI of pressure, the chalk piece would skip along the board at visible and measurable intervals. This assumes that the contact point for the chalk is true to the board, and has not been altered from this position. As the chalk is varied from the Z-axis (either favoring positive and/or negative X or Y values) the pitch of the chalk would disappear until the point at which the new surface that contacts the board gains enough surface area to which vibrations could start again. Obviously, given the apparent rigidity of the chalk piece and assuming that malleiation is not an option, the pressure applied at the 90 degree angle would be too much for the chalk's bending threshold and the piece would break. Therefore, if one factor of the experiment is changed, the other two factors are automatically changed. Therefore, the pitch that is produced will not resemble the original pitch produced.

A key factor in determining why a certain pitch is or is not produced is based on the amount of surface area used when running the test. A small surface area allows the chalk to be broken apart easily which produces an extremely high audio wave inaudible to the human ear. Therefore, the contact point varies with continual application. A large surface area, while able to make a noise, does not allow for an audible noise based on the extremely low frequency of the audio wave produced. The SIN's (or COS's wave, depending on how you look at it) peaks are so far apart that your ear cannot process it. Therefore the contact point does not change rapidly. Theoretically, if the wide surface area test is run long enough, an audio wave will be produced that can be heard.. In conclusion, when one is holding the chalk 'incorrectly,' they are rather holding the chalk perfectly, only now acutely aware of it.
Answered by: Bob Yarem, Normal person (I'm not a professor or anything)


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