Spider DSA User’s Manual
238
Use Octave Analysis as the template to create a CSA project when fractional
octave analysis is required. In both the Octave Analysis and Sound Level Meter
templates the user can see the frequency weighted readings (such as dBA) but the
reading results may be slightly different when comparing Octave Analysis and
Sound Level Meter results because the data is processed, and the values are
computed differently. In octave analysis, the A-weighted sound level (dBA) is
computed by applying the frequency weighting function to the output of each
individual filter bank; while in SLM, the A-weighted sound level is calculated by
applying the A-weighting filter to the entire time domain. The SLM template
should be used to obtain the dBA or similar overall readings for sound studies
that would be comparable to those measured with a traditional sound level meter.
Terms and Definitions
In this section, we will define the terminology used in the SLM software options.
Reference sound pressure is conventionally chosen as 20 μPa. This is the
threshold of hearing (at 1 kHz) for the average person and is used to compute the
sound pressure level in the dB scale.
Sound pressure level (in dB) is defined as twenty times the logarithm to the
base ten of the ratio of the RMS of a given sound pressure to the reference sound
pressure. Sound pressure level is expressed in decibels (dB); symbol L
p
.
Peak sound pressure is the greatest absolute instantaneous sound pressure
during a stated time interval.
Peak sound level (in dB) is defined as twenty times the logarithm to the base
ten of the ratio of a peak sound pressure to the reference sound pressure the peak
sound pressure being obtained with a standard frequency weighting. (Example
letter symbols are L
peak
, L
cpeak
)
Frequency weighting is the difference between the level (dB) of the signal
indicated on the display device and the corresponding level of a constant-
amplitude steady-state sinusoidal input signal, specified in the IEC or ISO
standards as a function of frequency. It accounts for the A, B and Z frequency
weightings discussed in the previous section.
Time weighting is an exponential function of time, of a specified time constant,
that weights the square of the instantaneous sound pressure. This is the same as
exponential averaging in the time domain of the instantaneous sound pressure.
It is a continuous averaging process that applies to the output of a frequency
weighting filter or one of the fractional octave filters. The amount of weight given
to past data as compared to current data depends on the exponential time
constant. In exponential averaging, the averaging process continues indefinitely.
In a sound level meter the time weighting exponential averaging mode supports
the following time constants:
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