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Sound Power v Sound Pressure

"Sound power" and "sound pressure" are two different but regularly confused characteristics of sound. They share the same unit of measure, the decibel (dB), and the term "sound level" is commonly used for each.

An Illuminating Analogy

The following comparison of sound and light helps illustrate the distinction between these terms. Think of sound power as the wattage rating of a light bulb; both measure a fixed amount of energy. Sound pressure corresponds to the brightness in a particular part of the room; both can be measured with a meter and the immediate surroundings influence the magnitude of each. In the case of light, brightness is more than a matter of bulb wattage. Asking for a 90 dBA motor is a lot like asking for a “light:” you don’t know what you are going to get. Most of us are much more familiar with light than sound. If someone says he has a 100-watt light bulb, you have some idea of the candlepower available, but if you want to read by the light, you want to know the light intensity level at the reading location. To determine the light intensity level you would need to know:

“How far away is the light?” If the light is a mile away, it is not much use. The analogous sound question is “How far away is the motor?”

“Is the light outdoors?” With no walls to reflect the light, all but the direct light radiates out into the free field of space. The analogous sound question is “Is the motor outdoors?”

“Are the room walls reflective if the light is not outdoors?” A room covered with black velvet would not reflect much light regardless of its size. The analogous sound question is “How reverberant are the walls?”

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Noise Travels in two ways....
.... transmitted
.... Airborne