79
IL = intensity level expressed in decibels
I = intensity of sound
I o  = the reference intensity usually taken as 10
−12
 W/m
2
2.6.5 Sound Pressure Level
The sound pressure level is defined by:
SPL
rms
o
=
(
)
20 10
log
/
P
P
Where
SPL = sound pressure level in decibels
P o  = the reference sound pressure levels of 0.00002 Pa
P rms  = root mean square pressure of sound
The instrument used to measure sound pressure level is called sound level meter.
2.6.6 Sound Power Level
The sound power level is a measure of the acoustic energy emitted from a
sound source.
Mathematically:
Table 2.10 Important wave terminologies
Sl.
no. Term
Definition/description
1.
Wave front
Imaginary surface representing similar points of a wave that vibrate in
unison
2.
Plane wave
A wave whose wave fronts are in finite parallel planes
3.
Diverging wave Wave in which the energy is spread over a larger and larger area
4.
Spherical wave Wave created by a sound source that radiates the energy equally in all
directions
5.
Progressive
wave
When there is a transfer of energy in the direction of transmission of the
sound, the wave is called as progressive
6.
Period of wave Time taken for the oscillation to repeat itself (Fig. 2.26)
7.
Amplitude of
wave
The maximum displacement of a vibrating particle (Fig. 2.26)
8.
Sound power
Rate at which sound energy is transmitted, emitted, received, or
reflected per unit time; the SI unit of sound power is watt
9.
Sound energy Form of energy generated when an object vibrates; in SI unit, it is
measured in joules
2.6 Acoustics