Casella CEL CEL-414 User Manual
Page 43
nected to a microphone. All other parame-
ters available from the instrument are ob-
tained by integrating this level over
periods of time.
A continuous display of sound pressure
level is available on the analog bar graph,
updated every 125 ms. This value can
also be selected for indication at finer
resolution on the digital display, re-
freshed every 500 ms.
Measurements can also be made using
the one second maximum profile mode
required by IEC 651 para 7.8 by adding
the connecting link as described in Sec-
tion 10.2.
9.2Duration of Measure-
ment Period (DUR)
The time since the instrument was last re-
set will be indicated on the digital display
in hours and minutes when DUR is se-
lected. Refer to Sections 3.2 and 3.3 for
details of the reset possibilities.
9.3Equivalent Continuous
Noise Levels (L
eq
)
This value is now widely used in the as-
sessment of noise deafness risk, and in
the investigation of environmental noise
problems. In some places it may also be
referred to as the average level L
avg
or L
m
.
The equivalent continuous noise level rep-
resents the steady continuous level that
would have the same energy content as
the varying noise level being measured.
It is obtained by integrating sound pres-
sure levels with respect to time:
L
eq
= 10log 1
T
∫
(
0
T
P
P
0
)
2
dt
Where:
P
= instantaneous sound
pressure,
T
= measured duration,
L
eq
= normally defined as
A-weighted, but may
also be an A-
weighted or linear
value that has been
octave band filtered.
It can also be shown that when the inte-
gration time is long compared to the time
weighting, both fast and slow time
weightings will yield the same results.
When LEQ is selected, the formula is
realized in these instruments by sampling
the RMS signal at high speed and inte-
grating the results.
The accumulated results are averaged
over time, so that a single index number
can be ascribed to a time varying signal.
This process starts when the instrument
is switched ON (or all parameters are re-
set by means of a general instrument re-
set as described in Section 6.1, step 7). It
continues until the period for the meas-
urement of all parameters is reset, until
the L
eq
register is reset by pressing the
RESET button while L
eq
is being dis-
played on the digital display, or until the
instrument is switched OFF.
When the L
eq
register is reset, using any
of these methods, the digital display indi-
cation will fall to minimum scale reading
as the new accumulation starts.
Pressing the PAUSE button interrupts
the accumulation and averaging process
to allow any unwanted transient events
to be excluded from the result. Pressing
the button again continues the process
from the same point.
General CEL Sound Level Meters Handbook - Page 39