Pink Noise Generator for Audio
Testing
By Rod Elliott - ESP
(Original Design)
(WebEE Note: Visit
Elliot Sound Products, www.sound.au.com,
for a number of Excellent Audio Circuits like this one)
For audio testing, a pink noise
source is an invaluable tool. It is essentially a flat frequency
response noise source, and will quickly show any anomolies in
speaker systems, room acoustics and crossover networks.
White noise (the sound you hear
when a TV is tuned to a non-existent station) has a frequency
characteristic which raises the power level by 3dB with each
increasing octave, and is not suitable for response testing (and
will probably blow your tweeters as well). By combining a 3dB /
octave filter and a white noise source, we can get a very good
approximation to "perfect" pink noise, where the power
in the octave (for example) 40 to 80Hz is exactly the same as in
the octave 10kHz to 20kHz.
Figure 1 shows the circuit diagram
for the filter, which uses the 1458 dual op-amp for economy. There
is no point using a low-noise device in something which is
specifically designed to make noise, so this op-amp is fine for
the purpose.
Figure 1 - Pink Noise Generator
for Audio Testing
The BC548 transistor is connected
so its emitter-base junction is reverse biased, which creates a
nice noisy zener diode. With the values shown, the average noise
output is about 30mV (broadband). The transistor zener voltage is
a bit iffy, mine runs at about 9V, but it could be anywhere from
5V up to 10V. The first op-amp stage acts as an amplifier /
buffer, providing a very high input impedance (so as not to load
the noise source), and having a gain of 11 (20.8dB). The positive
battery supply connects to pin 8 of the op-amp, and the negative
to pin 4 - don't mix up the battery polarity, or the op-amp will
die.
The second stage is a 3dB / octave
filter, which is quite linear across the frequency band 20Hz to
20kHz. This converts the white noise into pink noise, having equal
energy in all 10 octaves of the audio band.
Because of the comparatively high
zener voltage of the transistor, the supply voltage needs to be
somewhat higher - 2 standard size 9V alkaline batteries in series
(18V) should run the unit for far longer than you will ever want
to listen to it. Because of the limited capacity of the 9V
batteries, no indicator LED has been included, as this would draw
more current than the rest of the circuit. The power switch must
be a Double Pole, Single Throw (DPST) type, as both batteries must
be disconnected. The centre-tap of the batteries is the earth (or
ground) for the unit.
The entire circuit can be laid out
on a piece of prototype board, and mounted in a suitable plastic
or metal box. No special precautions are needed, other than
ensuring that polarised components (transistor, op-amp, and
electrolytic capacitors) are connected the right way 'round.
Values of components are not critical, so standard tolerance
components should be fine throughout. The use of 1% metal film
resistors to keep noise to a minimum is not required in this
circuit! The transistor can actually be any small signal type you
have handy, and so can the dual opamp (or a pair of single opamps
can be used - note that their pinouts are completely different).
If you have an oscilloscope or can
get access to one, check that the noise output is not clipping
(you won't be able to hear it, but if it clips, the energy
spectrum will be modified). There is no easy way to check without
a 'scope, and the noise output from transistors used in this way
tends to vary somewhat. If clipping is observed (or you suspect
it), increase the value of R3 or R4 (both 10k). Doubling the value
(of one or the other - not both) will reduce the output by half.
There are digital "pseudeo-random" noise generators
available, but I don't like them, since they have a cycle which
eventually repeats and this is very audible. By contrast, the unit
described is completely random, as only analogue can be.
Using A Noise Generator
Connect the generator to your
preamp, and slowly advance the level control until the sound level
is at about the level of normal speech (about 65dB). Carefully
listen for any "tonality" in the sound, such as a low
hum, or a point where the signal seems to disappear (sometimes
referred to as a "suckout"), or anything which does not
sound like pure noise. This will probably take a little practice -
if you have a graphic equaliser handy, this is a great way to
introduce peaks and dips to hear what they sound like.
Try listening through a good set of
headphones, and compare the result with the speakers and room
acoustics, you might be surprised at the result. I once read a
story where an engineer was trying to find out where the hum in
his noise generator was coming from. It turned out that the noise
generator had no hum at all, but he was hearing the bass resonance
from a badly designed loudspeaker - you can get results from these
little guys!
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