Page 48 - Master Handbook of Acoustics

TABLE 2-2 Use of 10 log and 20 log Equations

Reference Levels

As we have seen, reference levels are widely used to establish a baseline for measurements. For
example, a sound-level meter is used to measure a certain sound-pressure level. If the corresponding

sound-pressure level is expressed in normal pressure units, a great range of very large and very small
numbers results. As we have seen, by expressing levels in decibels, we compress the large and small
ratios into a more convenient and comprehensible range. Basically, a sound-level meter reading is a
certain sound-pressure level, 20 log (p/p ), as in Eq. (2-4). The sound-pressure reference p must
ref
ref
be standardized, so that ready comparisons can be made. Several such reference pressures have been
used over the years, but for sound in air the standard reference pressure is 20 μPa. This might seem
2
quite different from the reference pressure of 0.0002 microbar or 0.0002 dyne/cm , but it is the same
2
standard merely written in different units. This is a very small sound pressure (0.0000000035 lb/in )
and corresponds closely to the threshold of human hearing at 1 kHz. The relationship between sound
pressure in pascals, pounds/square inch, and sound-pressure level is shown in Fig. 2-1.