For most acoustical quantities, there is an internationally accepted reference value. For this reason, it is important to be aware of the context in which the term decibel or level is used. The decibel scale is used for many different descriptors relating to sound: source strength, sound level at a specified location, and attenuation along propagation paths each has a different reference quantity. Thus, the sound pressure level L p corresponding to a sound pressure is given by Equation (1) describes levels of power, intensity, and energy, which are proportional to the square of other physical properties, such as sound pressure and vibration acceleration. Therefore, the decibel (dB) scale is the preferred method of presenting quantities in acoustics, not only because it collapses a large range of pressures to a more manageable range, but also because its levels correlate better with human responses to the magnitude of sound than do sound pressures. The range of sound pressure in Table 1 is so large that it is more convenient to use a scale proportional to the logarithm of this quantity. Table 1 gives approximate values of sound pressure by various sources at specified distances from the source. The human ear responds across a broad range of sound pressures the threshold of hearing to the threshold of pain covers a range of approximately 10 14:1. Sound pressure is measured in pascals (Pa) (SI units are used here rather than I-P because of international agreement). Sound waves in air are variations in pressure above and below atmospheric pressure.
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