Larson Davis System 824, Technical Reference Manual

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G-6 824 Reference Manual 9/29/09
second. The Leq detector and the SPL detectors can be
totally isolated without any additional circuitry (the cost to
do this with analog circuitry would be very prohibitive). So
what is an Leq? It is the energy average of a signal over a
period of time. Now with separate digital detectors, the Leq
does not have to exhibit the decay of the SPL exponential
detector but can be true to its definition. That means that if
an Leq time-history is taken in this type of instrument, you
will not see any SPL decay effects.
Is this good or bad? It is different more than good or bad, but
there are benefits from that difference.
Good: Many scientists feel that this is good because Leq
time-histories show the true dynamics of the signal without
the residual decay effects of an exponential detector. For
example if you measure a sonic boom with a fast Leq time-
history, it can show the two impulses of the 'N' wave and
possibly even an echo or two. On the other hand, the older
instruments will show one pulse that may take upward of 20
seconds to return to the background level.
Bad(?): If you are comparing data from traditional
instruments it is important to be aware of this phenomenon,
but it is not necessarily bad. If the Leq values being
compared are of long duration (one minute or longer) then
there is little blur caused by the exponential detectors of the
older instruments and the data are very comparable.
Question #6
If I choose to save C-weighted SPLs every two seconds in
the LD824's Time-History, what am I getting?
Answer
You get the instantaneous SPL at the end of the two-second
period.
Question #7
How does this compare with C-weighted Leqs?
Answer
The Leq is the true equivalent level for that two-second
period as if the signal level were constant amplitude.
Detailed Explanation:
So what's the difference between sampled SPL and an Leq
for the same period? It depends on whether you are using