Casella CEL CEL-193 User Manual
Page 10
Casella USA
17 Old Nashua Rd #15
Amherst, NH 03031 USA
The true definition of Leq is based upon the integral of the instantaneous sound pressure levels; however, it
can be shown mathematically that an exact analogy is obtained by integrating the RMS sound pressure level
as long as the averaging time of the detector is short relative to the integration period. This condition is
satisfied because the sound level meter specifications call for averaging times in the order of milliseconds. It
also makes the assumption that the RMS level is exactly correct and here again the wide dynamic range and
impulse capability of the CEL-193 ensure a high degree of accuracy over the complete range of acoustic
conditions usually encountered. The integration is performed by sampling the RMS level and then digital
integration. To preserve the accuracy of the results an optimum sampling rate has been devised by
considering the maximum rate of change that can occur in the RMS level. This is, of course, controlled by
the averaging function of the detector so, as long as the sample speed is fast relative to the detector time
constant, a faithful representation of the RMS time history will be integrated. The 10msec sample rate fulfills
this requirement on the SLOW (1 sec), IMPULSE (35/1500msec) and FAST (125msec) settings. However,
some sampling errors are possible when the peak setting (50usec) is selected so this should not be used if
the accuracy of the Leq result is of prime importance.
There will be good correlation between L
eq
results obtained with either RMS FAST or SLOW time constants
employed but remember that consistently higher results will be obtained if IMPULSE is selected in
proportion to the impulse nature of the noise. When set in this mode the CEL-193 provides the impulse
weighted averages called for in certain noise control regulations.
Should sound levels go below the bottom limit of -5dB scale deflection their energy content will be very
small and they will not, therefore, affect the Leq answer to any significant extent. If the upper limit is ex-
ceeded at any time then the overload memory will be set and this will be displayed whenever the Leq or
MAX answers are selected.
It is important that an overload is not experienced during an Leq measurement as this may mean that
considerable noise energy is lost with the result that the answer will read low. For this reason it is wise to
adjust the measurement range such that the Leq answer comes in the lower half on the meter scale and so
giving the maximum range above the answer to accommodate large impulses and fluctuations. For instance,
should the Leq read 10dB on the meter scale than an overload would not occur unless levels rose tempor-
arily by 45dB and this is more than adequate in most measurement situations. To determine the correct
range selection it is wise to do an initial short Leq measurement and then adjust the range in the light of the
result. It is always wiser to be on a higher rather than lower range.
Changing the range switch, or any other switch other than the answer selector (b), will invalidate the
answer as the data stored prior to the change will have a different significance to the data stored after the
adjustment. Should it become necessary to make a range change the answer should be read from the
instrument, the adjustment made and the Leq function should then be reset by means of the RESET button
(f). The second part of the measurement should then be completed and the two results combined by the
relationship:
Page 10 of 20
Tel: (800) 366-2966
www.casellausa.com
2 Mar 2009
Fax: (603) 672-8053