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The squeal of chalk.

Nothing wakes up dozing students like the shriek of chalk sticking and slipping across a blackboard, and few sounds pain machinists more than the squeal of metal against stone during honing. It was the apparent similarity of these sounds that led Herbert L. Kuntz and Robert D. Bruce of Hoover Keith & Bruce, Inc., in Houston to study chalk squeal as a way of understanding and reducing the piercing noise generated during honing.

When an incorrectly held piece of chalk is drawn across a blackboard, the chalk at first sticks to the surface. Suddenly, when it crumbles, the chalk slips and vibrates, producing a squeal. As the vibrations die down and chalk dust rolls out of the way, friction between the chalk and the board increases until the chalk sticks once again, and the cycle is repeated.

Kuntz and Bruce found that the frequencies at which sound is radiated from squealing chalk depend on where, at what angle and how tightly a piece of chalk is held, as well as its length. For example, if the chalk is held just above the contact point and perpendicular to the board, the radiated frequencies are higher than if the chalk is held at a 45-degree angle. In the first case, vibrations are generated along the length of the chalk. In the second case, the chalk vibrates by bending.

The researchers found that although the blackboard itself produces very little sound, a stiff, smooth board permits chalk to radiate louder sounds than a pliant, rough board. A lump of putty atop the chalk reduces the squeal substantially.
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Title Annotation:experiments on sound made by chalk on blackboard
Publication:Science News
Date:May 11, 1985
Words:263
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