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Name: Drew
Status: student
Grade: 9-12
Country: Canada
Date: Fall 2012


Question:
The question that has been puzzling me for quite some time is why do objects make sound? When I say that, I do not mean "Why do cows go moo?" I mean why does something like closing a door make a different sound than putting a glass down on a table, or why does a slap sound different than a punch, or why does knocking on a wall sound different than knocking on a window. Does it have something to do with the molecules in the objects or the size/shape?

Replies:
Yes, it does have something to do with the molecules and size and shape. The bonding between molecules or atoms determines how rigid an object is, and how the motion of one part gets transmitted to other parts - how vibrations are conducted and damped, whether an object moves as a unit or in a wave. The details determine what vibration frequencies are supported, and which couple effectively to the air so that they can get to our ears. Size matters because it determines at what frequency sounds are beamed in one direction, or radiated in all directions. Shape matters because vibrations of surfaces form complicated patterns, and different patterns have different frequencies.

Tim Mooney


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