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Fundamentals of Sound
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An Analogy
ISound power
K Correlates to bulb wattage
ISound pressure
K Correlates to brightness
Figure 12
The following comparison of sound and light may help illustrate the distinction between these two properties. Think of sound power as the wattage rating of a light bulb. Both measure a fixed amount of energy. Whether you put a
Sound pressure corresponds to the brightness, from the light emitted by the light bulb, in a particular location in the room. Both sound pressure and brightness can be measured with a meter, and the immediate surroundings influence the magnitude of each. In the case of light, brightness depends on more than the wattage of the bulb. It also depends on how far the observer is from the light bulb, the color of the room, how reflective the wall surfaces are, and whether the light bulb is covered with a shade. These other factors affect how much light reaches the receiver, but do not affect the wattage of the light bulb.
Similarly, sound pressure depends not only on the sound power emitted by the source, but also on the characteristics of the surrounding environment. These might include the distance between the sound source and the listener, whether the room is carpeted or tiled, and whether the room is furnished or bare. Just as with light, environmental factors like these affect how much sound reaches the listener.
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