Casella CEL CEL-440 User Manual
Page 63
Once scanning has been started, each band will be measured in
turn for the preset duration, with the resultant L, L
mx
, L
eq
at the end of
the period saved in the store. The instrument then steps on to the next
band automatically and, after a suitable wait for the filter to stabilise,
starts the process of measurement again until all bands have been
measured. After measurement is completed, the results may be
examined on screen or downloaded as required.
The second method performs a self-timed autoscan. This
operates differently in that the length of time during which each filter
band is measured will be determined by how steady the readings in the
band are. The measuring process is the same as for the fixed interval
method, but the meter looks for a steady L
eq
value, and as soon as the
criteria defined for “steady” are detected, it stores this level and
immediately steps on to the next band.
Depending on the nature of the signal being analysed, the auto
scanned method may make for rapid stepping through all bands, possibly
noticeably quicker than the fixed interval method, however if the signal is
fluctuating considerably, it may take longer. This method is likely to
produce the most reliable reading of band levels when having equal time
period measurements is not of importance.
The criteria for concluding a band measurement and stepping on
to the next band are threefold.
1.
The last 4 readings of L
eq
must all be within 0.1 dB of each other.
2.
The last 4 readings of Leq must all be within 0.5 dB of each other
and have successively increased and decreased, or vice-versa,
within ±1 dB (e.g. the second reading is higher than the first,
but the third reading is lower than the second and the fourth is
higher than the third).
3.
The measurement has lasted for the maximum period set for the
band.
Note:
Measurement of L
AG
internally in the CEL-440 and CEL-480 occurs and
accumulates every 10 mSec. It is these readings that are used for satisfying
criteria 1 and 2. The internal computation is made to better than 0.01 dB.
Criteria 1 and 2 ensure that a steady or near steady reading has been
reached and provided the signal continues as in the immediate past, the
reading is the best L
eq
value that will be obtained.
CEL-440/480 Operators Handbook - Page 63
Specification