Norsonic nor140, Instructional Manual

Page: 167 / 220

149
Norsonic Nor140
Instruction Manual
149
Norsonic Nor140
Instruction Manual
Background noise
The average background noise level, L
b
, is obtained in
a similar way.
Entering vales for the background noise level is op-
tional. If the level in the receiving room is more than 10
dB above the level of the background noise, no correc-
tions will be made and the final result will be independ-
ent of having a background level or not. However, if the
difference between the averaged level in the receiving
room and the background noise is between 6 dB and
10 dB, a corrected level for the receiving room, L
2C
, will
be used in the calculation as described in ISO 16283-2.
The correction is limited to maximum 1,3 dB corre-
sponding to a measured level in the receiving room 6
dB above the background level.
Reverberation time
The acoustic losses of the receiving room used in
the calculation may be obtained by measuring the
reverberation time or by recalling earlier measured
values. See the paragraph describing measurement
of reverberation time. If the optional noise generator
is installed it may be used for the excitation, otherwise
impulse method has to be used. To start a measure-
ment, press
START
when the field cursor is in the field
for reverberation time marked “T”. After the measure-
ment you may inspect the resulting reverberation table
by pressing the
TBL
key.
Display of results
When the required information has been entered, the
cursor will automatically be placed in the field for cal-
culation of the final result. If you are satisfied with the
entered values press
ENTER
, or if you want to make
any correction, move the field cursor to the required
field and enter the corrected values or press
DEL
to
clear averaged values.
When you move the field cursor to the field
“Results”
and press
ENTER
, the following functions
are calculated for each of the one third octaves-bands
50 Hz to 5000 Hz:
• The normalised impact sound pressure level L
n
• The standardised impact sound pressure level L
nT
• The averaged room level L corrected for back-
ground noise.
• The reverberation time T
For the values L, L
n
and L
nT
, a frequency weighted value
according to ISO 717-2 is also calculated. Pressing the
FUNC
key repeatedly will display the different functions.
The weighted values are all rounded to the closest
whole dB. The
TBL
key switches between graphical
and tabular representation of the results.
If you want to go back to the menu for calculation,
press
ENTER
.
Press the
MODE
button if you want to return to nor-
mal mode of operation.