BRUEL & KJAER 1617 - DC or AC Recording; Writing Speed; Paper Speed

Tn (s)
0,1
0,3
3
10
30
100
Centre Frequency
(Hz)
2
3,15 10
31,5
100
315 1 k
3,15 k
1,5
0,9 0,5
0,9
0,3
0,3
oo{{\
0,5 0,3 ~'()(\
0,7
0,5
'--- SW-rl
0,3
Table 4. 1. Standard error (e) in dB for third - octave filters in combina -
tion with various averaging times . The framed values
correspond to the programs shown in Table 4.4 and
Fig.4 .3
10 k
0,3
770 4 12
If the bandwidth contains several sinusoidal components , which is possible, with third-
octave analysis , Equation (4 . 13) still applies when f is interpreted as the minimum sepa-
ration of any two of the components (lowest beat frequency) . When a large number of
components are contained within the bandwidth , as at higher frequencies the signal
should be treated as random.
4.3.2. DC or AC Recording
Generally, it is preferable to use DC recording when possible, but where no DC output is
available from the Measuring Amplifier, or where it has a dynamic range less than
required, AC recording must be employed. Using AC recording it is possible to achieve a
75 dB dynamic range with an appropriate potentiometer in the Level Recorder (not Type
2306) . Where a choice between DC and AC recording has to be made, the paper speed
obtained for either possibility should be investigated, as AC recording can be just as fast,
and may be preferable, to give averaging times less than 0, 1 second for example.
4.3 . 3. Writing Speed
For DC recording it is possible to select a writing speed of 250 mm / s on the Type 2306 ,
and 1 000 mm / s on the Type 2307 . In practice set as high a value as possible without
introducing instability .
The writing speed for AC recording is determined by the averaging time required . Suit -
able
writing speeds can be selected from the shaded portion of Fig.4 .2, which is valid
for both types of Level Recorders.
4.3.4. Paper Speed
42
The paper speed should be chosen so that the Recorder pen can attain the correct level
for each measuring band, and remain at that level for a while before the next filter band
is selected.
Normally, for DC recording it will be the rate of fall of the detector, with maximum
8,7 dB per selected averaging time , that limits the sweep rate. Table 4 .2 gives suitable