Casella CEL CEL-414 - Manual (Page 42)

Timing
The sound level meter outputs five bytes
of data containing the currently displayed
result and the current status every 500
milliseconds (the same rate as the dis-
play is updated).
When this data is available for output, the
“request to send” line is set low. As the
sound level meter also uses the serial
data lines for other purposes within the in-
strument, the receiving hardware or soft-
ware should be enabled ONLY when the
“request to send” line is low.
The instrument then interrogates the in-
terface 64 times every second and out-
puts the five data bytes when the “clear
to send” line is found to be low. As the in-
terface can output 32 characters in 500
milliseconds, the “clear to send” line can
also be used to hold up the sound level
meter for short periods, either between
individual bytes or between blocks of 5
bytes, while the receiving device proc-
esses the data.
As the sound level meter transmits digital
data regularly at 500 millisecond inter-
vals, the timing for the receiving device
can be derived from it, using 1 minute =
120 results.
This chapter contains details of the meas-
urement parameters available from the
sound level meters, shows how they are
derived and gives a brief explanation of
their use. The following parameters can
be read directly from the display.
SPL Sound Pressure Level,
DUR Duration over which
measurements taken,
L eq Equivalent Continuous
Level,
L max Maximum Level,
SEL Sound Exposure Level,
L Tm3 Cumulative average of
maximum values taken
from a series of 3 sec
periods (A, B Versions,
and CEL-493/3),
L Tm5 Cumulative average of
maximum values taken
from a series of 5 sec
periods (A, B Versions
and CEL-493.3),
L OSHA Averaged level with a
preset threshold and
q = 5 (C & D Versions),
L DOD Averaged level with a
preset threshold and
q = 4 (C & D Versions).
9.1Sound Pressure Level
(SPL)
The instantaneous sound pressure level
is defined in the relevant national and in-
ternational standards as:
L P = 20log
( P
P 0
)
Where:
P 0 = 2 x 10 -5 Pa.
And the sound power level is given by:
L W = 10log
( P
P 0
)
Where:
P 0 = 10 -12 W.
Sound pressure level (with various time
weightings) is the basic unit that is meas-
ured by the sound level meter when con-
9. EXPLANATION OF TERMS USED
Page 38 - General CEL Sound Level Meters Handbook