Larson Davis System 824, Technical Reference Manual

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9/29/09 FFT (Optional) 12-3
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 "Gain
Setting - (SSA)" on page 7-6 for
more details. The gain setting sets the measuring range of the instrument
in 10 dB steps by adjusting the amplification of the
microphone signal. This setting provides the following
options:
-20, -10, 0, +10, +20, +30, +40, +50
Bandwidth
The frequency resolution for each
line is the bandwidth divided by the
lines setting. With the bandwidth set
to 20kHz and 400 lines, the
frequency resolution is 50 Hz/line. The frequency range of a baseband FFT analysis extends
upwards from zero to a selected maximum frequency. Thus,
the bandwidth, or the frequency range covered by the
analysis, can range from a maximum of 20kHz to a
minimum of 200Hz. This setting provides the following
options:
200Hz, 500Hz, 1kHz, 2kHz, 5kHz, 10kHz, 20kHz
The following table shows resolution and FFT calculation
times as a function of the frequency bandwidth.
Bandwidth, Hz 200 500 1k 2k 5k 10k 20k
Resolution, Hz/line 0.5 1.25 2.5 5 12.5 25 50
Calcuation Time, Seconds 0.50 0.20 0.10 0.05 0.02 0.01 0.05