Casella CEL CEL-193 - Manual (Page 14)

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Amherst, NH 03031 USA
This signal was recorded -2dB on tape saturation so on replay it should be adjusted to be -2d8 below the
sound level meter overload point, is + 53dB scale deflection, This will give a replay level of 123dB and
hence all answers obtained on replay will have to be reduced by 10dB to give the correct result. Recordings
made via the unconditioned output with the instrument set to the 50-80 ranges or via the conditioned
output on the 60 range only need to be adjusted to take account of the -10dB replay k factor. If, however,
other settings of the attenuator were used during the recording additional correction must be made to take
account of the recording k factor. For example unconditioned recordings made on the 30dB range will have
to be corrected on replay by -30 to take account of the amplification applied during recording and -10 for
the replay correction, i.e. the correct result is the displayed answer -40. For conditioned recordings made on
the 80 range the correction on replay is +10 (+ 20 record k and -10 replay k).
10.2 Using the CEL-193 to drive Graphic Level Recorders
High speed level recorders that have been specifically developed for acoustic measurements are best fed
with AC signals and in these cases the output signal can be taken from either the conditioned or un-
conditioned outputs depending upon the type of signal that is required to record. The following cable sets
are available for the more popular types of recorders:
CEL-3137/2 Rion LR-04
CEL-3412/2 B & K 2306
CEL-3432/2 B & K 2305/7
The CEL-193 also provides a log DC output that has two important functions. Firstly, it enables the full
benefit to be taken of the excellent performance of its RMS circuits to be obtained when graphic recordings
are made of impulsive sounds. In the past problems have been encountered because of the traditional
concepts used in the writing systems employed in the B & K types of level recorders and the limited impulse
capability of the Rion recorders. The log DC output of the CEL-193 is available on pin 3 of the auxiliary
socket and it covers the full dynamic range of the indicator and will provide a drive to high speed DC level
recorders of waveforms having a high impulse content enabling them to record accurate RMS levels without
distortion due to incorrect writing dynamics or limited crest factors.
The other application for the DC output is in enabling low cost DC recorders to be used in measuring
general background levels. The log compression of the signal allows a full 65dB to be obtained in these
configurations.
10.3 Using the CEL-193 with Frequency Analysers
The CEL-193 has been specifically designed for use with the CEL-178 Octave and CEL-196 Automatic Third
Octave Analysers and full details of these instruments are published separately. The unconditioned output
may be fed to almost all types of frequency spectrometers, real time analysers and FFT devices. The cable
type CEL-3137/2 connects to a standard BNC type connector which is suitable for most devices of this type.
11 Technical Description
The heart of the CEL-1 93 is a unique new type of wide dynamic range RMS detector that greatly reduces
the crest factor and impulse capability limitations of earlier designs. It can respond to signal amplitudes
rising over 60dB above the background level without the need for complex feedback systems that have
been necessary to date making the response time independent of the detector's averaging time. Transient
responses can therefore be correctly measured in terms of fast, slow or impulse levels. Provision of a 50
microsecond averaging tinge also allows true peak levels to be measured.
The input to the detector is from a CEL-186 Precision Measurement Microphone via a CEL-2980 Preamplifier
which can, if required, be removed from the instrument and mounted on a CEL-2942/5 Extension Cable.
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