Brüel & Kjær 4423 User guide
Noise Dose Meter
Type 4423
When used in conjunction with a
Sound Level Meter, the Noise Dose
Meter provides analysis
of
time
varying noise levels to give the
Equivalent Continuous
L:evel
Leq
calculated
on
the Equal Energy
Principle. The Weighting Network is
selected
on
the Sound Level Meter,
and the Time Constant on the Noise
Dose Meter
(1
ms,
Fast, Slow
or
Impulse being available). The entire
system from microphone to Noise
Dose Meter can be calibrated using
a Sound Level Calibrator
or
Pistonphone.
NOISE DOSE METER
TYPE 4423
September 1972
CONTENTS
1.
INTRODUCTION
......................................
3
2.
CONTROLS
...........•..............................
5
2.1.
Front
Panel
2.2. Rear Panel
..................................
5
7
3.
OPERATION
........................................
10
3.
1.
Operation
with
aSound Level Meter . . . . . . . . . . . . . 10
Calibration
...............................
. . _
11
Measurement : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Use
of
the Conversion Slide Rule
................
13
3.2. Operation
from
a 12 V Battery
..................
13
3.3. Supply
of
Power
to
the
Sound Level Meter
........
14
3.4. Remote
Control
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.5. Analysis
of
Tape Recorded Noise
................
15
4.
MODE OF
OPERATION
................................
17
4.1. Principle
..................
.
................
17
4.2.
Input
Amplifier
.............................
17
4.3. Squaring
Network
............................
18
4.4.
Time
Constants
..............................
18
4.5. Noise Dose Counter
..............
.
...........
20
4.6. Elapsed
Time
and Overload Duration Counters
.....
22
5.
SPECIFICATIONS
....................................
24
1.
INTRODUCTION
.Noise
is
now
recognized
as
a pollutant. The noise levels in
our
present
day jndustrial society
are
constantly increasing. High noise levels
can
result
at worst in irreparable hearing
loss,
and at best in annoyance
or
interruption
of
work,
leisure activities
and
sleep and interference
with
conversation. A
large research
effort
has
been
brought
to
bear on the problem
of
noise
pollution
to
discover the relevant parameters
for
noise measurement.
It
has
been
found
that
the human ear
has
a reduced response
to
low
and
very high
frequencies. This effect
is
produced in noise measurement
by
using a fre-
quency weighting
network
of
similar frequency response
to
the human
ear.
There
are
several
such weightings,
but
the A weighting
is
in most common
use.
A second result
has
been
that
hearing
loss
and annoyance
are
a
function
not
only
of
the noise level,
but
also the
duration
of
the exposure, thus
introducing the concept
of
Noise Dose.
For
asteady continuous noise
it
is
a
simple matter
to
define the Noise
Dose
as
being the sound level
for
the
specified duration.
Time
varying sound levels are, however, more
difficult
to
define, and
some
trading relationship between level
and
duration
must
be
specified. Such a relationship
would
allow the reduction
of
a randomly
varying sound level
to
an
equivalent continuous sound level
for
the measure-
ment duration. The
commonly
accepted trading relationship is:
(1)
where
Leq
is
the equivalent continuous level
T
is
the measurement period
P
0
is
the reference pressure
of
20MPa
(1
Pa
=1
N/m
2)
and P(t)
is
the
time-
varying sound pressure level
The parameter
"q"
requires
further
discussion. Basically q
is
the number
of
dB
by
which a noise level must
be
increased
to
give the
same
Noise Dose
when the duration
is
halved.
It
is
at this
point
that
the various recommenda-
tions and legislations on noise control must
be
considered,
as
different
views
exist on the correct value
of
q.
3
Five
of
the main standards
for
noise control
are
listed below:
1.
DIN
45 641. Averaging
of
Time-
Varying Sound Levels (German).
2.
ISO 1996. Assessment
of
Noise
with
Respect
to
Community
Re-
sponse.
3.
ISO 1999. Assessment
of
Occupational Noise Exposure
for
Hearing
Conservation Purposes.
4. Code
of
Practice
for
Reducing the Exposure
of
Employed Persons
to
Noise (English).
5.
The Walsh Healey
Act
(USA federal law concerning, amongst
other
things, hearing conservation in government supplying industries).
For
more detailed
information
regarding the contents
of
these standards,
the original references should
be
consulted.
Dl N, ISO and the Code
of
Practice all operate
with
q =3, i.e., a noise
Jevel
must
be
increased
by
3 dB
to
give
the
same
Noise Dose when the
duration
is
halved. This corresponds
to
energy averaging, and
is
often
refer-
.
red
to
as
the Equal Energy Principle. The Walsh-Healey
Act,
however, speci-
fies q =
5.
(5 dB increase in level
for
half
the duration). Comparison on the
basis
of
90
dB(A)
for
an
8 hour day shows the Walsh-Healey
Act
to
be
more
permissive at high sound levels than those standards specifying q =
3.
The
Noise
Dose
Meter (Type 4423) operates
with
q =3.
Rewriting
with
q = 3 in Eqn. 1 gives:
{ 1 T
[P(t)l
2
Leq
= 10 log
10
T £
~J
(2)
which
is
the fundamental equation
for
the determination
of
Leq
from
a
time-varying noise using the Equal Energy Principle,
and
the
basis
of
opera-
tion
of
the Noise Dose Meter.
4
2.1. FRONT PANEL
OVERLOAD
-20
dB
TIME
CONSTANT
OVERLOAD
DURATION
ELAPSED
TIME
2.
CONTROLS
' BrUel & Kj
<:e
r NOISE
c._.,
DOSE
COUI\IT
INHIBIT
FUNCTION
Fig.2.1.
Front
Panel
Counts the number
of
hours
for
which
the overload
limit
of
the instrument
was
exceeded.
For
the effect
of
OVER-
LOAD
DURATION
on measurement
ac-
curacy
see
section 4.6.
Counts the measurement duration in
hours.
5
NOISE DOSE COUNTER
RESET
OVERLOAD
OVERLOAD
-20
dB
FUNCTION
CALIBRATION
SIGNAL
INPUT
6
Counts the
total
noise dose occuring
during
the
measurement duration. The
meaning
of
the numerical value,
and
the
calculation
of
the Equivalent Continu-
ous Level (Leq)
is
given in sections 4.5
and 3. 1.3 respectively.
Reset
push buttons
for
the OVER-
LOAD
DURATION
and
ELAPSED
TIME
COUNTERS
are
provided on the
actual counters. These push buttons
have
locking levers underneath. The RE-
SET
button
for
the NOISE DOSE
COUNTER
is
immediately below the
counter. This RESET
button
will
only
operate
with
the
FUNCTION knob on
"Reset".
An
orange lamp which lights when the
overload
limit
of
the instrument
has
been exceeded.
A
white
lamp which lights when a level
20 dB lower than the overload
limit
has
been exceeded.
Acts
as
an
On/Off
switch, and allows
switching between
"Calibration",
"Re-
set", "Stand
by"
and "Measurement"
modes.
FINE and COARSE screw adjustments
for
calibration
of
the instrument. The
calibration procedure
is
given in
sec-
tion
3.1.1.
For
connection
of
the signal
for
analysis
from
the Sound Level Meter, Measuring
Amplifier
or
Frequency Spectrometer.
Two
sockets
are
provided, a standard
B & K socket which accepts plug
JP
0101
and
a miniature socket which
accepts the plug on cable
AO
0037.
TIME
CONSTANT
2.2. REAR PANEL
MAINS
POWER
SOCKET
AC POWER
VOLTAGE
SELECTOR
POWER SWITCH
Allows
selection
of
the required
RC
value in the
time
constant
circuitry,
"1
ms",
"Fast",
"Slow"
and
"Impulse"
being available.
POWER SWITCH
Fig.2.2. Rear Panel
SLM
POWER
SIGNAL
INPUT
SIGNAL
OUTPUT
For
connection
of
AC mains supply.
For
selection
of
the correct AC mains
supply voltage, unscrew the central fuse
holder, unplug the black annular por-
tion,
and rotate
it
until
the protruding
metal piece
is
adjacent
to
the required
voltage setting and replace.
Only
the
required voltage should
be
visible.
Re-
place the fuse holder.
For
switching between AC mains power
supply
("
AC")
and
12 V battery supply
("DC").
7
FUSE
5A
For
instrument
protection
when using
12 V
DC
battery supply.
12 V
DC
POWER REM. CONTR. 8-pin
DIN
socket which accepts plug
JP
0002
for
connection
of
12
V battery
supply and remote
control
leads.
The
connections
are
given in Fig.2.3.
+
----------------~
12 V
DC
Remote 172287
Control
Switch
Fig.2.3. External View
of
4423
12
V DC POWER REM. CONTR. socket
with connections
FUSE 0.5 A
For
protection
of
the Sound Level
Meter when powered
by
the Noise Dose
Meter.
+4.5V
I+
1.5v-----------..,
To SLM
172288
Fig.2.4. External View
of
4423
SLMPOWER socket
with
connections
8
SLM POWER
SIGNAL
INPUT
SIGNAL
OUTPUT
7-pin DlN socket which accepts plug
JP
0703
for
the supply
of
1.5 V
or
4.5 V
DC
to
the Sound Level Meter.
For
connections
see
Fig.2.4.
A standard B & K socket and a minia-
ture
socket
for
signal
input
from
the
Sound Level Meter, Measuring
Amplifier
or
Frequency Spectrometer. These sock-
ets can
be
used instead
of
the similar
pair on the
front
panel.
For
output
of
the DC Level
from
the
Time
Constant
circuitry.
9
3.
OPERATION
3.1. OPERATION WITH A SOUND LEVEL METER
The Noise Dose Meter can
be
used
in conjunction
with
any
of
the B & K
Sound Level Meters (also
with
B & K Measuring Amplifiers, Frequency
Spectrometer and Frequency Analyzer). The
range
of
Sound Level Meters
is
given in Table 3.1.
Group Type No Description Weighting
2203 Precision Sound Level Meter A,
B,
C,
Lin.
(a)
2204 Impulse Precision Sound Level Meter A,
B,
C,
D, Lin.
2209 Impulse Precision Sound Level Meter A,
B,
C,
D, Lin.
2205 Sound Level Meter
A,B,C
(b)
2206 Precision Sound Level Meter A,
B,C
2208
Noise
Event Meter A
072040
Table 3.1. Classification
of
Sourrl Level Meters
Group
(a)
contains the larger Sound Level Meters which
can
be
used
with
Octave
Filter
Sets (Type 1613) and group (b) contains
the
smaller pistol
grip Sound Level Meters. The Weighting
Network
facility
of
each
instrument
is
also included
as
this determines the method
of
calibration which
will
be
discussed later.
For
further
information
on the operation and capability
of
these instruments the relevant Instruction Manual should
be
consulted.
The
OUTPUT
of
the Sound Level Meter in
use
(AC
OUTPUT
for
the
2204 and 2209) should
be
connected
to
one
of
the
SIGNAL
INPUT
sockets
on
the Noise
Dose
Meter.
10
3.1.1. Calibration
The Sound Level Meter and Noise Dose Meter should
now
be
calibrated
using a Sound Level Calibrator, Type 4230, which produces a 1 kHz tone at
94 dB
or
a Pistonphone, Type 4220, which produces a
250Hz
tone
at
124 dB. These levels
are
nominal
only,
and vary
with
atmospheric pressure
for
the 4220, and effectively
with
certain microphone types
for
the 4230.
It
should also
be
noted
that
only
the C weighting
and
Lin. have no attenuation
at
250Hz,
and
only
Sound Level Meters
with
these weightings available
should
be
calibrated using the Pistonphone.
The
following
procedure should
be
followed
to
obtain maximum
meas-
urement accuracy when the actual calibration levels
differ
from
the nominal
levels. Special instructions
for
the 2204 are given in brackets after
the
instructions
for
the other Sound Level Meters. These special instructions
are
necessary
to
avoid the
distortion
present at the
AC
OUTPUT
of
the 2204
mentioned in the
Instruction
Manual.
For
the 2209 a METER SWITCH
setting
of
either
"Slow",
"Fast"
or
"Batt."
can
be
used,
but
the
same
switch
·position must
be
used
for
both
calibration
of
the Noise
Dose
Meter and
measurement. The switch setting should
not
be
altered during the measure-
ment.
1.
The Sound Level Calibrator
or
Pistonphone should
be
applied
to
the
microphone and started. The Sound Level Meter should then
be
ad-
justed
to
give a meter
scale
deflection
of
+ 4.0
dB
(2204 -
use
METER SWITCH position
of
"Fast").
2.
The Noise
Dose
Meter FUNCTION knob should
be
switched
to
"Cali-
bration-
SLM dB
4",
and the COARSE screw adjustment turned
fully
anti-clockwise using a small screwdriver. (2204 -
use
METER
SWITCH setting
of
"Batt.
(Rec.)"). The
two
red
light
emitting diode
digits which
form
the right hand
portion
of
the NOISE DOSE
COUNTER should alternately show a count and return
to
zero. The
OVERLOAD
-20
dB lamp should stay on when the Calibrator
or
Pistonphone
is
emitting
a tone.
3.
The COARSE and
FINE
adjustments should
now
be
used
to
give a
Noise Dose Count
of
99. A
count
of
over 100
will
be
indicated
by
the
orange
OVERLOAD
lamp flashing.
4.
The Sound Level Meter should
finally
be
adjusted
to
the exact level
of
the Calibrator
or
Pistonphone (2204 -return
to
"Fast"
METER
SWITCH setting), and the Calibrator
or
Pistonphone removed
from
11
the microphone. Calibration
can
also
be
performed using the Electro-
static Actuators Types
UA
0033 (Half-inch) and
UA
0023 (One-inch)
or
the Rain Cover
UA
0393. For calibration using these instruments,
adjust the calibration level
to
give a deflection on the Measuring
Amplifier
of
+ 10 dB, switch the Noise
Dose
Meter FUNCTION
knob
to
"Calibration
-SLM
dB
10",and
adjust
to
aNoise
Dose
Countof99.
3.1.2. Measurement
12
1.
Reset
the
OVERLOAD
DURATION
and ELAPSED
TIME
counters
by pressing
fully
down the buttons under
each
counter.
Reset
the
NOISE DOSE COUNTER
to
zero
by
switching the FUNCTION knob
to
"Reset"
and pressing the RESET
button
under the counter. The
button
should
be
held
pressed
for
a few seconds
to
allow complete
clearance
of
the counter.
2.
The weighting
network
on the Sound Level Meter, and the
TIME
CONSTANT on the Noise
Dose
Meter should
be
selected
to
follow
the
Standard being
used.
Switch the FUNCTION knob on the Noise
Dose
Meter
to
"Stand-by"
(2204-
select
"BATT.
(REC.)").
3.
Adjust the RANGE knob on the Sound Level Meter
until
the OVER-
LOAD
-20
dB lamp flashes
intermittently.
This procedure ensures
that
the sound level
is
varying about the centre
of
the dynamic
range
of
the Noise
Dose
Meter.
If
the
OVERLOAD
-20
dB lamp
is
perma-
nently
on, then the RANGE setting
is
too
low, and overload condi-
tions could occur during the measurement.
If
the lamp
is
permanently
off,
then the RANGE setting
is
too
high, which could lead
to
low
signal
levels
and
inaccurate results due
to
poor
resolution.
4.
To
start the measurement, the FUNCTION knob should
be
switched
to
"Measure".
If
it
is
thought
that
extraneous high transient levels
are
about
to
occur during the measurement, then the
INHIBIT
button
should
be
used.
Pressing
this
button
will
halt the NOISE DOSE
COUNTER and the
OVERLOAD
DURATION
COUNTER,
but
not
the ELAPSED
TIME
COUNTER.
On
releasing the
button,
normal
counting
will
be
resumed. The complete measurement can
also
be
temporarily halted by switching
to
"Stand-by"
and re-started
by
switching
to
"Measure"
without
spurious switching transients being
introduced.
5.
When
the required Elapsed
Time
has
been
counted, the FUNCTION
•
knob
should
be
switched
to
"Stand-by".
The Overload Duration,
Elapsed
Time
and Noise Dose
Count
should now
be
suitably recorded
along
with
all other relevant
information,
including the Sound Level
Meter RANGE setting (SLM
Att.)
the weighting
network
and
TIME
CONSTANT
used.
3.1.3.
Use
of
the
Conversion Slide Rule (QH 0007)
Fig.3.1 shows a typical calculation using the slide rule.
Fig.3.
1.
Use
of
the Conversion Slide Rule
Find the Noise Dose
Count
on the lowest
scale
of
the slide rule
(e.g.
450 000). Place opposite this the Elapsed
Time
(10 h).
Read
off
the highest
scale
opposite the arrow marked
"Add
to
SLM
Att.
dB"(+
5 · 5 dB). This
figure should then
be
added
to
the Sound Level Meter RANGE setting in dB
to
give the required equivalent continuous level Leq·
3.2.
OPERATION
FROM A
12
V
BATTERY
The Noise Dose Meter may
be
powered using a 12 V battery. The current
drawn
will
be
1.5 A, and hence
for
a
full
8
hour
working
day measurement
of
Lew
12
Ah
minimum
battery capacity
will
be
required. Connections
to
the 12 V
DC
POWER/REM. CONTR. socket should
be
made
as
in Fig.2.3 in
Section 2.
The POWER SWITCH
at
the back
of
the instrument should
be
switched
to
DC. Under 12 V
DC
operation the Noise Dose Meter
is
protected
by
a
5 A fuse.
If
the instrument fails
to
operate after
this
procedure
has
been
followed, then the 5 A fuse should
be
removed, checked
and
replaced
if
necessary. The connections between
the
battery and the Noise Dose Meter
should also
be
checked
if
fuse-blowing occurs, and corrected
if
necessary.
13
3.3. SUPPLV OF POWER TO THE SOUND
LEVEL
METER
The Sound Level Meter
used
with
the Noise
Dose
Meter can
be
powered
from
the Noise
Dose
Meter regardless
of
whether AC
or
DC power
is
being
used
for
the
4423
itself. Battery Adaptors
for
the various Sound Level
Meters
are
available according
to
the accessory numbers given in Table 3.2.
Type Battery
Adaptor
Voltage
Number Number
(V
DC)
2203
UA
0363
+4.5
2204 + 4.5
UA
0364
2209
2205
2206
UA
0365 + 1.5
2208
072041
Table
3.2. Sound Level Meter Battery Adaptors
The Battery Adaptors
are
supplied
with
a standard cable length
of
3m.
Longer lengths
are
most easily obtained
by
cutting the cables and suitably
inserting
the
required length
of
cable, checking
that
the correct
polarity
is
maintained. The connections
to
the Noise Dose Meter are shown in Fig.2.4.
Plug the 7-pin DlN plug
of
the battery adaptor
into
the SLM POWER socket
at the rear
of
the Noise
Dose
Meter and screw
or
slide
the
Adaptor
into
the
Sound Level Meter. The Sound Level Meter should
now
operate when
the
Noise
Dose
Meter
is
switched on.
If
this
is
not
so,
remove the 0.5 A fuse
holder, check the fuse and replace
if
necessary.
3.4. REMOTE CONTROL
Remote Control
of
the Noise Dose Meter
"Stand-by"
function
can be
achieved
by
connecting the 7-pin
DIN
plug
to
a remote switch
as
shown in
Fig.2.3.
The Noise Dose Meter FUNCTION
knob
should
be
switched
to
"Meas-
ure". Opening the remote
control
switch
will
automatically start the
meas-
urement, and closing
it
will
end the measurement.
14
•
3.5.
ANALYSIS
OF TAPE RECORDED NOISE
A complete measurement set-up
for
the
analysis
of
tape recorded noise
is
shown in Fig.3.2.
Power Amp.
2706
Measuring
Amp.
2606 Noise
Dose
Meter
4423
172289
Fig.3.2. Measurementset-up
for
the Analysis
of
Tape
Recorded Noise
The Power
Amplifier
and Loudspeaker are provided
for
audible
monitor-
ing
of
the signal,
and
the Level Recorder
to
provide a visual representation
of
the signal variation
if
this
is
required. The tape recording should consist
of
a calibration tone, using either a Sound Level Calibrator
or
Pistonphone
on the Sound Level Meter Microphone
followed
by
the
signal
to
be
ana-
lyzed. The Power
Amplifier
ATTENUATOR
and
GAIN
CONTROL
knobs
should
be
adjusted
to
give
a reasonable sound level
from
the loudspeaker.
Also, a sound level
scale
should
be
inserted in
the
Measuring
Amplifier.
As
it
will
not
usually
be
possible
to
match the attenuator setting illuminated
on
the
scale
to
the recorded sound level (because
of
signal amplification in
the
Sound Level Meter) any Sound Level
scale
can
be
used.
15
The calibration tone should now
be
played,
and
the Measuring
Amplifier
INPUT SECTION
ATTENUATOR
knob adjusted to give a meter deflection.
The deflection should then
be
adjusted
to+
4
dB
using
the
SENSITIVITY
adjustment
for
the Dl RECT INPUT. The difference between the total
Spectrometer reading and the nominal calibration
level
should
be
noted,
as
values
of
Leq
obtained
will
have
to
be
adjusted by this amount. The Noise
Dose
Meter FUNCTION knob should now
be
switched
to
"Calibration
-4
dB"
and the
count
adjusted
to
99. The Measuring
Amplifier
should now
be
recalibrated
to
give a deflection corresponding to the exact level emitted
by the Calibrator
or
Pistonphone. The
system
is
now calibrated.
The Noise
to
be
analyzed should now
be
played back and the INPUT
SECTION
ATTENUATOR,
and
if
necessary
the OUTPUT SECTION AT-
TENUATOR,
should
be
adjusted
so
that
the
OVERLOAD
-20
dB lamp on
the Noise
Dose
Meter flashes on
and
off.
The
Noise
can
now
be
analyzed
following the procedure given in sections 3.1.2
and
3.1.3.
16
4.
MODE OF OPERATION
4.1. PRINCIPLE
The
basic formula
for
the
calculation
of
the
Energy
Equivalent
Continu-
ous
Level
for
a time-varying noise
(from
chapter
1)
is:
{ 1
T[P(t)J2
Leq =
10
log10 T £
~
(2)
The
Noise Dose Meter calculates
the
value
of
Leq relative
to
the
Sound
Level Meter
Attenuator
setting,
LAtt.·
Introducing
this
into
Eqn.
(3) gives:
(3)
where
P
Att
is
the
pressure
corresponding
to
LAtt·
Simply,
the
Noise Dose Meter
accepts
the
voltage
corresponding
to
P(t)/P
Att.
from
the
Sound
Level Meter,
squares
it
and
integrates
to
obtain
the
Noise Dose.
The
analysis
time
is
also
measured
to
give
the
Noise Dura-
tion
(T),
and
these
two
values are used
to
calculate
the
value
of
Leq·
A
block
of
diagram
of
the
Noise
Dose
Meter
is
shown
in Fig.4.1.
4.2. INPUT AMPLIFIER
The
signal
to
the
Noise Dose Meter
is
assumed
to
have no
DC
level (as
this
will lead
to
clipping).
The
output
from
the
B & K
Sound
Level Meters,
Frequency
Spectrometers
and
Measuring
Amplifiers
conform
to
this
speci-
fication.
The
maximum
input
voltage
of
the
Noise Dose Meter
is
70
V
peak.
This
input
signal
is
fed
to
the
Input
Amplifier,
which
acts
as a
buffer
amplifier
and
has variable gain
to
allow
for
calibration
adjustment.
The
input
impedance
of
the
Noise Dose
Meter
is
at
least
10
kil.
17
Signal
Input
o-----
---j
100
to
240 V
AC
'-------'-'
+5V
+ 12 v
+ 15 v
-15 v
Fig.4.
1.
Block
Diagram
of
the Noise
Dose
Meter
4.3. SQUARING NETWORK
Noise
Dose
Count
Elapsed
Time
Overload
Duration
Overload
Lamp
Overload
-
20dB
L
amp
Power
The
output
from
the
Input
Amplifier
is
fed
to
the Squaring
circuitry.
The Squaring
circuitry
is
divided
into
two
channels. Squaring in
each
chan-
nel
is
achieved
by
feeding the
signal
to
both
input
terminals
of
an
integrated
circuit
multiplier.
The signal in
one
channel
is
squared directly,
but
in
the
second channel
is
first
amplified by 26 dB, squared and then attenuated
by
52 dB.
Automatic
electronic switching between channels
is
provided de-
pending on
the
signal
size.
By
this process the accuracy
of
squaring
of
small
signal levels
is
increased and
the
dynamic
range
extended
to
40 dB.
4.4. TIME CONSTANTS
The
output
from
the
Squaring
circuitry
is
fed
to
the
Time
Constant
circuitry.
The
time
constants available
are"
1 ms"
"Fast",
"Slow"
and
"Im-
pulse". The modes
"Fast"
and
"Slow"
are
basically defined in terms
of
meter deflection on a Sound Level Meter
for
certain tone burst
input
sig-
nals.
In
equipment
using a
different
type
of
read-out, such
as
Type
4423,
"Fast"
and
"Slow"
represent approximations
to
the basic
definition.
"Im-
pulse",
on
the
other hand,
is
specified in terms
of
electrical networks featur-
ing one
RC
=
35
ms
time
constant followed by
square
root
extraction and a
separate 3 second discharge
time
constant. The latter becomes 1.5 seconds
in Type 4423 due
to
the absence
of
root
extraction.
18
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