Model 820 Technical Reference Manual
C-13
Preamplifier
A part of the sound level meter that matches a particular model of
microphone to the meter. It must be chosen in conjunction with a
microphone and a cable that connects them.
Projected Noise Dose
It is the Noise Dose expected if the current rate of noise exposure
continues for the full Criterion Duration period.
Single Event Noise Exposure Level (SENEL, L
AX
)
The total sound energy over a specific period. It
is a special form of the Sound Exposure Level where the time
period is 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 phe-
nomenon 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 emo-
tional overtones associated with the word
noise
. Some sound met-
rics 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 specific
time period. It is expressed in Pascals-squared seconds.
where p
A
is the sound pressure and T
2
- T
1
is the Measurement
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
∫
=