Pressure Amplitude: Quantitative Measurement of Sound As we will discuss in this section decibels are a means of creating a logarithmic scale relative to some reference. Decibel scales are by no means confined to acoustics, there are decibel scales defined for use in electronics and optics. Even in acoustics there are a variety of different definitions of the decibel scale depending upon the quantity being used as a reference.  Thus, to begin our discussion I want to cover the question of what parameters are measured in determining sound levels. The most physically understandable quantity used in determining the size of a sound signal is the Pressure Amplitude.  Pressure amplitude is a measure of the size of the variation in air pressure caused by a sound wave.  In pure silence there is a constant pressure--atmospheric pressure.  Atmospheric pressure is measured in newtons/meter2 and is approximately 105 N/m2.  The atmospheric pressure varies a small amount over the space of hours or days--this is what they mean by high and low pressure centers on the weather map--however, we can consider it to be essentially constant on the time scale of sound waves. A sound wave creates a variation in the normally constant air pressure that oscillates above and below the normal atmospheric level of 105 N/m2.  The average size of the pressure variation away from the constant background level is the Pressure Amplitude of the sound wave.  I am hiding a bit of complication in the word average; the pressure goes above (positive) and below (negative) the constant atmospheric background so a straight average would tend to give zero.  The real average is a root mean square average which you might be familiar with if you have done some electronics.  For a pure tone--a pure sine wave--the averaging process leads to a pressure amplitude that is reduced from the maximum amplitude by a factor of 0.707 (equivalent to dividing by the square root of 2).  The relation between amplitude of the sine wave (measured from the background level to one extreme) to the pressure amplitude (average pressure excursion from the background) is shown in the figure below.