BRUEL & KJAER 2226 - CALIBRATION

(Normalised to 60s) being indicated
on a flashing display to warn that
the measurement is still in prog-
ress. On completion of a measure-
ment the display stops flashing and
the timer allows a one-minute read-
out period before turning the instru-
ment off unless a reset occurs dur-
ing this time . Operating any switch
causes a reset.
The important advantage of dis-
playing the normalised Leq is that
SEL(LAx ) can be obtained easily as
soon as the event is over, simply by
adding 10 log
10
60, (approx . 18 dB)
to the reading . SEL (Sound Expo-
sure Level) is the sound level
which , if maintained constant for a
period of 1 second, would contain
the same A-weighted energy as the
actual event. This is a useful par -
ameter for comparing single events
of different levels and duration, or in
building up a long term Leq from a
knowledge of individual events and
their frequency of occurrence.
Calibration
Calibration is easily carried out us-
ing Sound Level Calibrator Type
4230, which fits directly over the
microphone producing 93,8 dB
(± 0,25 dB).
A
windscreen is supplied and
s hould always be used when mak-
mg outdoor measurements.
Description
A composite block diagram is re-
pro duced in Fig.2. A Prepolarized
Co ndenser Microphone is fitted,
which, combined with the small
size of the meter case, gives the
overall frequency and directional
chm acteristics shown in Figs. 3 and
4 . The attenuator, which splits the
dy na mic range into four overlapping
40 dB measuring ranges, is fol-
low ed by a permanently connected
A -w e ighting network. The weighted
signal is fed to a logarithmic recti-
fier which performs a full - wave rec-
tific il tion and outputs the result in
logar ithmic form.
Th e detector, which performs true
RMS detection, has the two standar-
dized time constants "Fast" and
" Slow ". In addition Type 2225 has
a " Pe ak" hold with 30 JJ.S rise time ,
and the Type 2226 an "Impulse"
dete ctor to IEC and DIN specifica-
Overload
Detector
~
~
(2226
Display
only)
Microphone
•
p-
A·
Peak (2225) or
Pream-
...
Attenuator
~
Weighting
~
Log.
~
Impulse (2226)
~
DC
plifier
Rectifier Detector
Amplifier
Network
and
Integrator
Jt
t
D~
-=--
DC
Reset
Function
- Timer
f-4-
and
.......
-
""'[_____
Converter
Control
Selector
Output
780791
Fig . 2. Composite Block Diagram of Types 2225 and 2226
dB
6
4
2
0
-2
-4
-6
10
dB
8
6
4
2
0
-2
-4
-6
-8
~
'
'
./
100
Frequency (Hz)
.............
:""'.....
--
("'
I
I
l
20
100
l I
~
IEC 651 Type 2
1--
-
""""
---
v
ll
0° incidence
.... ~
r-
1-o...
......
...
1k
.,..-
...... ~
......
v
I-'
1-"'
...... ~
.............
.........
...........
1k ?...
'-
10k
20k
790096 / 1
AN~I s\,4- ~97~-
Type S2A --=
£
,
Random incidence
"-
\'
I
I
10k
20k
Frequency (Hz)
7 900 95
Fig.3. Free-Field frequency response of the complete instrument to sine waves with 0° inci-
dence (upper) and random incidence frequency response (lower)
Fig . 4. Directional Characteristics of the complete instrumen t
tions. For calculating a short-term
average sound level in the "60s
Leq" mode, the detector output is
squared and integrated , the process
being controlled by the timer . Crest
factor capability of the detector is
high, up to 50dB . (See Speci fica-
tions).
3