Casella CEL CEL-193 - Manual (Page 15)

Casella USA
17 Old Nashua Rd #15
Amherst, NH 03031 USA
Block diagram of CEL-193 Sound Level Meter
Within the main instrument there is an input amplifier with selectable frequency weighting networks and,
attenuators for setting the measurement range. The output from the RMS detector is fed through a circuit
that takes the logarithm in order to provide a signal that is linear in decibels thereby providing a clear easy
to read scale graduation. This signal is fed to the meter whenever the SPL mode is selected.
Whenever the instrument is operating the output of the detector is sampled every 10 milliseconds in order
to provide data to the integrator. This sample rate is fast relative to the averaging time of the detector and
hence a very accurate representation of the time history is integrated. These samples are first digitized,
antiloged, squared and then added into a cumulative noise energy data store.. A separate data acquisition
system is used to monitor elapsed time and in order to provide Leq results the contents of the energy store
is divided by the contents of the elapsed time store. This takes place continually on a 330 millisecond cycle
providing an updated Leq result 3 times a second. In order to provide an answer in dB the log of the
resultant is taken and this is fed to the meter whenever the Leq mode is selected. To provide the impulse
weighted average LTM (takt maximum) a maximum hold function is added in front of the RMS sampling
gate which is preset on a 3/5 second duty cycle. This arrangement causes the maximum occurring in each
period to be integrated for the ensuing period as required by the LTm formula. The alternative method
recommended for impulse weighting averages is also available by selecting the impulse time constant and
the Leq mode of integration. In order to provide the single event noise exposure index LAX the elapsed time
counter is locked to 1 second enabling a direct reading of LAX to be obtained.
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