Casella CEL CEL-160 - Manual (Page 22)

When the L eq option is selected the CEL-160
prints 'LEQ ON' and waits for the beginning of
the next whole minute interval before taking
any samples. The program then antilogs and
accumulates over the L eq period and then
averages the result prior to displaying as either
LOG or LIN bar graph. Annotation is similar to
normal SLM mode. Time markers are output
every L eq with a time print every four L eq
periods. During the first period no result is
produced as the calculation is still in progress.
The result for the first period is
displayed throughout the second period; and
so on throughout the measurement.
Since an L eq value is calculated after a
period has ended then bar graph period L eq
values would be out of synchronisaton with
the time axis of the recorder.
This problem is overcome by
reprogramming of the time clock when
selecting L eq mode and results in complete
synchronisation of the bar chart L eq and time
annotation. When the L eq option is deselected
the CEL-160 will print 'LEQ OFF' and return
the clock to its original state.
10.3.2 Operation
To commence profile L eq operation select
RMS level recording (see section 5.3) and
select the required L eq profile by paper speed
selection as outlined in the above table. During
the minute preceding the required start time
select the OPTION position with the option
switch.
The Leq output can be annotated in
logarithmic or linear axes and determined by
the LOG/LIN switch. Once the L eq option
program is running any switch or key change
will terminate the measurement.
If the required operating changes are
made and the recorder is left in RMS and
OPTION modes the program will restart a new
measurement at the beginning of the next
whole minute.
10.4 CEL 160/2B Reverberation Time
(RT60) Measurement Option
10.4.1 Introduction
This option package extends the range of
functions provided by the instrument to
include a direct printout of the rever-
beration time (RT60) in both analogue and
digital formats. It also includes all the
necessary command functions to
automatically control the CEL-213 Random
Noise Generator and CEL-196 Third Octave
Filters in both the source and receiving signal
paths. It will therefore provide a very compact
and convenient measurement package that is
ideally suited to field measurements.
The instrument may be operated in
either an impulse or continuous excitation
mode. In the former a signal line on the
recorder auxillary socket is raised when the
system is ready for the excitation and will then
wait until it detects the rapidly rising level and
when its maximum value is passed the decay
calculation procedure is initiated. In the
continuous mode the same line is used to
switch on and off the noise source. When the
measurement procedure commences in this
mode the source is activated and a period of
approximately 2.2 seconds is allowed for the
levels to normalise; then the average value of
the generated level is calculated and this value
subsequently used in the RT60 calculation. The
'source off' signal is then activated and the
calculation procedure commenced.
In both modes of operation prior to the
sound source being activated a short period of
background level is measured and averaged
and this information is subsequently used to
validate results obtained. When the CEL-213
Random Noise Generator is used in a
measurement configuration the source will be
automatically switched by the programme and
in addition a 'ready' light is provided for the
impulse excitation mode.
The reverberation time may be
calculated from measurements made over
either a 20 or 30 dB decay range and this
parameter should be preset at the beginning of
a measurement sequence. In the case of
impulse exicitation 8 mS is allowed after the
detection of the maximum level to ensure that
the shock wave has passed and then decay is
timed from a point -5 dB on that level. For the
Paper Speed
mm/S L eq period Column
Width mm Number of
Samples
1B - 4B 1 10 S 10 2 479
5B 0.3 1 min 18 15 215
6B 0.1 5 min 30 76 559
7B 0.03 10 min 18 153 455
8B 0.01 1 hr 36 921 455
Figure 13: Leq period determination
Page 22 - CEL-160 Graphic Recorder