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A sound generator is an object that makes sounds. These sounds may be produced in a nearly infinite number of ways, ranging from a foot stomping on the floor to a fire engine siren. Regardless of the origin of the sound, it is always the same thing — vibrations in the air that make a pattern recognizable by aural senses. While the ways to make sound are nearly limitless, practically all sound-generating objects fall within two main categories: single-tone and multi-tone. Each of these processes makes a full and complete sound pattern, but in different ways.
Sound may seem like many different things since a kitten mewing and a lawnmower cutting grass seem so different — but it isn’t. Sound is simply vibrations moving through the air from the sound generator, such as a lawnmower, to an aural receptor, such as the human ear. These vibrations actually disrupt the air itself, which generally means that, if there is no air, then there is also no sound.
When a sound generator functions, the way it disrupts the air determines what it sounds like. Large, violent disruptions become large and violent sounds. As the generator operates, it creates a wave of energy that causes the disruption. As the wave moves further from the object, it flattens out until it no longer has the energy to disrupt the air. This causes diminishing volume and slower repetition over distance, which is why a siren gets disproportionally louder, higher-pitched, and faster the closer that a person is to the source.
There are two basic forms of sound generator: a single-tone generator and a multi-tone generator. A single-tone generator works through a fixed interaction between two objects. When one object interacts with another, a sound is made, and that sound is the only possible sound available for that interaction. A good example of this type of generator is a xylophone bar; a strike on the bar results in one note that does not vary at creation. As the wave moves through the air, there may be some variation, but that is independent of the sound generator.
A multi-tone generator can actually vary as it is being produced. In this case, when the sound is made, one or more parts of the generator are mutable. This allows tonal variation in the sound and can result in a warbling effect that single-tone generators cannot produce. A common example of this is a trumpet; a user can alter airflow quantity and speed by changing the shape of his or her lips while playing a continuous note.