Larson Davis 831, User Manual

Page: 62 / 548

Model 831 Manual Setup Tabs 4-7
OBA Range Setting
In the Low range, the full scale level is reduced by 33 dB on
the display. The default display ranges will be as follows:
• Normal Range: 20 to 140 dB
• Low Range: -10 to 110 dB
However, the graph scaling range can be modified by the
user as described in “Adjust Graph Scale” on page 5-29.
OBA Bandwidth Setting
The user can select to utilize a single 1/1 Octave analysis
module, a single 1/3 Octave analysis module or both
simultaneously.
Select Off to have no real-time analysis module active.
OBA Frequency Weighting
The user can select that the 1/1 and/or 1/3 Octave frequency
analysis modules process data from the A, C or Z weighting
filters.
See “Spectral Statistics” on
page 24-5 for a more detailed
description of spectral Ln
measurements. Spectral Ln Mode
Spectral Ln values can be measured using either 1/1 or 1/3
octave bandwidths, depending on the OBA bandwidth
selected, as shown below.
• OBA = 1/1 Octave: Spectral Ln Mode is 1/1 Octave
• OBA = 1/3 Octave or 1/1, 1/3 Octave: Spectral Ln
Mode is 1/3 Octave
Setting the spectral mode Off will reduce the memory used
for data storage.
Ln Tab
The Ln value is the measured sound level which was
exceeded n% of the measurement time. For example, a value
of L
90
= 35 dB means that the measured sound level was
above 35 dB for 90% of the measurement period. These
statistical values are commonly used to describe the
characteristics of non-steady sound such as environmental
noise.
In order to be able to calculate Ln values, the Model 831
creates an amplitude distribution table over the range 0 to
200 dB, in amplitude increments of 0.1 dB. These data