SoundAdvisor Model 831C
Creating an FFT Setup File On the Meter
140
Step 4.
On the General tab, the
Default Data File
name you enter here is a prefix for
each data file generated using this Setup.
Step 5.
To create an ISO compliant measurement, use the
Measurement Description
field to enter a thorough description of the measurement site, mic location,
source location, etc., (See ISO 1996-2, section 12). This can also be easily
managed in G4 after the measurement is downloaded.
Step 6.
Press the
right softkey
(
//*
) to set preferences on the FFT
tab.
Step 7.
On the FFT tab, choose a
Frequency Span
, which specifies the upper end of the
frequency spectrum that should be sampled.
Step 8.
From the
Lines
drop-down menu, choose the number of Lines of sampled data
to be collected for each frequency.
Your selection for Frequency Span and Lines results in the Frequency
Resolution. This is done by dividing your selection for frequency span by the
number of Lines, sometimes called “bins.” The result is the bandwidth of each
line (bin) in Hz/line or bin. Also, note that selecting a greater number of lines
increases the FFT computation time.
TAKE NOTE
For your selected frequency resolution, the SoundAdvisor 831C
calculates the FFT measurement accurately to within 0.1 dB.
Step 9.
Choose a
Window
method from the drop-down menu. The following options
are available:
Rectangular
: The windowing method distinguishes frequencies that are close
together in the examined spectrum, but compromises on amplitude details.
Use this method to view sounds within a particular spectrum.
Sampled Data and the Windowing Function
With fast Fourier transform mode,
when the measured signal doesn’t use
up an even number of lines or when
the number of lines is not a multiple of
the sampling frequency, the resulting
data “smears” into the adjacent lines.
This results in additional high-
frequency values that were not part of
the original signal.
The Windowing function
help
s to
account for this smeared data
(Spectral Leakage) leading to a more
accurate representation of the
measured sound. It does this by
zeroing the time domain function at
the beginning and end of each record
and by representing the wave form in
an alternative sine wave format. See
the following figures for more detail.
Summary of Contents for 831C-FFT
Page 1: ...Larson Davis SoundAdvisor Model 831C Sound Level Meter Reference Manual...
Page 214: ...SoundAdvisor Model 831C A 50 FIGURE A 47 Model 831C with 377C20 Microphone...
Page 215: ...SoundAdvisor Model 831C A 51 FIGURE A 48 Model 831C with 377C20 Microphone...
Page 216: ...SoundAdvisor Model 831C A 52 FIGURE A 49 Model 831C with 377C20 Microphone...
Page 223: ...SoundAdvisor Model 831C A 59 A 10 Declaration of Conformity...
