Casella CEL CEL-160 - Manual (Page 21)

RMS, or to 100% so that 100% is equivalent
to 2 g RMS.
AC Input
Where the signal is provided to the CEL-160
from an external amplifier then the calibration
procedure should follow the techniques
outlined above with the following further
considerations.
i) If the overload light on the B & K 1621
becomes lit switch to the 1 V RMS Max
voltage.
ii) For logarithmic recording the CEL-160
may have its amplitude scale
information adjusted using the `D' key to
display the correct dB values as
indicated on external amplifiers.
10.2.5 Limitations of Use
System Noise
The CEL-160 is equipped with a built-in sound
level meter which is using the B & K 1621 as
an external filter, before graphically recording
the DC output of the sound level meter.
The dynamic range of the B & K 1621
Filter is limited to 35 - 40 dB below its
maximum signal input by its own self-noise.
Adjusting the range control of the CEL-160 will
in most switch positions amplify the noise and
reduce the signal to noise ratio for analysis.
Selection of the 120 dB range position will
enable a dynamic range of at least 30 dB to be
used. Selection of the 140 dB range position
will enable a maximum signal 109 times that
on the 120 dB range to be analysed but will
not improve the signal to noise ratio.
These three ranges are recommended
for analysis and permit a total analysis range of
at least 50 dB - 140 dB.
Signal Peak Enhancement
Dynamic Range Switch
The dynamic range switch displays either the
whole 50 dB across the paper the top 20 dB
across the paper or the top 10 dB across the
paper. Where the signal is almost lost in noise
then the noisy part of the signal may be placed
off scale by selecting either 10 dB or 20 dB as
appropriate and the signal now viewed across
the whole of the paper.
Recording Mode Log or Linear
Because of the logarithmic nature of the
decibel the top 10 dB of a 50 dB logarithmic
recording would occupy the top 66% of the
paper on a linear recording. The effect of
logarithmic recording is therefore to visually
increase the significance of low amplitude
signals. Signal peak enhancement on the other
hand is obtained in the linear recording mode.
Sweep Rate Considerations
The CEL-160 Frequency Analysis programme
incorporates a software RMS averaging time
which is decreased in two steps at 5 third-
octave intervals as the frequency sweep is
undertaken. The basis for reducing the
averaging time is that as frequency increases
then a statistically valid result can be obtained
in decreasing time.
The initialising conditions for the
programme are established by the start speed
for the frequency sweep and the typical start
speeds are given in the Sweep Initiation
paragraph, 10.2.3 (b). The programme will
commence sweeping the B & K 1621 at the
0.3 mm/S or 0.03 mm/S start speed and will
finish the sweep at 3 mm/S and 0.3 mm/S
respectively. The instructions are satisfactory
for periodic signals, but where a random signal
is to be analysed then the start speed may
require setting at a lower level bearing in mind
the frequency content of the signal to be
analysed.
In all cases the CEL-160 Frequency
Analysis programme will determine the
necessary averaging time constant and the
detector time constant and the detector time
constant should be set in the FAST position.
10.3 CEL-160/2A Integration Software
Options (L eq Option)
10.3.1 Introduction
The provision of this facility enables the
CEL-160 Graphic Recorder to operate as an
Integrating Graphic Level Recorder. It can
therefore produce profile L eq bar charts
without the need for an associated integrating
sound level meter.
L eq periods may be user selected
between ten seconds and one hour and the
dynamic range selected as either 10, 20 or
50 dB. L eq values are calculated from 4 mS
samples of the RMS level. Fully annotated
traces are provided giving both time and range
information. The Leq period is determined by
the set paper speed as follows.
CEL-160 Graphic Recorder - Page 21