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You can use this Pink Noise
Source as an aid to cali
brating the Sound Level Meter
described last month. It can
also be used as a general
purpose signal for setting the
balance between loudspeakers
in a multichannel (2, 4 or
more channels) system and for
PA adjustments.
By JOHN CLARKE
BUILD THIS
While noise is usually considered a nuisance, it
can be useful in some cases. In audio applications
it provides us with a signal which covers the entire
audible spectrum. This means that there is every
conceivable frequency from 20Hz up to 20kHz, all
in the one signal.
Armed with this type of signal we can obtain
frequency response measurements and a wideband sound level output for loudspeakers. Also it
provides a standard sound for subjective listening
tests. With an analyser and equaliser we can also
adjust the frequency levels from a loudspeaker in
a particular room so that it provides a flat response
across the audible spectrum.
All of these measurements assume that the noise
source has a flat frequency response or an equal
energy per octave. This is called “pink” noise.
The energy from 20Hz to 40Hz must be the same
as that from 10kHz to 20kHz even though there is
40 Silicon Chip
Pink Noise
Source
For sound level meter
calibration & signal balancing
AUDIO PRECISION SCNOISE AMPL(dBr) vs BPBR(Hz)
20.000
29 AUG 96 14:15:39
•
•
•
•
15.000
10.000
Main Features
Pink noise signal output
Battery operated
0dB and -60dB levels
Power-on LED
5.0000
0.0
-5.000
-10.00
-15.00
-20.00
20
100
1k
10k
20k
Fig.1: the spectrum (signal output versus frequency) of the Pink Noise Source.
Since the noise source is random, a second response test would no doubt reveal
a slightly different result, with perhaps dips in response where slight peaks are
shown and vice versa.
only a 20Hz difference in frequency for
the lowest octave and a 10kHz range
for the upper octave. Fig.1 shows the
spectrum (ie, signal output versus
frequency) of the Pink Noise Source
featured in this article.
By contrast, the noise from electronic circuits is “white”. It has a 3dB
rise in output per octave of frequency
since it has equal energy per constant
bandwidth. So the octave band from
20Hz to 10.02kHz will have the same
energy level as the octave between
10kHz and 20kHz.
Rose-coloured filter
To convert white noise to pink
noise we need a filter which has a
3dB/octave or 10dB/decade rolloff.
This is a little tricky since a normal
single pole low pass filter will roll off
at 6dB/octave (or 20dB per decade).
A “pink” filter is achieved by rolling
the signal off in four discrete steps,
introducing fur
ther filtering as the
frequency rises.
Fig.2 shows the pink noise circuit.
It uses a transistor noise source, two
op amps for amplification and some
passive filtering.
An NPN transistor, Q1, is connected
for reverse breakdown between the
emitter and base, with current limiting
provided by the 180kΩ resistor from
base to ground. This provides a good
white noise source but it only produces a low signal level.
Op amp IC1a amplifies this noise
by a factor of 101. IC1a is AC-coupled
and biased to the 4.5V half supply
rail to provide a symmetrical swing
at its output, pin 1. The 0.27µF input
capacitor and bias resistor roll off the
response below 0.6Hz. Similarly, the
2.2kΩ resistor and 100µF capacitor
in the feedback path at pin 2 roll off
response below 0.7Hz. High frequency
rolloff above 153kHz is provided by
the 4.7pF capacitor across the 220kΩ
resistor.
Following pin 1 of IC1a is a passive
RC filter to roll off the frequency response at 3dB per octave. This filter
Fig.2: the pink noise circuit uses a transistor noise source, two op amps for
amplification and some passive filtering.
January 1997 41
220k
Fig.3 (left): the component
layout and wiring details.
Note that the two switches
are mounted on PC stakes
and be sure to mount all
polarised components with
the correct orientation.
Capacitor Codes
❏
❏
❏
❏
❏
❏
Fig.4: check your etched PC board against this full-size artwork before
installing any of the parts.
Performance
Output levels ..................................60mV RMS at 0dB; 60µV at -60dB
Maximum output load .....................1kΩ (for <1dB error in 60dB attenuator)
Frequency spectrum ......................<0.25dB 20Hz to 20kHz (see Fig.1)
Power supply ..................................7.6 to 9V at 7mA
Value
0.27µF
.047µF
.033µF
10pF
4.7pF
IEC
270n
47n
33n
10p
4p7
is accurate to ±0.25dB from 10Hz
to 40kHz, assuming the use of close
tolerance capacitors. The spectrum
response shown in Fig.1 is that of the
prototype using normal 10% tolerance
capacitors.
Note that the signal levels shown in
Fig.1 are the actual levels at the instant
the measurement was taken. Since
the noise source is random, a second
response test would no doubt reveal a
slightly different result, with perhaps
dips in response where slight peaks
are shown and vice versa.
The pink noise output is AC-coupled into op amp IC1b which has a
gain of 46. This has a low and high
frequency response rolloff similar to
IC1a.
IC1b’s output is AC-coupled to
switch S2. Note that a non-polarised
Resistor Colour Codes
❏
❏
❏
❏
❏
❏
❏
❏
❏
❏
❏
❏
No.
2
2
1
2
2
1
1
3
1
1
1
42 Silicon Chip
Value
1MΩ
220kΩ
180kΩ
100kΩ
10kΩ
6.8kΩ
3kΩ
2.2kΩ
1kΩ
300Ω
100Ω
4-Band Code (1%)
brown black green brown
red red yellow brown
brown grey yellow brown
brown black yellow brown
brown black orange brown
blue grey red brown
orange black red brown
red red red brown
brown black red brown
orange black brown brown
brown black brown brown
EIA
274
473
333
10
4.7
5-Band Code (1%)
brown black black yellow brown
red red black orange brown
brown grey black orange brown
brown black black orange brown
brown black black red brown
blue grey black brown brown
orange black black brown brown
red red black brown brown
brown black black brown brown
orange black black black brown
brown black black black brown
NOISE OUT
0dB
+
-60dB
OFF
+
+
ON
PINK
NOISE SOURCE
Fig.5: this is an actual size artwork for
the front panel.
The construction is easy since all parts except for the RCA output socket are
mounted on the PC board.
(NP) capacitor is specified. This is
because the noise source is designed
to connect to the Sound Level Meter
which would reverse polarise a normal
electrolytic type. Switch S2 selects the
full output (0dB) or a divide by 1000
using the 100kΩ and 100Ω resistors
for a -60dB output.
The 4.5V half supply is derived
from a 10kΩ resistive divider which
is decoupled using a 100µF capacitor.
The power LED is driven via a 2.2kΩ
resistor while the whole supply is
decoupled using a 100µF capacitor.
Construction
The Pink Noise Source is housed
in a plastic case measuring 130 x 67
x 41mm. The circuitry fits onto a PC
board coded 04312962 and measuring
104 x 60mm. The wiring details are
shown in Fig.3.
Begin construction by checking the
PC board for defects. This done, install
the resistors and install PC stakes at
the switch positions. The PC stakes
are required to allow the switches to
be mounted above the PC board.
The capacitors can be mounted next,
while ensuring correct orientation of
the electrolytics. The 10µF NP capacitor can be mounted either way around.
LED1 is mounted with its leads at full
length, so that it can protrude through
the front panel lid. Splay the leads
slightly to give the LED some vertical
adjustment, without one lead shorting
to the other.
Next, insert transistor Q1 and IC1.
Attach the battery holder using small
self-tapping screws from the underside
of the PC board. The toggle switches
can be soldered in place on top of the
PC stakes.
Attach the Dynamark adhesive label
on the lid of the case and drill out the
holes for the switches, LED bezel and
January 1997 43
PARTS LIST
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1 plastic case, 130 x 67 x 41mm
1 PC board, code 04312962,
104 x 60mm
1 self-adhesive label, 61 x
123mm
2 SPDT toggle switches (S1,S2)
1 panel mount RCA socket
1 9V battery holder
1 9V battery
1 3mm LED bezel
8 PC stakes
3 small self-tappers for the battery holder
Semiconductors
1 TL072 dual op amp (IC1)
1 BC548 PNP transistor (Q1)
1 3mm red LED (LED1)
Capacitors
4 100µF 16VW PC electrolytic
1 10µF NP PC electrolytic
1 1µF 16VW PC electrolytic
3 0.27µF MKT polyester
2 .047µF MKT polyester
1 .033µF MKT polyester
1 10pF ceramic
1 4.7pF ceramic
Resistors (0.25W 1%)
2 1MΩ
1 3kΩ
2 220kΩ
3 2.2kΩ
1 180kΩ
1 1kΩ
2 100kΩ
1 300Ω
2 10kΩ
1 100Ω
1 6.8kΩ
Card No.
corner mounting locations. Also drill
a hole in the end of the case for the
RCA socket. Attach the socket and
clip the PC board in place against the
integral side pillars of the box. Wire
up the RCA socket as shown in Fig.3.
Finally, insert the battery and attach
the lid with the LED bezel in place.
Take care to ensure that the LED
protrudes through the bezel before
tightening the case screws.
Signature_______________________________ Card expiry date______/______
Testing
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You can test the unit by connecting
the output to an amplifier and speaker.
Apply power and listen to the noise
which should occur after several seconds. Alternatively, look at the signal
on an oscillo
scope. A multimeter
should give an AC reading of around
60mV on the 0dB range and 0V on the
SC
-60dB position of S2.
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