Larson Davis System 824, Reference Manual

Page: 520 / 536

D-16 824 Reference Manual 11/3/00
defined as the start and end times of a noise event such as an
aircraft or automobile passby.
Sound
The rapid oscillatory compressional changes in a medium
(solid, liquid or gas) that propagate to distant points. It is
characterized by changes in density, pressure, motion, and
temperature as well as other physical quantities. Not all
rapid changes in the medium are sound (wind noise) since
they do not propagate.
The auditory sensation evoked by the oscillatory changes.
Difference between sound and noise: Sound is the physical
phenomenon associated with acoustic (small) pressure
waves. Use of the word sound provides a neutral description
of some acoustic event. Generally, noise is defined as
unwanted sound. It can also be defined as sound that causes
adverse effects on people such as hearing loss or annoyance.
It can also be defined as the sound made by other people. In
every case, noise involves the judgment of someone and
puts noise in the realm of psychology not physics.
Rules:
1. Use word sound to describe measurements to remove the
emotional overtones associated with the word noise. Some
sound metrics use noise in their name and it is proper to use
the name as it is.
Sound Exposure (SE)
It is the total sound energy of the actual sound during a spe-
cific time period. It is expressed in Pascals-squared seconds.
where p
A
is the sound pressure and T
2
- T
1
is the Measure-
ment Duration (specific time period).
When applied to hearing damage potential, the equation is
changed to
where k is the Exposure Factor. See Exchange Rate.
Standard: ANSI S1.25
SE p
A
2
t
( )
t d
T
1
T
2
∫
=
SE p
A
2
t
( ) [ ]
k
t d
T
1
T
2
∫
=