Page 53 - The Master Handbook Of Acoustics

28 CHAPTER TWO

Table 2-2. Use of 10 log and 20 log (Continued).
Eq(2-2) Eq(2-3)
b 1
a 1
Parameter 10 log 10 20 log 10
a 2 b 2
Electric
Power X
Current X
Voltage X
Distance
(From source-SPL; inverse square) X

Sound pressure is usually the most accessible parameter to mea-
sure in acoustics, even as voltage is for electronic circuits. For this rea-
son, the Equation 2-3 form is more often encountered in day-to-day
technical work.

Reference Levels

A sound-level meter is used to read a certain sound-pressure level. If
the corresponding sound pressure is expressed in normal pressure
units, a great range of very large and very small numbers results. Ratios
are more closely related to human senses than linear numbers, and
“the level decibels approach” compresses the large and small ratios
into a more convenient and comprehensible range. Basically, our
sound-level meter reading is a certain sound-pressure level, 20 log
(p 1 /p 2 ), as in Equation 2-3. Some standard reference sound pressure
for p 2 is needed. The reference p 2 selected must be the same as that
used by others, so that ready comparisons can be made worldwide.
Several such reference pressures have been used over the years, but
for sound in air the standard reference pressure is 20 µPa (micropas-
cal). This might seem quite different from the reference pressure of
2
0.0002 microbar or 0.0002 dyne/cm , but it is the same standard
merely written in different units. This is a very minute sound pres-
sure and corresponds closely to the threshold of human hearing. The
relationship between sound pressure in Pascals, pounds per square
inch, and sound pressure level is shown in the graph of Fig. 2-1.