This is only a preview of the December 1996 issue of Silicon Chip. You can view 28 of the 104 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "Build A Sound Level Meter":
Items relevant to "Build An 8-Channel Stereo Mixer; Pt.2":
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Build This
Sound Level
This Sound Level Meter adaptor will
measure sound pressure levels from
below 20dB up to 120dB with high
accuracy. It connects to any standard
digital multimeter and has inbuilt
filters for A and C-weighting.
Noise can have a huge affect on the quality of our lives. A
reliable measuring instrument is a must for those interested
in finding out just how much noise is in their environment.
Just how much noise is present at any time is very
subjective. If you are confined to a soundproof room for
a period of time, even the sound of a pin dropping will
seem quite loud. But if you are in a normal home or office
environment, the dropping of a pin is likely to be completely inaudible. And even the sounds of people on
the telephone or using computers may be completely
drowned out if a semi-trailer passes down your street
or a jet flies overhead.
The above examples show just how exceptional
our ears are in responding to the possible range of
sounds in our environment. In fact, we could expect
to experience a sound pressure range of about three
million to one. Because of this huge range of values sound pressure levels are usually expressed in
decibels, a logarithmic ratio where 20dB (decibels)
is equivalent to 10:1; 40dB is 100:1 and 60dB is
1000:1, all compared to a reference level. The
overall 3,000,000 to 1 range can then be expressed
as 130dB (20 log 3,000,000).
Since the dB is a ratio it must be referenced to
•
•
•
•
66 Silicon Chip
66 Silicon Chip
Main Features
Connects to any digital multimeter
Calibration method uses loudspeaker & pink noise source
A and C weighting plus flat (unweighted) filters
Slow, Fast and Peak response
By JOHN CLARKE
Meter
a particular pressure level of 20.4µPa
(micro Pascals). Usually sound pressure levels are quoted as so many dBSPL, indicating that the 0dB reference
is 20.4µPa.
On the dBSPL scale, 0dB is virtually inaudible, 30dB might be the
sound level in a quiet rural area with
no wind while a noisy home kitchen
might be 80dB or more. Heavy traffic
can easily be 80-90dB while a suburban train in a tunnel can produce
100dB. Electric power tools or pneumatic drills can easily run at 110dB
and some can go into the pain level
at 120dB.
Measuring SPL
The S ILICON C HIP Sound Level
Meter is designed to produce accurate
readings of sound pressure which are
displayed on a digital multimeter. It
Fig.1: this graph shows the differences between A and C-weighting and flat
(unweighted) responses in the Sound Level Meter.
comprises a handheld case with a
short tube supporting the microphone
at one end of the unit. Flying leads
with banana plugs connect to the
multimeter.
A slide switch provides A-weighting
and C-weighting filters to tailor the
measurement readings. A-weighting
is called for in many measurements to
Australian standards although it is not
really appropriate for louder sounds
where C-weighting or a flat response
(unweighted) can give more meaningful results. Fig.1 shows the differences
between A and C-weighting and flat
(unweighted) responses in the Sound
Level Meter.
Slow and fast response times are
provided as well, so that sudden noise
can be filtered out, if need be.
A “peak detect” facility has also been
included which will give an indication
Fig.2: the block diagram of the Sound
Level Meter. IC4b controls the gain
of IC2 so that the output from the
full-wave rectifier is constant. IC4b’s
output is attenuated by IC3b and fed
to an external multimeter.
December 1996 67
Fig.3: apart from the use of a VCA (IC2), an unusual feature of the circuit is
the use of IC5 to evenly split the 18V supply. This has been done because the
negative rail is subjected to a higher current drain than the negative rail, which
would shorten the life of battery B2.
of the noise waveform shape. If there
is no or little difference between the
peak and the fast reading then the noise
waveform can be assumed to be relatively sinusoidal. If, however, the peak
level is greater than the fast reading,
then the noise waveform has a lot of
transient bursts. These may result in a
low average value as shown on the slow
68 Silicon Chip
or fast response settings but are easily
captured by the peak detect circuitry.
The cost of the Sound Level Meter
has been kept low by using a multi
meter as the display.
Logarithmic conversion
As already noted, the Sound Level
Meter will read from below 20dBSPL
to 120dBSPL, a range of 100dB. That’s
a pretty stiff requirement. The circuit
has to provide a direct logarith
mic
conversion over 100dB, producing an
output of 10mV per dB.
In practice, the signal fed to the multimeter ranges from 200mV at 20dB
to 1.2V at 120dB. This means that all
readings can be made on the 2V range
of the multimeter; there is no need to
switch ranges.
Fig.2 shows the block diagram of
our sound level meter. Signal from
the microphone is amplified by op
amp IC1a and then fed to either the A
or C-weighting filters which involve
switch S2 and op amp IC1b.
IC2 is a voltage-controlled amplifier
(VCA) which can either amplify or attenuate the signal from IC1b, depending on the voltage at its control input.
This input operates in a logarithmic
fashion so that small control voltage
changes can produce large variations
in the output signal.
IC2’s output is full wave rectified
by IC3a & IC4a and the rectified signal
fed to the Slow, Fast or Peak filters
involving switch S3. The resulting
DC voltage is compared in error amplifier IC4b against a 20mV reference.
IC4b’s output then controls the VCA
so that it produces a constant output
regardless of changes in the microphone signal.
As well as driving the control input
of the VCA, IC4b drives op amp IC3b
which modifies the signal so that it
provides the required 10mV per dB,
to drive the external multimeter.
Circuit description
Fig.3 shows the complete circuit
for the Sound Level Meter. It uses five
ICs, three of which are dual op amps
(IC1, IC3 & IC4). IC2 is the VCA, which
can be considered as an op amp with
a DC gain control. IC5, a TL071 single
op amp, is used to accurately split the
18V battery supply; more of that later.
The microphone is an electret type
which is biased via a 10kΩ resistor
from the +9V supply. Its signal is coupled to op amp IC1a which has a gain
of 7.9 (+18dB), as set by the 68kΩ and
10kΩ feedback resistors. This gain has
been selected for the specified microphone and will need to be altered if
other types are used.
IC1a drives both the C and A-weighting filters. These are selected at positions 1 and 2 of switch S2a respectively. Position 3 selects IC1a’s output
directly for the flat or unweighted
signal mode. IC1b is simply a unity
gain amplifier to buffer the filters and
prevent loading of the filter signal.
IC1b’s output is fed to IC2 via switch
S2b and a 10µF coupling capacitor.
Note that in positions 1 and 3 of S2b,
the 4.7kΩ and 12kΩ resistors are connected in series while for position 2,
the 4.7kΩ resistor is bypassed. This
allows a 3dB higher gain for IC2 when
A-weighting is selected. The gain adjustment is necessary to maintain the
Fig.4: waveforms from the precision full-wave rectifier. The top trace (Ch1)
shows the input sinewave while the lower trace (Ch 2) is the rectified version.
Note that the RMS values are slightly different due to small offsets in the op
amps.
same 1kHz signal level applied to IC2
for all positions of switch S2.
IC2 is an Analog Devices voltage-controlled amplifier (VCA). It
has a dynamic range of 117dB, .006%
distortion at 1kHz and unity gain, and
a gain control range of 140dB. The DC
control input operates at -30mV per
dB gain change. IC2’s gain is set by
the voltage at pin 11 and the ratio of
resistance between pins 3 and 14 and
the input at pins 4 & 6.
The 100kΩ resistor between pin 12
and the +9V rail sets the bias level for
the output at pin 14. This bias can be
selected for class A or B operation.
Class A gives lower distortion but
slightly higher noise. We opted for
class B bias for best noise performance. A .001µF capacitor between
pins 5 & 8 compensates the gain control circuitry.
Precision rectifier
IC2 is AC-coupled to the precision
full wave rectifier formed by op amps
IC3a & IC4a. For positive signals the
output of IC3a goes low to reverse bias
diode D1. Positive-going signals are
then summed in inverter IC4a via the
20kΩ resistor R1 to produce a negative
output at pin 7. The gain is -1. Diode
D2 and the 20kΩ series resistor limit
the op amp’s negative excursion.
For negative signals D1 conducts
and IC3a acts as an inverting amplifier
with a gain of -1 to sum into IC4a via
R5. Negative-going signals are also
summed in IC4a via R1. Since the
voltages across R1 and R5 are equal
but opposite and the value of R5 is
exactly half R1, the net result of the
sum into IC4a is a negative output with
an overall gain of 1.
So for positive signals applied to
the full wave rectifier the gain is -1
and for negative signals the gain is 1.
Thus IC3a and IC4a form a precision
full wave rectifier. The 10kΩ and
5.6kΩ resistors at IC3a’s and IC4a’s
non-inverting inputs minimise any
offset voltages in the op amps.
Fig.4 shows the oscilloscope waveform of the precision full wave rectifier. The top trace shows the input
sinewave while the lower trace is the
rectified version. Note that the RMS
values are slightly different due to
small offsets in the op amps.
The switched feedback across IC4a
provides filtering of the rectified signal
as well as gain control. In the ‘slow’
setting of S3a, the 20kΩ resistor sets
the gain and the 470µF capacitor
controls the response. Similarly, for
the ‘fast’ setting of S3a, the 100µF capacitor sets the response. In the ‘peak’
position of S3, diode D3 charges the
10µF capacitor to the peak value of the
waveform while the 12kΩ resistor sets
December 1996 69
Fig.5: follow this diagram
when installing the parts on
the PC board and take care to
ensure that all polarised parts
are correctly oriented. Note
that REF1 and a number of
capacitors must be laid flat on
the PC board (see text).
the gain. This is lower than the 20kΩ
value used in the other S3 positions
so that the output at the wiper of S3b
is the same as for the slow and fast
settings when a sinewave is applied.
VR1 allows precise adjustment of
this calibration, providing a divide
by 4.6 to 1.8 range. VR2 is the offset
adjustment.
Error amplifier
If, after reading the circuit description so far, you are unclear about its
operation, do not despair. Let’s summarise what really happens. Op amp
IC4b, the error amplifier, is really the
The filter signal at the wiper of
S3b is monitored with error amplifier
IC4b. This has a gain of -100 (ie, it is
an inverting amplifier) and compares
the rectified signal from switch S3b
against the -20mV reference at the
non-inverting input, pin 3. IC4b’s
output drives pin 11 of IC2.
The -20mV reference is derived
from the 2.49V reference REF1 via
560kΩ and 4.7kΩ resistors. REF1 is
an LM336-2.5 preci
sion reference
diode which has facility for a small
amount of adjustment although it is
not used here.
REF1 is also used to provide a calibration offset for op amp IC3b. IC3b
attenuates the logarithmic DC control
voltage for IC2 to convert its nominal
30mV/dB calibration to 10mV/dB.
70 Silicon Chip
The big picture
CAPACITOR CODES
❏
❏
❏
❏
❏
❏
❏
❏
❏
❏
❏
Value
0.56µF
0.22µF
0.18µF
0.15µF
.047µF
.0027µF
.001µF
100pF
33pF
12pF
IEC
EIA
560n
564
220n
224
180n
184
150n
154
47n
473
2n7
272
1n
102
100p
101
33p 33
12p 12
heart of the circuit. It continually adjusts the control voltage fed to IC2 so
that the negative DC voltage fed from
the wiper of S3b to its pin 2 is always
very close to the -20mV at its pin 3.
In fact, VCA IC2 does not really
operate as an amplifier for most of the
time. For example, when a signal of
120dBSPL is fed to the microphone,
the output of IC1a and IC1b is close to
clipping; ie, around 14V peak-to-peak
or 5V RMS. This is heavily attenuated
by IC2 so that around 30mV RMS (see
Fig.4) is applied to the input of the
precision rectifier, IC3a.
Actually, it is only for signals of
around 20mV or less from IC1b that
the circuit involving IC2 has any gain;
the rest of the time it is attenuating
and the actual degree of attenuation
depends on the size of the signal coming from IC1a. Typically, the control
voltage delivered by IC4b ranges from
about +3V, corresponding to maximum
attenuation in this circuit, to about
-1V, corresponding to maximum gain.
Hence, IC4b makes sure that its two
inputs are very similar, and in doing
so, it produces a control voltage which
happens to be 30mV/dB. This is then
further attenuated by IC3b to produce
an output of 10mV/dB which can be
read out as a measure of the sound
pressure level. Looked at this way, the
output voltage read by the external
multimeter is almost just a byproduct
of the overall circuit operation.
The assembled PC board
is secured to the base of
the case using four small
self-tapping screws.
Battery supply
Two 9V batteries in series provide an
18V supply. The 18V is divided using
two series connected 10kΩ resistors,
to produce a 0V reference and this is
buffered by op amp IC5. IC5’s output
feeds a 100Ω resistor and two 100µF
capacitors. These decouple the op
amp’s output and ensure that it has
a very low output impedance at all
frequencies of interest. The result is a
dual-tracking supply which is nominally ±9V.
Now why go to all that trouble when
we could have used the midpoint of
the two 9V batteries to do the same
thing? The reason is that there is more
current drain from the negative rail
in this circuit and so the negative 9V
battery would normally be discharged
faster than the positive 9V battery.
This would be a problem because the
circuit require more negative output
swing.
By using the op amp split supply
RESISTOR COLOUR CODES
❏
No.
❏ 1
❏ 1
❏ 1
❏ 3
❏ 1
❏ 1
❏ 1
❏ 1
❏ 6
❏ 1
❏ 2
❏ 9
❏ 1
❏ 1
❏ 1
❏ 2
❏ 2
❏ 2
❏ 1
Value
2.2MΩ
560kΩ
180kΩ
100kΩ
68kΩ
33kΩ
22kΩ
24kΩ
20kΩ
18kΩ
12kΩ
10kΩ
8.2kΩ
6.8kΩ
5.6kΩ
4.7kΩ
3.9kΩ
150Ω
100Ω
4-Band Code (1%)
red red green brown
green blue yellow brown
brown grey yellow brown
brown black yellow brown
blue grey orange brown
orange orange orange brown
red red orange brown
red yellow orange brown
red black orange brown
brown grey orange brown
brown red orange brown
brown black orange brown
grey red red brown
blue grey red brown
green blue red brown
yellow violet red brown
orange white red brown
brown green brown brown
brown black brown brown
5-Band Code (1%)
red red black yellow brown
green blue black orange brown
brown grey black orange brown
brown black black orange brown
blue grey black red brown
orange orange black red brown
red red black red brown
red yellow black red brown
red black black red brown
brown grey black red brown
brown red black red brown
brown black black red brown
grey red black brown brown
blue grey black brown brown
green blue black brown brown
yellow violet black brown brown
orange white black brown brown
brown green black black brown
brown black black black brown
December 1996 71
shown in Fig.5, to allow room for
the battery to lie on top of the PC
board. For the same reason, the
.001µF capacitor near IC2, the
0.18µF capacitor near VR2 and
the 100pF capacitor near VR1
should be inserted so that they
lie flat on the board.
The electrolytic capacitors
must be oriented as shown.
Insert and solder LED1 at the
end of its leads to allow it to
protrude through the front panel
when assembled. Insert trimpots
VR1 and VR2 and cut the ‘A’
PC stakes slightly higher than
the trimpot height. This will
prevent the batteries pressing
on the trimpots and altering the
set values.
Now fit the assembled PC
This battery holder was made by soldering several pieces of double-sided PC board
board into the base of the case
material together. The three smaller pieces fit into the integral slots moulded into the
and secure it with four small
lid of the plastic case.
self-tapping screws. Wire up the
9V battery clips and multimeter
method, the current drain from the
assembly of components. Begin by leads as shown. Prepare the two wires
two 9V batteries must always be the
inserting the two links and all the for switch S1.
same and the battery life will be exresis
tors. The accompanying table
Fit the Dynamark adhesive label to
tended. For the same reason, LED1 is
can be used as a guide for the resistor
the lid of the case and drill and file
connected across the full 18V supply
colour codes. Alternatively, use your
out the holes for the switches and
via a 10kΩ resistor.
multimeter to check each resistor as
LED. Attach S1 with the screws and
it is installed.
connect its wiring.
Construction
Next, insert and solder in the PC
The rear end panel can be drilled
stakes. These are located at all external
The SILICON CHIP Sound Level Meto accept a small grommet. Pass the
wiring points, the ‘A’ positions and for
ter is housed in a plastic case measmultimeter leads through the gromuring 150 x 80 x 30mm and uses a PC the eight switch terminal locations for metted hole and attach the banana
S2 and S3.
board coded 04312961 and measuring
plugs to it.
67 x 120mm. The microphone is held
Next, the ICs can be inserted and
Microphone mounting
inside a copper tube which protrudes soldered in. Take care with the orifrom the front of the case. This is done entation of each and make sure that
An 80mm length of 12.7mm copper
to prevent sound reflections from the
IC5 is the TL071 (or LF351). Diodes tube is soldered to a 12 x 30mm piece
case from upsetting the reading.
D1-D4 can now be inserted, taking care
of 1mm thick copper sheet (or PC
to ensure that they are also correctly board). The copper sheet becomes a
Fig.5 shows the component layout
oriented. Switches S2 and S3 can be flange for easy attachment to the front
for the PC board. You can start construction by checking the PC board mounted by soldering their pins to the end piece of the box. Drill holes in
top of the PC stakes.
for any shorts or breaks in the copthe flange and front end plate to allow
per tracks. Repair any faults before
REF1 is mounted on its side as it to be secured with two screws and
Fig.6: this is the set up
used for calibrating
the Sound Level Meter.
It relies on using a
speaker of known
sensitivity. Most
manufacturers quote
sensitivity figures for
their loudspeakers.
72 Silicon Chip
nuts. Also drill a hole central to the
flange and end plate for the shielded
cable to pass through the tube. The
tube and flange can be painted if
desired.
Connect the microphone using
shielded cable and attach some heat
shrink tubing around its body. Shrink
the tubing down with a hot air gun
and insert the wire and microphone
into the tube.
Leave the microphone flush with
the end of the tube. The flange can be
attached to the end plate of the case
with the screws and nuts. The shielded cable is clamped with a solder lug
attached to one of the screws.
The batteries are held in place on
the lid of the case using three pieces
of double-sided PC board (73 x 5mm)
which are inserted in the integral
slots. Two pieces of double sided PC
board, measuring 30 x 15mm, are soldered in place between the transverse
pieces so that they provide a snug fit
for the battery and clip assemblies.
Check that the lid will fit onto the
base of the case.
Voltage checks
Switch on and connect the red
multimeter lead from the Sound Level Meter to the common input of the
multimeter and then measure voltages
on the circuit with the other lead of the
multimeter. Check that there is +9V at
pin 8 of IC1, IC3 and IC4; at pin 7 of
IC5; and at pin 2 of IC2. There should
be -9V at pin 4 of IC1, IC3, IC4 & IC5
and at pins 10 & 16 of IC2.
REF1 should have -2.49V at its
anode and pin 3 of IC4b should be
-20mV. LED1 should also be lit.
Connect both output leads from the
sound level meter to the multimeter.
Performance
‘A’ response .......................................... -18dB at 100Hz, -10dB at 20kHz (see Fig.1)
‘C’ response ......................................... -5dB at 20Hz, -13dB at 20kHz (see Fig.1)
Overall flat response (input
versus multimeter reading) .................. -3dB at 28Hz and 50kHz
Log conversion accuracy at
multimeter output ................................ <0.5dB over a 100dB range from 0.550V
RMS to 5.5µV input level
Temperature stability ............................ <10mV (1dB) change per 30°C
Slow response time constant ............... 9.4 seconds
Fast response time constant ................ 2 seconds
Peak response ...................................... 1.5ms attack; 120ms decay
Power ................................................... 12-18V at 32mA
Microphone Performance (ECM-60P A version)
Sensitivity �������������������������������������������� -56dB ±3dB with respect to 0dB+1V/µbar <at>
1kHz
Microphone response .......................... within ±3dB from 50Hz to 3kHz and ±6dB
from 3kHz to 8kHz. Above 8kHz and below
50Hz unspecified.
Maximum SPL ..................................... 120dB
Note: filter responses measured at VCA output with control input (pin 11) grounded.
it is greater than 400mV, rotate VR1
slightly clockwise.
Conversely, if the multimeter reading is less than 400mV, rotate VR1
slightly anticlockwise. Now measure
the difference again with the 0dB/
-60dB switch. You will note that the
reading will now not be 1V for the 0dB
setting. However, what we are look-
ing for is a 600mV change between
the 0dB and -60dB pink noise level
settings (ie, 10mV per dB). After some
repeat adjustments of VR1 it should
be possible to obtain close to 600mV
variation between the 0dB and -60dB
settings.
Calibration now only requires the
offset adjustment trimpot VR2 to be
Calibration
Calibration is done in two steps and
a pink noise source is required for both
steps. We will describe a suitable pink
noise source in next month’s issue of
SILICON CHIP and we assume that you
will also build that or have access to
an equivalent source.
First, connect the pink noise source
to the electret microphone input of
the sound level meter. Select 0dB on
the pink noise source (equivalent to
60mV RMS) and adjust trimpot VR2
for a reading on the multimeter of
1V DC. Now switch to -60dB on the
pink noise source and check that
the multimeter reading is 400mV. If
Fig.7: check your etched PC board against this full-size artwork before installing
any of the parts.
December 1996 73
PARTS LIST
1 plastic case, 150 x 80 x 30mm
1 PC board, code 04312961, 67 x
120mm
1 front panel label, 75 x 144mm
1 ECM-60P type A electret
microphone (sens. -56dB with
respect to 1V/1µbar at 1kHz)
3 pieces of double sided PC
board, 73 x 5mm
2 pieces of double sided PC
board, 30 x 15mm
1 DPDT slider switch and mounting screws (S1)
2 DP3P slider switches (S2,S3)
1 50kΩ horizontal trimpot (VR1)
1 100kΩ horizontal trimpot (VR2)
2 9V battery snaps
2 9V batteries
1 black banana plug
1 red banana plug
1 250mm length of shielded cable
1 500mm length of black hookup
wire
1 500mm length of red hookup
wire
1 50mm length of 0.8mm tinned
copper wire
30 PC stakes
2 3mm x 10 screws and nuts
4 small self-tapping screws (to
secure PC board)
1 solder lug
1 small rubber grommet
1 small cable tie
1 SSM2018P voltage controlled
amplifier (IC2)
1 TL071, LF351 op amp (IC5)
4 1N914 signal diodes (D1-D4)
1 LM336-2.5 2.5V reference
(REF1)
1 3mm red LED (LED1)
Semiconductors
3 LM833 dual op amps
(IC1,IC3,IC4)
Miscellaneous
12mm diameter heatshrink tubing,
solder.
Capacitors
1 470µF 16VW PC electrolytic
5 100µF 25VW PC electrolytic
1 47µF 16VW PC electrolytic
3 10µF 16VW PC electrolytic
1 0.56µF MKT polyester
1 0.22µF MKT polyester
1 0.18µF MKT polyester
2 0.15µF MKT polyester
1 .047µF MKT polyester
2 .0027µF MKT polyester
1 .001µF MKT polyester
1 100pF ceramic
1 33pF ceramic
1 12pF ceramic
Resistors (0.25W 1%)
1 2.2MΩ
2 12kΩ
1 560kΩ
9 10kΩ
1 180kΩ
1 8.2kΩ
3 100kΩ
1 6.8kΩ
1 68kΩ
1 5.6kΩ
1 33kΩ
2 4.7kΩ
1 24kΩ
2 3.9kΩ
1 22kΩ
2 150Ω
6 20kΩ
1 100Ω
1 18kΩ
set. This is done using the setup shown
in Fig.6.
You will need an amplifier, the pink
noise source and a woofer or tweeter
with known sensitivity. All manufacturers of loudspeakers provide a sensitivity rating for their units and these
are specified as a dBSPL when driven
at 1W and at 1m on axis. Note that if
you use a tweeter, the manufacturer’s
specified filter should be used when
making the measurement.
For example, a loudspeaker may be
rated at 88dB when mounted on a baffle and driven from a 2.828V AC source
at a distance of 1m. The loudspeaker
impedance is 8Ω. Note that 2.828V
into 8Ω is equivalent to 1W.
Use your multimeter to measure the
voltage applied to the loudspeaker and
set the amplifier’s volume control to
deliver 2.828V AC for an 8Ω system
and 2V AC for a 4Ω speaker. Be sure
to set your amplifier’s tone controls to
the flat settings (ie, centred or switched
off) and make sure that the loudness
switch is off.
Now connect the multimeter to the
sound level meter (with the unweight
ed and slow settings selected) and
with the microphone at 1-metre and
on axis to the speaker. Adjust trimpot VR2 to obtain the loudspeaker
sensitivity. For our 88dB example,
the multimeter should read 0.88V or
880mV DC.
Alternatively, if you have a calibrat
ed sound level meter, adjust VR2 for
the same readings. Make sure that both
sound level meters are set with the
same filtering and responses.
SC
74 Silicon Chip
(10mV/dB)
CONNECT TO
MULTIMETER
FILTER
C-WEIGHTING
A-WEIGHTING
UNWEIGHTED
+
SOUND
LEVEL
METER
RESPONSE
SLOW
FAST
PEAK
+
OFF
+
ON
+
Fig.8: this is the
actual size artwork
for the front panel.
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