Larson Davis System 824, Reference Manual

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11/2/00 FFT (Optional) 11-3
Settings Descriptions
Title Setting
Use the title setting to create a title or heading for printed
reports. This title will be placed at the top of each FFT report
you print. You are allotted 30 characters. Printing of FFT
reports is not currently available.
Gain Setting
-20 & -10 dB gain are achieved by
desensitizing the microphone by
reducing its polarization voltage
from 200 volts to 20 volts. See
appendix D of the 824 Reference
manual for more details. The gain setting sets the measuring range of the instrument
in 10 dB steps by adjusting the amplification of the micro-
phone signal. This setting provides the following options:
-20, -10, 0, +10, +20, +30, +40, +50
Bandwidth
The bandwidth for each line is the
bandwidth setting divided by the
lines setting. With the bandwidth set
to 20kHz and 400 lines, the band-
width for each FFT line is 50Hz.
This is sometimes called frequency
resolution. The frequency bandwidth can be reduced from 20kHz to
200Hz. This setting provides the following options:
200Hz, 500Hz, 1kHz, 2kHz, 5kHz, 10kHz, 20kHz
The following table shows line bandwidth for various set-
tings:
Avg Count
The 824 FFT is guaranteed to have
no more than 75% overlap. The average count is the number of FFTs taken and averaged
together.
Bandwidth in Hertz
200 500 1k 2k 5k 10k 20k
Lines
100 2 5 10 20 50 100 200
200 1 2.5 5 10 25 50 100
400 .5 1.25 2.5 5 12.5 25 50