BRUEL & KJAER 2212 User guide

Noise Limit Indicator Type 2212
Type 2212 Noise Limit Indicator is an instrument
containing six identica I input amp I ifiers and six
output amplifiers and a control panel. It is mainly
intended for traffic noise monitoring, f. inst . at
airports or in road traffic surveys.
BHUEL & KJJER
Naerum, Denmark . ~ 80 05 00 . ~ BRUKJA, Copenhagen . Telex: 5316
B B 2212

Noise Limit Indicator.
Type 2212
November 1966

Contents
1. Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Input Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Filters and Output Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Overall Frequertcy Characteristic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Rectifier and Relay Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Control Panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Meter Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Power Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.
Identification of Control Knobs, Terminals, etc •. . ..... .. ........... 10
3. Operation of the Noise Limit Indicator Type 2212 as Traffic Noise
Monitor ....... ..... ................................ . .......... .. 12
General . ...................................................... . . 12
Preadjustment of Noise Limit Indicator Type 2212 .................. 13
Connection between Noise Limit Indicator Type 2212 and Outdoor
Microphone System Type 4920 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Adjustment of the complete Measuring Set-up . . . . . . . . . . . . . . . . . . . . . . 18
Checking by means of the Pistonphone Type 4220 . . . . . . . . . . . . . . . . . . 20
Adjustment of the Sensitivity Increase . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4. Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2'2

1. Description
General.
The Noise Limit Indicator Type 2212 is a modified version of the well-known
Noise Limit Indicator Type 2211 mainly intended for traffic noise monitoring.
It contains six identical, separate input amplifiers and six output amplifiers.
Furthermore two "sets" of six microphones each ( or six vibration pick-ups)
can be successively switched to the input amplifiers by means of a remotely
controlled relay circuit. It is thus possible to quickly check 12 measuring
positions-and continuously monitor six.
The input circuit of the amplifiers c ontains a transformer which allows con-
nection to a symmetrical transmi s sion system. The input transformer has
been included, however, not only to allow connection to a symmetrical
transmission system but also to facilitate the remote control of the calibra-
tion oscillator in a connected Outdoor Microphone System Type 4920 over
a two-wire system .
In the output amplifiers provision is made for the insertion of filters.
The Noise Limit Indicator consists of the following major parts.
1. Input Amplifiers.
2. Filters and Output Amplifier s.
3.
Rectifier and Relay circuits.
4. Control Panel.
5. Meter Circuit.
6. Power Supply.
Input Amplifiers.
The Type 2212 contains six input amplifiers which are identical and
interchangeable. Each of them contains a four-stage transistor amplifier
preceded by a 60 dB attenuator. The attenuator can be varied in 10 dB
steps and is intended to be used in conjunction with the 48 dB attenuator
in the output amplifier.
The first amplifier stage is designed as an emitter follower in order to
obtain a sufficiently high input impedance. The input impedance is, for the
least sensitive position of the attenuator, approximately 1.5 Mohms. For
the next position it is approximately o.5 Mohms, and for the remaining
positions 150 kohms. The next two stages are RC-coupled, while the last
stage contains two transistors in a single-ended push-pull arrangement. The
output signal is fed via a relay to the input of the filters in the output
3

n
n
4.
I 5.
: 6. 1
L
_________ J
4.
5.
I 6. I
L---------------------- -- -J
When the pushbutton marked RELEASE is operated the lamps are reset ond INPUT I or
INPUTn disconnectod.
r
I
I
I
0 0
0 0 0
I I
I Meter circuit I
L ________ j
162.?76
Fig. 1.1. Block diagram of the Noise Limit Indicator.
amplifiers. The relay is controlled from the INP UT I or INPUT II buttons
on the Control Panel and serves to break or connect the input circuits to the
filter inputs . See Fig. 1.1. To obtain a high degree of stability and to ensure
a distortion of less than o.1 %, a large amount of negative feedback is
introduced in the amplifier. The feedback can be altered with a potentio-
meter to obtain fine adjustment of the sensitivity. A second potentiometer
can be switched into the feedback circuit by means of a relay which is
controlled from the SENSITIVITY INCREASE knob on the control panel.
This potentiometer can be set to increase the sensitivity by between 0 and
5 dB, without influencing the frequency response of the amplifier. The total
amplification with increased sensitivity is 45 dB.
Filters and Output Amplifiers.
Weighting networks or filters can be used in conjunction with the Output
Amplifiers.
The filters can be by-passed by a toggle switch on the front plate of the
amplifier. After filtering, the signal is fed to a 48 dB attenuator with 1 dB
steps. Two independent tappings of this attenuator make it possible to adjust
the attenuation to two different settings, one for each of the two inputs.
The pushbutton switches on the control panel, which are used to select the
desired input, also select the correct section of this attenuator ( with the aid
of a relay), so that INPUT I corresponds to attenuator tap No. 1 and INPUT
II with attenuator tap No. 2 (see Fig. 1.3 and block diagram).
4

+1
db
01-"
-1
-2
-3
-4
-51
2 5 10 2 5 100 2 5 1000 2 5 10000 2
C/5 5 100000
Frequency 1'
2289
Fig. 1.2. Frequency Response of Input Amplifier without input transformer.
The first stage of the amplifier is designed as an emitter follower in order
to obtain an input impedance high enough to have no influence on the
attenuator. The impedance loading the filter is thus determined by the
attenuator and is approximately 146 kohms which gives optimum performance
of the filter. The following stages are RC-coupled amplifier stages, stabilized
by negative feedback. Three feedback circuits are employed, two of which
are used to shape the frequency response curve and to minimize the input
capacity. The third one can be varied so as to alter the amount of am-
plification ( this is the sensitivity adjustment on the front plate of the
amplifier).
Control Panel
r n
,Q.QOOO
/ >( '
II
To Output
Amplifier 1
To Output
Amplifier 2
~66H4-
Fig. 1.3. By depressing one of the INPUT buttons, the correct input and also
the correct tapping of the INDICATOR LEVEL attenuators are chosen by
means of relay circuits.
5

+
d
1
~
1/ .
2
...
~
I
5
I-I-
- ....
""
2 5 10
5 100 5 1000 2 5 10 K C/5 5 100 K
162177
Fig. 1.4. Frequency Response of output amplifier.
Overall Frequency Characteristic:
The overall frequency response of the instrument ( without filters in the
output amplifiers ) is shown in Fig. 1.5 and is mainly determined by the input
transformer. Also a loss in amplification of approximately 1 dB takes place
in this transformer.
The curves shown in Fig. 1.5 are valid for various generator impedances, R1.
To allow a clear picture of the change in characteristic with generator im-
pedance and voltage, however, the curves are drawn with a fictive distance
between the flat parts of 2 dB.
l I
Rg= 60..n.
I
600mV
v-
I. _I
:10mV
60mV
60 6mV
6mV Rg=200.n.
600mV7
~""""""
l _I
\\
60mV Rg=600.n.
60-
soo~n~
6mV
~
6mV~
6mV-
600mV -;?
60mV I
2!8
Olrmv
r---
6mV i
t
10 20 50 100 200 500 1000 2
000 5 000 10 000 20 000 50000 100 000
Frequency Hz(cfs)
Fig. 1.5. Frequency chamcteristic for the entire instrument.
Rectifier and Relay Circuit.
Following the amplifier is a quasi-RMS rectifier circuit. It provides full-wave
rectification and gives accurate RMS indication for sinusoidal signals as
well as for white noise and other signals with complex waveforms, including
asymmetrical pulses with crest factors up to 3. By means of a switch
6

'
two different damping characteristics can be introduced . "Standard" or "low
damping" is meant for the measurement of steady signals with frequencies
above 20 Hz while "high damping" is meant for the measurement of
fluctuating signals, i.e. frequencies below 20 Hz or measurements where
fluctuating background noise would otherwise disturb the meter reading.
The signal is now capable of energizing the relay which makes the indicator
lamp light-up. This relay has two stable positions. However, as even the best
relays do not always operate on precisely the same amount of current-flow,
another stage has been added so that the relay invariably switches over at a
certain input signal voltage. This stage has a characteristic as shown in
Fig. 1.6, and as seen the steep slope of the curve provides a large current
increase for a slight voltage increase, thus making the energizing point
much more consistent. To obtain a stable working point for this stage a
zener-diode is introduced, and also a 20 kHz signal is presented to give an
independent bias voltage. This signal is derived from a 20 kHz generator
located in the power supply unit and thus common for all the amplifiers.
This bias is so adjusted as to make the relays operate at an AC input
voltage of 2.24 V. · when none of the INPUT buttons on the Control Panel
are depressed, the relays are held in their passive position by means of a
" holding voltage" of approximately 12 V DC.
Before \ the rectifier circuit is an AC output lead to a multisocket on the
rear panel of the instrument. The output impedance at the pins of this
I Output
Relay {
Current
Tolerance
2.
24 v
Voltage
tolerance
--j~b
Einput
162212
Fig. 1.6. Ei/1. characteristic of the current shaping stage just before the relay.
socket is approximately 25 ohms (to ground). The maximum voltage to be
obtained is approximately 7 Volts AC (RMS). A voltage of 2.24 Volts AC
at this point energizes the relay.
7

Control Panel.
The control panel contains the five pushbuttons intended to be used by the
operator, as well as the indicating lamps for each channel.
The two input selectors bring the proper input into circuit and also keep the
relays of the output amplifier in the passive position when not depressed.
The SENSITIVITY INCREASE button controls the relays which influence the
amount of feedback round the input amplifiers, thereby increasing the
sensitivity. This button only functions while kept depressed.
The RELEASE button releases the other pushbuttons and thus resets the
"holding" voltage used to keep the relays of the output amplifiers in the
passive position. Furthermore, the RELEASE button serves to check the
proper functioning of the output relays and the indicator lamps, as all the
lamps are lit up when the switch is pressed down.
The INPUT I, INPUT II and RELEASE buttons operate via a relay. The
two INPUT buttons are furthermore delayed in operation, so that the "holding
voltage" used to keep the output relays in their passive position is not
switched off till approximately 1 second after the input amplifiers are
switched in. It this way the disturbing effect of transients are avoided.
A green light indicates when the instrument is operating and at which input
channel.
Parallel to the RELEASE button is a springloaded RESET button which
serves to momentarily reset the output relays to their passive positions
without releasing the INPUT or SENSITIVITY INCREASE buttons. Its
purpose is to recheck the lamp indication in case of unwanted noise, e.g.
footsteps, doors slamming etc.
Each of the red lamps is permanently connected to a 12 V AC source and is
grounded through its corresponding output relay. In this way the lamps
cannot be lit when no amplifier is inserted.
There is a relay in parallel with the lamps. The relay contacts change over
whenever anyone of the lamps light-up. The switch so obtained is connected
to pin "T" and "U" of the remote control plug and may be used to remotely
control buzzers, alarm bells, or other auxiliary equipment.
Meter Circuit.
The meter circuit of the apparatus serves mainly as an aid in the proper
setting of the attenuators and to check amplification by means of the built -
in reference voltage. The meter can be connected through a switch to any
of the outputs of the amplifiers so as to read the output level before
rectification. The rectifier circuit of the meter is identical to that of the
output amplifiers, as is the damping circuit.
To provide aural or visual indication of the signal that reaches the meter
circuit, two test sockets are placed beneath the meter. These sockets are in
parallel with the meter circuit, allowing headphones , oscilloscopes or other
instruments to be connected to them.
8