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Measure
sound and
vibration
way below
human
hearing
by
Allan Linton-Smith
and Ross Tester
Infrasound
Detector
Photo: Harvey McDaniel
Wikipedia
Are wind turbines making you sick? Is building vibration making
you nauseous? Or do you just want to measure infrasound in your
environment? You don’t need to spend thousands of dollars to do it
properly; just build our low-cost but accurate Infrasound Detector.
T
here’s been a lot of press lately
about infrasound, particularly
as it applies to wind turbines.
But until now, you’ve needed tens
of thousands of dollars worth of test
equipment to detect and measure it.
Our Infrasound Detector can be built
for less than a hundred dollars yet will
give very accurate results. You can
either read the sound pressure directly
or store and analyse readings on your
computer!
So what exactly is infrasound?
It can defined as sound below the
range of normal human hearing. That’s
30 Silicon Chip
generally reckoned to be below about
20Hz.
Below that, you can perhaps sense
or even “feel” sound but you can’t
actually hear it.
In practice, infrasound involves frequencies from about 20Hz to 0.5Hz but
some natural phenomena can cause
infrasound down to the millihertz
(.0001Hz!) region.
When people complain about illeffects from infrasound (and there
are legions of those reports), many
acoustic consultants have taken the
attitude that “if you can’t hear it, it
can’t be doing you harm”.
We disagree – and the publisher of
this magazine even wrote an editorial
on the subject back in February 2010.
Reported human reaction to infrasound from wind turbines is varied
but some of the reports associate
infrasound with a general feeling
of malaise, nausea, vertigo, blurred
vision, memory problems, tinnitus,
anxiety, uneasiness, extreme sorrow,
nervous feelings of revulsion or fear,
chills down the spine and feelings of
pressure on the chest.
Others have reported headaches and
migraines, major sleep disorders and
even self-harm tendencies.
siliconchip.com.au
Some researchers have even given it
a label: wind turbine syndrome.
Wind turbines are one example but
you’ll also find infrasound caused by
traffic noise, heavy surf, engines/motors (especially things like compressors), building vibrations being excited
by wind, machinery and so on.
Large animals such as whales,
crocodiles, alligators, elephants and
emus communicate with infrasound
so if you want to record amorous
crocodiles, our Infrasound Detector
is a good way to go about it (from a
safe location!).
Other source of infrasonics are
heavy artillery, the calving of icebergs
from glaciers and earthquakes.
In fact, there is a theory that the
buildup of stresses with the earth’s
crust before a major earthquake causes
infrasound – which could explain why
birds and some other animals appear
to have some warning of an imminent
quake.
Want another example? The very act
of opening or closing a door produces
infrasound waves. But that is transitory – you don’t normally stand there
for hours opening and closing doors!
Whatever the infrasonic phenomenon you want to investigate, our
Infrasound Detector is an effective and
low-cost way to do it and it compares
more than favourably with commercially available equipment.
While it’s economic, it’s also acFig.1: the testing unit is based on a modified PreCHAMP preamplifier which detects sound
via the electret microphone, then removes all but signals below 20Hz. This signal can then be
analysed by a computer running “Fatpigdog” software, or it can be fed to a modified CHAMP
amplifier which drives a multimeter in its AC range to deliver readings of sound pressure levels.
'PRE-CHAMP’ PREAMP
(MODIFIED)
*10k RESISTOR
ADDED TO
POWER
ELECTRET MIC
22k
100k
10k*
Q1
BC548
SHIELDED
LEAD
B
1000F
VR1
100k
C
8.2k
150k
1uF
MKT
470F
16V
TO PC
SOUND
CARD
1k
VR2
10k
*SEE TEXT
SC
siliconchip.com.au
220F
16V
4700F
8
16V
1
IC1
LM386N
(SEE
TEXT)
2
5
10F
10V
12V
LED
100nF
A
68
2.2k
INFRASOUND TESTING UNIT
9V
BATTERY
7
4
K
10
LED
COMPONENTS IN RED ARE CHANGED/ADDED
2011
6
3
120pF
100
470F
16V
POWER
100F
16V
B
S2*
39k
S1
ANALYSER
GAIN
100F
Q2
BC558 E 16V
C
E
ELECTRET
MICROPHONE
0.778
'CHAMP' AMPLIFIER
(MODIFIED)
2.2k
10F
16V
JAYCAR
QM1327
MULTIMETER ON
FREQUENCY
RANGE
K
A
BC548, BC558
ELECTRET
B
OUTPUT
EARTH
E
C
March 2013 31
.
curate and reliable – we believe it can
be just as accurate and reliable as commercial gear.
In fact, while our unit should cost
well under $100 to build and is easy to
put together, it took hundreds of hours
to develop and test. That is because
infrasound sweeps can take hours to
settle, measure and average – and some
very specialised and expensive equipment was required to design and test it.
If you wanted to buy that commercial
equipment yourself, you’d have little
change from $30,000!
We also had to develop a method
for testing and calibrating high levels
of infrasound without upsetting the
neighbours!
How it works
The output from a wide-range
electret microphone is fed to a verylow-frequency bandpass filter. The
infrasound signal is amplified and fed
to a “virtual” spectrum analyser which
then plots the amplitude of the infrasound signal on the vertical (Y) scale
versus frequency on the horizontal (x)
scale using a principal known as Fast
Fourier Transform (FFT).
A computer can then be used to
analyse the signal and/or a direct frequency readout can be obtained if used
in the field.
Our Infrasound Detector is built into a small diecast box, with an old
microphone shield attached to the front. Inside this shield is a low-cost electret
mic insert. The terminals at left are the output to a frequency counter (or in
our case, a budget multimeter) while a socket is provided on the right side for
output to a PC sound card. Suitable analysis software is quite cheap.
Specifications
Microphone frequency response G-weighted:........... ±2.0dB corrected (0.5Hz-26Hz)
Microphone frequency response C-weighted:........... ±2.0dB (10Hz-20kHz)
Microphone intermodulation distortion:......................... 0.8% <at> 100dB SPL
Preamplifier frequency response: ....................................... ±0.2dB (0.5Hz-20kHz)
Power amplifier frequency response: .............................. ±0.2dB (0.5Hz-20kHz)
Power amplifier output (before clipping): .................... 200mW into 8Ω
Frequency response of virtual instrument: ................. ±0.4dB (0.5Hz-20kHz)
Overall measuring accuracy –
Without calibration table:................................................. ±15dB (20Hz-20kHz)
Using calibration table: ..................................................... ±1.0dB (2Hz-20kHz)
THD+N preamplifier: ........................................................................ 0.102% at 1kHz (5Hz-22kHz)
THD+N power amplifier:................................................................. 0.40% at 1kHz (5Hz-22kHz); 250mW
Preamp input maximum: .......................................50mV
Preamp input minimum: .............................................................. 1.0mV
Power amp input maximum:...................................................... 500mV
Power amp input minimum:....................................................... 30mV
Preamp phase distortion:.......................................±6.35° (below 200Hz)
Preamp intermodulation distortion:.......................0.095% (88mV output 70Hz/7kHz)
Preamp S/N ratio:...................................................-107dBV (10Hz-80kHz ref 630Hz 25mV)
32 Silicon Chip
Good grief: The CHAMP is
back!
After constructing many circuits
which offered good theoretical performance we discovered that the good
old PreCHAMP preamplifier, combined
with the equally elderly CHAMP audio
amplifier, could be easily modified to
do the job admirably.
Yes, we know, we said only two
months ago that our new CHAMPION
amplifier would kill off the PreCHAMP
and CHAMP but there’s a good reason
for resurrecting it here: low quiescent
current.
The PreCHAMP and CHAMP draw
only about 4mA each on idle, so prolonged operation (which you’ll need
for field checks) is quite practical using
only a 9V battery.
By comparison, the CHAMPION
draws up to 60mA so your 9V battery
wouldn’t last long at all!
If you built the CHAMPION project
(based on the Panasonic AN7511), you
could use it for infrasound with only a
few modifications but you’ll probably
need to use it with an external supply.
siliconchip.com.au
As used here, the modified PreCHAMP now has much improved frequency response; within +/-0.2dB from
2Hz – 20kHz. The modified CHAMP
also gives a flat frequency response at
around 0.25-0.5W – so you can feed any
oscilloscope or low frequency counter.
Optional CHAMP
The CHAMP is optional – it has been
included so that you can take quick
measurements in the field.
The PreCHAMP is set up as a bandpass filter and high gain amplifier
which is approximately G-weighted,
ie, its centre frequency is around 10Hz
with -3dB points at 500mHz (0.5Hz)
and 26Hz.
A selector switch is provided for
switching to “C” weighting (ie, flat
response) so that the unit can easily be
calibrated at 1kHz.
The infrasound signal from the PreCHAMP is fed to the CHAMP amplifier which has been modified to give a
flat frequency response from 0.5Hz to
20kHz and is set at high gain so that the
signal output to a frequency counter is
over 130mV at 1Hz.
Electret Microphones
The electret microphone is pretty inefficient at frequencies below 25Hz, hence the very high amplification.
There are lots of electret microphone inserts available
but we are specifying a particular Jaycar model (Cat AM4011) because we found it to be a very good match for
this project. However, you can see from the graphs below
that even these specific Jaycar mics are not all the same –
some are more sensitive than others due to manufacturing
variations – so you may need to buy a few to experiment.
Frequency counter?
Whoops! Haven’t got one of those?
That little problem is solved very
cheaply with a multimeter – specifically the Jaycar QM1327 auto-ranging
multimeter, which can read read down
to 0.1Hz and sells for only $34.95.
While its specs state it needs a minimum of 3V RMS AC before it will show
a frequency reading, we found that it far
exceeded its specification and 130mV
was sufficient. Few frequency counters
go below 10Hz so the Jaycar meter represents good value in this application.
If you use the CHAMP together with
the Jaycar multimeter, then you will
be able to determine SPL (the sound
pressure level in dB) by switching the
DMM to AC volts. This will give an
approximate SPL in dB (decibels) as
described.
For signals below 0.5Hz this approach will not be accurate but this will
be more than sufficient for the majority
of applications.
Because they have flying pigtails changing them is a pretty easy soldering task.
Each electret will need to be calibrated as described below in the “Calibration”
section to enable you to assess sound pressure level (SPL).
By the way, we averaged the frequency response of several of the Jaycar electret
microphones combined with the Pre-CHAMP and compared them with an accurately
calibrated Bruel & Kjaer microphone/preamp (expensive!) – and found that the Jaycar
electret was actually better at infrasound frequencies!
Fatpigdog again!
The direct readout is very handy in
the field but if you want to do some
real analysis, you’ll need a computer
and suitable software.
Readers may remember “fatpigdog”
from our feature article on measuring
siliconchip.com.au
March 2013 33
We used this sweep to show that 1Hz was easily detectable with a resolution of 0.5Hz. By correction, the sound source is
100dB. You need to be patient because the analyser sometimes sets the sweep time to 10 seconds automatically and you
have to wait before you can make adjustments.
Loudspeaker Frequency Response in
the December 2011 issue.
We’re using this software again but
it has since been updated considerably
(the latest version is 4.04) and has more
usable functions than the original version. You can purchase and download
the software for around $30 from www.
fatpigdog.com.
On their website, you will also find
various dedicated bench top spectrum
analysers for sale but the virtual instrument is about 99.9% cheaper!
Fatpigdog is fun to use, easy to manage and includes all sorts of extras
such as a waterfall display, spectrum
analysis to 22kHz, BMP capture and
much more.
The PreCHAMP output is simply
fed to the sound card input of your
computer.
You could feed the spectrum analyser from the ‘CHAMP’ output but
we don’t recommend this because
your computer soundcard is usually
set up for microphone-level inputs (ie,
Using the Agilent 35670A, the sweep gives the lower
response for the G-weighted PreCHAMP down to 0.1 Hz…
that’s 1 cycle every 10 seconds! -3dB points are 0.5Hz and
26Hz. Mains hum is not a problem at these frequencies!
34 Silicon Chip
millivolts not volts). Any large voltages
will usually result in clipping and
consequently the spectrum analyser
will show multiple peaks from the odd
harmonics.
The Jaycar multimeter is an option if
you wish to have a hand held detector
for quick infrasound detection without
having to set up a computer and adjust
the software.
It is fed from the Pre-CHAMP output via the 10k preset pot. You can
set the maximum output from the
Fatpigdog spectrum of a 15-inch speaker fed with 200W.
The resolution is set at 1Hz and the sweep time is one
second. You could actually feel the sound – and it was not
nice!
siliconchip.com.au
The Jaycar QM1327 Multimeter works fine as a frequency
meter and also an AC voltmeter. It’s simply held in place
on the back of the Infrasound Detector with self-adhesive
hook’n’loop tape (usually sold under the “Velcro” brand).
“CHAMP” by setting the preset fully
anti-clockwise.
The only other modification is the
addition of a 68Ω “dummy load” resistor which prevents the output capacitor
from building up a DC charge, which
would otherwise result in false readings. You could attach a loudspeaker
instead but you won’t hear much below
25Hz (and it will drain the battery more
quickly).
By changing the parameters on the
analyser – such as sweep time, start
and stop frequencies and resolution
bandwidth, you can save and print your
spectra for further analysis.
Furthermore, by setting the spectrum
analyser to “max hold” you will be able
to observe any infrasound which occurs
during an extended period of time.
Using the virtual spectrum analyser
requires some practice and patience
(just like a real benchtop spectrum analyser) but if you experiment, you will
learn to master it all fairly quickly. We’ll
have much more to say on this later.
This spectrum shows the maximum sound level for
suburban Pacific Highway traffic. The microphone is
a good 5-10 metres away from vehicles and there is
significant noise at 2Hz! Note also the peak at around
20Hz – probably from engines.
siliconchip.com.au
Construction
The “hardware” is built into an
aluminium diecast box (to minimise
noise) measuring 119 x 93.5 x 34mm
(eg, Jaycar HB5067). Inside this are the
PCBs for the modified PreCHAMP (and
CHAMP if you wish to use it) and a 9V
battery in suitable holder.
Layout is not particularly critical
but given the very high amplification
of the PreCHAMP/CHAMP combina-
You can set up the detector and leave it running for up to
an hour. We caught a distant thunder clap at 5Hz and a
calculated 84dB. The resolution was set at 1.0Hz and the
sweep time was one second. The maximum hold function
runs continuously and updates every second.
March 2013 35
9V BATTERY
1 F
120p
S2
CON1
ELECTRET
MIC
TO SPECTRUM
ANALYSER
PRE-CHAMP
9V BATTERY
HOLDER
OUTPUT
+V
IN
14970110
CS
GND
GND
NEW 39k RESISTOR
SOLDERED UNDER
PCB
VR1
14920110
INPUT
4700F 16V
220F
OUT
GND
CS
S1
S1
'CHAMP' AMPLIFIER
(MOUNTED SIDE-ON)
12V LED
(MOUNTED
ON CASE)
+V
GND
68
OUTPUT TO FREQ COUNTER
SPST switch to switch the larger capacitor in and out of circuit but the
arrangement shown (using a DPDT
switch) allows easy mounting of the
two external capacitors: they are simply
wired across the outside terminals and
the wires back to the PCB are wired to
the centre terminals.
Assuming you want to include the
“CHAMP” power amplifier, to provide
sufficient voltage to the Jaycar Frequency meter (multimeter), construct
it as per the kit instructions (or refer to
SILICON CHIP February 1994).
The modifications we have made
to give a flatter frequency response
involve changing two capacitors. You
will find that the 4,700µF capacitor is
large but fits neatly on the PCB. However, it is a little too tall and the finished
amplifier will have to be put on its side
so it can easily fit in the diecast box.
Now you can drill and mount all
the hardware on the diecast box using
the picture as a template and solder all
the wires up according to the diagram.
Fitting a tripod adaptor
K
A
To enable easy use in the field, we
wanted to be able to attach the unit
to a photographic tripod. So we fitted
Fig.2 (above): component layout is not
critical but this diagram should give
you a guide. Both the PreCHAMP and
CHAMP PCBs are held in place with
double-sided foam pads. The photo at
right shows the same internal view,
together with the T-nut tripod adaptor
we fitted to the end of the diecast case.
tion (about 4000 times), outputs should
be kept relatively clear of inputs, as is
normal practice for an amplifier.
Start by constructing the Pre-CHAMP
pre-amplifier as per the instructions
given with a kit (or refer to SILICON
CHIP July 1994).
See Figs.1&2 for the modifications
required – you will only need to change
the values of three capacitors and these
will easily fit on the PCB.
The 39kΩ resistor should be soldered
to the underside of the board input or
across the input pins.
To the two holes on the board marked
“1n5” solder two leads and connect
these leads to the two central pins of
DPDT switch S2.Then solder the 1uF
capacitor to one side and the 120pf
capacitor to the other (see photo).
Then run leads to CON3 and VR2 as
shown in the wiring diagram.
We could have used just a simple
36 Silicon Chip
siliconchip.com.au
As it has a 1/4-inch Whitworth
internal thread (same as most tripods)
we used one of these furniture T-nuts
from a hardware store, flattened out
the points, and drilled the box to suit.
Then we glued it in place with some
2-part epoxy.
our box with a 1/4-inch threaded bush
(Whitworth thread; standard on most
tripods/cameras/etc).
In fact, we used a “T-nut” fitting
intended for furniture and shelf hardware (pictured) which has an internal
1/4-inch Whitworth thread. It had four
punched points intended to help it grip
timber – we simply flattened these out
with a hammer, then glued it in place
with epoxy inside and out, making sure
no epoxy got inside the thread.
T-Nuts are available from most hardware stores and they are really cheap!
However, you need to ensure you do
get 1/4 Whitworth – it appears that
5/16 and 3/8 are much more common.
If you must use 3/8-inch, 3/8 to 1/4inch adaptors are available from better
photographic stores.
Finishing off
It will be easier to solder the wires to
the boards first, then solder the wires
to all the switches and sockets before
mounting them inside the box.
Because the circuit boards are tiny
and sometimes have no provision for
normal screw mounts, you will have
to use some good quality, thick double
sided foam pads. Cut it to cover the
bottom of the “pre-CHAMP” board then
Here’s how we mounted
the electret microphone,
using an old dynamic mic
windshield as the base. The
insert is held in place with
an adhesive foam tab.
press it firmly in place, allowing plenty
of room for everything to clear.
Then fit the “CHAMP” amplifier by
putting double sided tape to the side
4,700µF capacitor and the side of the
board and then pressing it all into place
as shown.
Check again to see if any wires have
come loose then mount the battery in
its holder and switch on. The current
drain should be about 8mA or so. If all
is OK, put the lid on and plug in your
computer, set up the software and start
testing.
The microphone
For the microphone assembly, drill a
hole large enough for the electret in the
base of the box, solder a short length
of shielded cable to the microphone
with the shielding to earth (the side
connected to the outer case of the electret) and the other end to the the input
terminals of the PreCHAMP.
We are looking at frequencies below
30Hz on the G-weighting setting so
hum should not be a problem until you
switch to C-weighting
We cut the top off an old dynamic
mic and mounted it on the box, then
attached the electret to the side with
double sided tape as shown.
We maintained the original mic
thread to allow us to attach a wind
shield and also to calibrate our setup
and to make quick changes to test
various microphones without having
to unscrew the box all the time.
But this is not critical and you can
just stick the electret to the inside of
the box with double-sided tape or even
solder it directly to the input pins and
just have an appropriate hole in the
diecast box.
Whatever you do, you should be
able to access the electret to enable
Parts List –
Infrasonic Detector
1 PreCHAMP Kit
1 CHAMP Kit [optional - see text]
1 Diecast box (eg, Jaycar HB5067)
1 frequency-reading multimeter (eg,
Jaycar QM1327) [optional - see
text]
1 SPST miniature toggle switch (S1)
1 DPDT minature toggle switch (S2)
1 3.5mm mono socket, panelmounting
1 banana socket - red
1 banana socket - black
1 short red wire fitted with banana
plugs each end
1 short black wire fitted with banana
plugs each end
1 electret microphone insert (eg
Jaycar AM-4011) [see text]
1 microphone
1 1/4-in Whitworth T-nut for tripod
mount [see text]
1 9V battery
1 U-shaped 9V battery holder
1 3.5mm to 3.5mm shielded audio
cable (to connect to sound card)
Short lengths hookup wire and
shielded audio cable
Double-sided adhesive foam pads
Self-adhesive hook & loop tape, etc
Epoxy glue (for tripod adaptor)
1 Fatpigdog Virtual Analyzer program
(download from www.fatpigdog.
com [approx. $30]).
Semiconductors
1 LED, panel mounting 12V type
Capacitors
1 4700µF 16V electrolytic
1 1000µF 25V electroyltic
2 470µF 16V electrolytic
1 1µF MKT
1 120pF ceramic
Resistors
1 39kΩ
1 8.2kΩ
1 68Ω
1 100kΩ (or 50kΩ) log pot
1 knob to suit pot
quick changes because there is significant variation between electrets
as the graphs will show and having it
mounted on shielded cable makes it
easier to solder and unsolder.
Checking it out
Once everything is done connect the
output from the pre-CHAMP to your
computer Mic input making sure your
siliconchip.com.au
March 2013 37
Frequency
ADD dB to
(Hz)
measurement
0.5.................................. 41
1.................................... 29
2.................................... 17
3.................................... 11
4...................................... 8
5...................................... 5
6...................................... 4
7...................................... 3
8...................................... 2
9...................................... 1
10..................................... 0
11..................................... 0
12..................................... 0
13..................................... 0
14..................................... 0
15..................................... 0
16..................................... 0
17..................................... 0
18..................................... 0
19..................................... 0
20.................................. 0.5
21..................................... 1
22..................................... 1
23.................................. 1.5
24.................................. 1.8
25.................................. 2.5
26..................................... 3
27.................................. 3.2
28.................................. 3.3
29.................................. 3.4
30.................................. 3.5
Table 1: Correction table for a Jaycar
AM-4011 electret mic insert.
sound card mixer is set flat; ie, no bass
or treble boost.
Check to see if the microphone is
working by switching to C-weighting
and then talking or whistling. Measure
the output with a DMM set on AC or
plug the output into an amplifier or
oscilloscope.
Alternatively, you might like to plug
the output of the Pre-CHAMP into the
mic socket of your computer soundcard and view your “whistle” on the
spectrum analyzer. Your whistle should
give you a peak at around 1-2kHz, plus
harmonics at 2 and 3kHz.
Once all your checks are done switch
it to G-weighting and observe the LED
(assuming you have added the CHAMP)
It should flash in time with the signal
and you can open and shut a door to
test it (a car door opening is approx 0.52Hz). If all goes well you will finally be
38 Silicon Chip
ready to fine tune it all and try some
infrasound testing.
Plug in the Jaycar multimeter, switch
it to the Hz range and read off the
frequency. On C-weighting you will
probably see something in kHz but on
the G-weighting setting you should see
frequencies below 20Hz.
The frequencies will probably jump
around a bit and you can vary the gain
control to stabilise the readings.
In the SILICON CHIP office, we saw 8Hz
coming up consistently on the counter
and also on the spectrum analyser.
It disappeared when we switched off
our air conditioner but it was a hot day
so we put up with the 8Hz (although it
was less than 75dB [SPL]).
Calibration
As we mentioned before, calibration
is only really needed if you want to
establish sound pressure level .
Frequency calibration is already
inbuilt in the software and multimeter
and is not required for our purposes.
It is fairly straightforward but it will
help if you already have a sound level
meter (like the Jaycar QM1591) and an
audio oscillator but if you don’t have
these items and you don’t calibrate, you
will still get a pretty good idea from
the relative dB levels indicated in the
spectrum analyser.
For example our leaf blower is rated
at SPL 70dB at one metre. We set the
detector to C-weighting and found
that the fatpigdog analyser indicated
-15dB at 35Hz at 1 metre, so switching to G-weighting will mean that any
infrasound frequency BELOW 26Hz
will also be 70dB, if you see -15dB on
the analyser.
Sure, it’s a rough measurement but
there are many devices which have a
dB rating on their label such as mowers,
snippers, saws etc and you can check
these out.
For a more accurate calibration,
feed a tone (say 1kHz) through an
amplifier and loudspeaker and check
your C-weighted result against your
C- weighted sound level meter. Try various levels, incrementing them by 5dB.
Most sound level meters have absolutely no response below 35Hz so there
is no point checking the G-weighted
setting.
If you don’t use the fatpigdog software, don’t worry because you can
switch the Jaycar multimeter to “AC
volts”, making sure the gain control
is fully advanced and just take note of
the reading at various sound pressure
levels. Our setup showed approx 0.9v
AC at 94dB.
For frequencies below 7Hz the accuracy falls off somewhat but if you
are looking at 0.5Hz, just switch it to
DC volts and watch the rise and fall!
Other unique applications –
vibration anaysis
This instrument is very useful in
checking out vibration problems as we
found with our 8Hz air conditioning.
Sometimes these problems go undetected for years and some have claimed
that they may be responsible for nausea,
headaches, sleep problems or just a
general sense of unpleasantness.
Additionally, traditional methods
of sound level monitoring have only
focussed on the audible spectrum and
have not even considered infrasound
effects on the human (or animal) body
and the access to infrasound measuring devices has been both difficult and
expensive.
Any vibrating device will give off
sound and our setup will detect it
and/or datalog it. Not only that but for
a few dollars it could be used in just
about any industrial situation where
vibrations may be destructive – such
as engines, chassis, suspensions even
buildings and bridges!
Data logging with
waterfall analysis
The software will also enable you to
do waterfall analysis and this is really
a way of viewing a spectrum analysis
as it varies over time.
It can be used as a datalogger for
infrasound and audio signals. The vertical scale shows the frequencies of the
various harmonics while the horizontal
scale is time so the whole chart is a
record of a few minutes.
TO SETUP FOR WATERFALL CHARTS
The wiring setup is virtually the same as
for testing spectrum analysis microphone
“Pre-champ” output (for voice prints)
The setup for the virtual instrument is:
Click on “preset”
Then “display”
Then “waterfall F2”
Then “rotate”
Then try different sweep times and
resolution bandwidths (Res. Bw…..).
And try different colour schemes by
clicking on “jet”
Press BMP to save the image you want.
siliconchip.com.au
Setting up and operating the Virtual Analyzer
We assume you have downloaded the spectrum analyzer
software from www.fatpigdog.com/spectrumanalyzer (or
updated if you’re using an older version).
The originator, Spyro Gumas, is very communicative
and can assist if you have any problems.
To start, open and run the program. We used Windows
XP but check the website first for compatibility with Vista,
Windows 7, 8, etc.
Initially, you will see the black and white MS-DOS screen
appear. You may have to wait (perhaps two minutes or so)
and the instrument will appear similar to the trace below:
This sweep shows the frequency response of the
modified preCHAMP: the top line is C weighted and
is flat from 10Hz-20kHz. The middle line is an unmodified pre-CHAMP (not used) and bottom line (red)
is the G-weighted response which joins the top line at
10Hz.Calibration can be carried out at 1kHz on the C
setting and then is the same when the unit is switched
to G, up to a max of 20Hz!
NOTE: Our Audio Precision analyser cannot go
below 10Hz.
The analyser is now ready to do a ten-second sweep of
your sound source from 0.5Hz to 100Hz with a resolution of
0.5Hz and will continuously update itself with the maximum
signal. For example we set it going during a thunderstorm
to record the sound over a period of 20 minutes
You can save an image anytime by pressing “BMP” (bitmap).
You can play around with the RBW (resolution bandwidth)
which you can set as low as 0.1Hz!
Refer to the fatpigdog manual provided if you have difficulty
because some computers have different delay arrangements
with the soundcard and you may need to compensate this
with “tstupid”.
When you are happy with a particular trace, you might like
to activate the marker to examine point of interest.
Click on “marker” then “ON” and then click “peak”. The
marker will then indicate the dominant frequency
You will see a red dot appear on the trace, then move the
marker to the area you want to measure by clicking on “<”
(backward) or “>” (forward) keys.
The marker reading appears at the top of the page e.g “Mrk
2.558Hz, -86.2dB”.
Once you have measurements of the points you are interested in, go to Table 1 and add or subtract the dB value at
the frequency of interest. For example if you measured -10dB at
5Hz from the chart you have to ADD 5dB, ie -10+5=-5
Now during calibration, for our setup we found that -15dB on
our spectrum analyser was 74dB SPL so we have to add 10dB
(because -5dB is 10dB louder than -15dB).
So SPL=74+10
SPL=84dB
Accuracy
The figures quoted in this article are those achieved on
a PC fitted with a generic sound card (ie, nothing special!)
so we have every reason to believe that you should achieve
similar results. However, no guarantees can be given! SC
Once the virtual instrument pops up, plug the output from the
prechamp into your soundcard mic input, switch to G weighting
then set up as follows.You can attach the multimeter to the
CHAMP output if you wish but in this case it is redundant.
On the virtual analyser:
Click on “reset” to clear any previous settings.
Click on frequency
Click on start (F2) and type in “0.5” <enter>.
Click on stop key (F3) and type “100” <enter> (This sets the
range to 0.5Hz-100Hz)
Click on Lin/Log key (F4) so you see lin/(log) – now the
frequency range is set to a logarithmic scale.
Then:
Click on bandwidth
Click on RBW and type in “0.5” <enter>
Click on sweep and type “10000” <enter>
Click on “trace” and then “max hold”
The analyser will then sweep continuously and indicate
the number of averages at the top of the page.
siliconchip.com.au
This shows the actual frequency response of the finished
setup using the Jaycar electret and is usable down to
1 Hz! You need to allow for falloff by using the table
provided. Eg, for 1Hz you need to add 29dB to your base
figure to obtain the correct SPL.
March 2013 39
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