Spider DSA User’s Manual
237
Figure 147 Octave Spectra and FFT APS Processed at same time
Sound Level Meter
A sound level meter measures sound pressure level. A standard sound level meter
is more correctly called an exponentially averaging sound level meter because the
AC signal from the microphone is converted to an RMS level, requiring a duration
of integration termed the “time constant”. Three of these time-constants have
been standardized, 'S' (1s) originally called Slow, 'F' (125 ms) originally called Fast
and 'I' (35 ms) originally called Impulse.
The output of the RMS detector circuit is a linear voltage proportional to pressure.
This is passed through a logarithmic converter to give a readout in decibels (dB).
The pressure dB is 20 times the base 10 logarithms of the ratio of a given root-
mean-square sound pressure to the reference sound pressure. (The standard
reference sound pressure in air or other gases is 20 µPa, which is usually
considered to be the threshold of human hearing at 1 kHz).Root-mean-square
(RMS) sound pressure is calculated using standard time and frequency
weightings.
With the advent of digital technology and the increasing accuracy of electronic
circuits, sound level meter functions are now frequently calculated in the digital
domain. High dynamic range is one of the most important measures of the quality
of an acoustic analyzer as it assures that both weak and strong signals can be
calculated and observed. The Spider provides 130dB dynamic range, which is
exceptional for this application.
Traditional sound level meters only include 1/1 and 1/3 octave filters. The CoCo
systems ability to do octave analysis and other advanced analysis functions
provides more flexibility and computation power than a traditional sound level
meter.
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