Spider DSA User’s Manual
230
Acoustic Analysis
The Acoustics Data Acquisition option includes Fractional Octave Filter Analysis,
Sound Level Meters and Microphone Calibration functions.
The Fractional Octave Filter Analysis function applies a bank of real-time 1/n
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octave filters to the input time streams and generates two types of responses at
the same time: 1/N
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octave spectra, and the RMS time history of each 1/N
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octave filter band. The output of each real-time filter bank is in fact a 3D waterfall
signal that is arranged with the x-axis as logarithmic frequency and the z-axis as
time. Frequency weighting is applied in the frequency axis and time-weighting is
applied in the time axis.
The Sound Level Meter (SLM) (also referred to as Overall Level Meter) also uses
octave filters during acoustic data acquisition. The SLM applies ONE frequency
weighting filter to the input signal and time weighting to the output. Various
measures are then extracted from both the input and output signals of this
frequency weighting filter.
Octave Filters
Acoustics Analysis provides 1/N
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octave analysis using true real-time digital
filters in accordance with ANSI std. S1.11:2004, Order 3 Type 1-D and IEC 61260-
1995 specifications. A, B and C weighting filters can be applied to the input data.
Output results are weighted or un-weighted RMS values. The output can be
normalized with a calibration value. The results can be plotted on log or linear
axes and exact or preferred frequency values are supported.
Each band filter is designed in accordance with ANSI S1.11 and IEC 61260
specifications. The original analog signal is transferred to the digital domain by
means of the bilinear transform. The filter order can be specified, and the
frequency ratio can be calculated using the binary or decimal system.
The RMS reading of each octave filter is usually represented by a “bar” in the
spectrum plot. Keep in mind that the octave filters are actually somewhat wider
than the bars depict. Just like the analog filters they emulate, digital filters have
tapered pass-bands or “skirts”; they are imperfect frequency selectors. The filter
bands are not as sharp as the bars depict them, hence adjacent filters always
overlap one another. For this reason, a sine tone at 1 kHz will not only excite the
filter with center frequency at 1 kHz, but also all of the other filters as well, albeit
to much lower levels.
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