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CASELLA USA
CEL-240 DIGITAL SOUND LEVEL METER
FREQUENTLY ASKED QUESTIONS
How quickly does the digital display update during measurements?
The digital display shows samples of the continuous rms sound pressure level updated twice every
second. This is a compromise between updating the display too quickly making it too difficult to see
the readings and being too slow for the update routine in which case the display would appear very
sluggish. In a non-integrating instrument tests have shown about 2 or 3 times a second to be about
the optimum rate that the brain can interpret without missing valuable information.
Is the time history trace stored in the CEL-240?
The CEL-240 has a display mode that allows the user to see the last 1 minute or 5 minute trend of
the changing instantaneous noise levels as they vary. These samples are not stored in the
instrument but are available at the mini USB connector at the bottom of the meter for output to a
suitable data logging device such as a computer.
What outputs are available from the CEL-240?
The 2.5 mm three-pole jack socket on the bottom of the case of the CEL-240 provides both the ac
and log dc signals to external equipment. The ac output signal is used for connection to an audio
tape recorder or spectrum analyser. The dc output is used to provide a signal to a data logger or
paper chart recorder.
What are the characteristics of the ac output?
The ac output is a conditioned signal that has been passed through the selected frequency
weighting. This means that it will give either an 'A' or 'C' response signal that represents the pressure
waveform at the microphone. The 'C' setting is recommended for making audio recordings since it
has a better (wider and flatter) frequency response than the 'A' weighting. The ac signal is available
on the 2.5 mm jack socket at a level of up to 0.85 V rms for full scale deflection. This means that for a
sound level of 100 dB measured on the Lo range of the meter the output would be 0.85 V rms. A
signal at half the range FSD would output a voltage of 0.425 V rms and so on. A signal may still be
present below the minimum scale deflection but may not still be linear at such low levels.
What are the characteristics of the dc signal?
The dc signal is a more slowly varying voltage after the time weighting network in the meter.
Therefore, it will represent the same signal as seen on the display. It will have the selected 'A' or 'C'
frequency and either the Slow, Fast or Impulse time weighting. The signal is optionally available on
the 2.5 mm stereo jack socket at a level of up to 1.3 V rms at full scale deflection on each range. It
increases at a rate of nominally 18.6 mV/dB for each range. A noise level of 100 dB on the Lo range
would produce an output of 1.3 V rms while the same noise level but measured on the Lo range
would produce an output of 0.742 V rms.
What are the characteristics of the digital output?
The digital output is produced every 1 second and provides both the A and the C frequency weighted
sound level with the selected time response (S, F or I). The CEL-240 has been designed to appear
as an industry standard memory card format when connected to a personal computer. The optionally
available program for the CEL-240 called dB24 produces a standard text file format data file in the
form of a list of date, time and the 1 second A and C sound levels with a carriage return. This can be
imported into many standard office programs such as word processors or spreadsheets for further
manipulation and graphing.
Casella USA
Page 4 of 6
(800) 366-2966
31 Jan 2008
Info@CasellaUSA.com
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