Hunterhigh80

Measuresound andvibrationway belowhumanhearingbyAllan Linton-Smithand Ross TesterInfrasoundDetectorPhoto: Harvey McDaniel WikipediaAre wind turbines making you sick? Is building vibration makingyou nauseous? Or do you just want to measure infrasound in yourenvironment? You don’t need to spend thousands of dollars to do itproperly; just build our low-cost but accurate Infrasound Detector.There’s been a lot of press latelyabout infrasound, particularlyas it applies to wind turbines.But until now, you’ve needed tensof thousands of dollars worth of testequipment to detect and measure it.Our Infrasound Detector can be builtfor less than a hundred dollars yet willgive very accurate results. You caneither read the sound pressure directlyor store and analyse readings on yourcomputer!So what exactly is infrasound?It can defined as sound below therange of normal human hearing. That’s30 Silicon Chipgenerally reckoned to be below about20Hz.Below that, you can perhaps senseor even “feel” sound but you can’tactually hear it.In practice, infrasound involves frequencies from about 20Hz to 0.5Hz butsome natural phenomena can causeinfrasound down to the millihertz(.0001Hz!) region.When people complain about illeffects from infrasound (and thereare legions of those reports), manyacoustic consultants have taken theattitude that “if you can’t hear it, itcan’t be doing you harm”.We disagree – and the publisher ofthis magazine even wrote an editorialon the subject back in February 2010.Reported human reaction to infrasound from wind turbines is variedbut some of the reports associateinfrasound with a general feelingof malaise, nausea, vertigo, blurredvision, memory problems, tinnitus,anxiety, uneasiness, extreme sorrow,nervous feelings of revulsion or fear,chills down the spine and feelings ofpressure on the chest.Others have reported headaches andmigraines, major sleep disorders andeven self-harm tendencies.siliconchip.com.auSome researchers have even given ita label: wind turbine syndrome.Wind turbines are one example butyou’ll also find infrasound caused bytraffic noise, heavy surf, engines/motors (especially things like compressors), building vibrations being excitedby wind, machinery and so on.Large animals such as whales,crocodiles, alligators, elephants andemus communicate with infrasoundso if you want to record amorouscrocodiles, our Infrasound Detectoris a good way to go about it (from asafe location!).Other source of infrasonics areheavy artillery, the calving of icebergsfrom glaciers and earthquakes.In fact, there is a theory that thebuildup of stresses with the earth’scrust before a major earthquake causesinfrasound – which could explain whybirds and some other animals appearto have some warning of an imminentquake.Want another example? The very actof opening or closing a door producesinfrasound waves. But that is transitory – you don’t normally stand therefor hours opening and closing doors!Whatever the infrasonic phenomenon you want to investigate, ourInfrasound Detector is an effective andlow-cost way to do it and it comparesmore 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 soundvia the electret microphone, then removes all but signals below 20Hz. This signal can then beanalysed by a computer running “Fatpigdog” software, or it can be fed to a modified CHAMPamplifier which drives a multimeter in its AC range to deliver readings of sound pressure levels.'PRE-CHAMP’ PREAMP(MODIFIED)*10k RESISTORADDED TOPOWERELECTRET MIC22k100k10k*Q1BC548SHIELDEDLEADB1000001eFVR1100kC8.2k150k1uFMKT470001eF16VTO PCSOUNDCARD1kVR210k*SEE TEXTSCsiliconchip.com.au220001eF16V4700001eF816V1IC1LM386N(SEETEXT)2510001eF10V12VLED100nFA68001d2.2kINFRASOUND TESTING UNIT9VBATTERY74001cK10001dLEDCOMPONENTS IN RED ARE CHANGED/ADDED001f201163120pF100001d470001eF16VPOWER100001eF16VBS2*39kS1ANALYSERGAIN100001eFQ2BC558 E 16VCEELECTRETMICROPHONE0.778'CHAMP' AMPLIFIER(MODIFIED)2.2k10001eF16VJAYCARQM1327MULTIMETER ONFREQUENCYRANGEKABC548, BC558ELECTRETBOUTPUTEARTHECMarch 2013 31.curate and reliable – we believe it canbe just as accurate and reliable as commercial gear.In fact, while our unit should costwell under $100 to build and is easy toput together, it took hundreds of hoursto develop and test. That is becauseinfrasound sweeps can take hours tosettle, measure and average – and somevery specialised and expensive equipment was required to design and test it.If you wanted to buy that commercialequipment yourself, you’d have littlechange from $30,000!We also had to develop a methodfor testing and calibrating high levelsof infrasound without upsetting theneighbours!How it worksThe output from a wide-rangeelectret microphone is fed to a verylow-frequency bandpass filter. Theinfrasound signal is amplified and fedto a “virtual” spectrum analyser whichthen plots the amplitude of the infrasound signal on the vertical (Y) scaleversus frequency on the horizontal (x)scale using a principal known as FastFourier Transform (FFT).A computer can then be used toanalyse the signal and/or a direct frequency readout can be obtained if usedin the field.Our Infrasound Detector is built into a small diecast box, with an oldmicrophone shield attached to the front. Inside this shield is a low-cost electretmic insert. The terminals at left are the output to a frequency counter (or inour case, a budget multimeter) while a socket is provided on the right side foroutput to a PC sound card. Suitable analysis software is quite cheap.SpecificationsMicrophone 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 SPLPreamplifier frequency response: ....................................... ±0.2dB (0.5Hz-20kHz)Power amplifier frequency response: .............................. ±0.2dB (0.5Hz-20kHz)Power amplifier output (before clipping): .................... 200mW into 8ΩFrequency 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); 250mWPreamp input maximum: .......................................50mVPreamp input minimum: .............................................................. 1.0mVPower amp input maximum:...................................................... 500mVPower amp input minimum:....................................................... 30mVPreamp 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 ChipGood grief: The CHAMP isback!After constructing many circuitswhich offered good theoretical performance we discovered that the goodold PreCHAMP preamplifier, combinedwith the equally elderly CHAMP audioamplifier, could be easily modified todo the job admirably.Yes, we know, we said only twomonths ago that our new CHAMPIONamplifier would kill off the PreCHAMPand CHAMP but there’s a good reasonfor resurrecting it here: low quiescentcurrent.The PreCHAMP and CHAMP drawonly about 4mA each on idle, so prolonged operation (which you’ll needfor field checks) is quite practical usingonly a 9V battery.By comparison, the CHAMPIONdraws up to 60mA so your 9V batterywouldn’t last long at all!If you built the CHAMPION project(based on the Panasonic AN7511), youcould use it for infrasound with only afew modifications but you’ll probablyneed to use it with an external supply.siliconchip.com.auAs used here, the modified PreCHAMP now has much improved frequency response; within +/-0.2dB from2Hz – 20kHz. The modified CHAMPalso gives a flat frequency response ataround 0.25-0.5W – so you can feed anyoscilloscope or low frequency counter.Optional CHAMPThe CHAMP is optional – it has beenincluded so that you can take quickmeasurements in the field.The PreCHAMP is set up as a bandpass filter and high gain amplifierwhich is approximately G-weighted,ie, its centre frequency is around 10Hzwith -3dB points at 500mHz (0.5Hz)and 26Hz.A selector switch is provided forswitching to “C” weighting (ie, flatresponse) so that the unit can easily becalibrated at 1kHz.The infrasound signal from the PreCHAMP is fed to the CHAMP amplifier which has been modified to give aflat frequency response from 0.5Hz to20kHz and is set at high gain so that thesignal output to a frequency counter isover 130mV at 1Hz.Electret MicrophonesThe electret microphone is pretty inefficient at frequencies below 25Hz, hence the very high amplification.There are lots of electret microphone inserts availablebut we are specifying a particular Jaycar model (Cat AM4011) because we found it to be a very good match forthis project. However, you can see from the graphs belowthat even these specific Jaycar mics are not all the same –some are more sensitive than others due to manufacturingvariations – so you may need to buy a few to experiment.Frequency counter?Whoops! Haven’t got one of those?That little problem is solved verycheaply with a multimeter – specifically the Jaycar QM1327 auto-rangingmultimeter, which can read read downto 0.1Hz and sells for only $34.95.While its specs state it needs a minimum of 3V RMS AC before it will showa frequency reading, we found that it farexceeded its specification and 130mVwas sufficient. Few frequency countersgo below 10Hz so the Jaycar meter represents good value in this application.If you use the CHAMP together withthe Jaycar multimeter, then you willbe able to determine SPL (the soundpressure level in dB) by switching theDMM to AC volts. This will give anapproximate SPL in dB (decibels) asdescribed.For signals below 0.5Hz this approach will not be accurate but this willbe more than sufficient for the majorityof 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 electretmicrophones combined with the Pre-CHAMP and compared them with an accuratelycalibrated Bruel & Kjaer microphone/preamp (expensive!) – and found that the Jaycarelectret was actually better at infrasound frequencies!Fatpigdog again!The direct readout is very handy inthe field but if you want to do somereal analysis, you’ll need a computerand suitable software.Readers may remember “fatpigdog”from our feature article on measuringsiliconchip.com.auMarch 2013 33We used this sweep to show that 1Hz was easily detectable with a resolution of 0.5Hz. By correction, the sound source is100dB. You need to be patient because the analyser sometimes sets the sweep time to 10 seconds automatically and youhave to wait before you can make adjustments.Loudspeaker Frequency Response inthe December 2011 issue.We’re using this software again butit has since been updated considerably(the latest version is 4.04) and has moreusable functions than the original version. You can purchase and downloadthe software for around $30 from www.fatpigdog.com.On their website, you will also findvarious dedicated bench top spectrumanalysers for sale but the virtual instrument is about 99.9% cheaper!Fatpigdog is fun to use, easy to manage and includes all sorts of extrassuch as a waterfall display, spectrumanalysis to 22kHz, BMP capture andmuch more.The PreCHAMP output is simplyfed to the sound card input of yourcomputer.You could feed the spectrum analyser from the ‘CHAMP’ output butwe don’t recommend this becauseyour computer soundcard is usuallyset up for microphone-level inputs (ie,Using the Agilent 35670A, the sweep gives the lowerresponse for the G-weighted PreCHAMP down to 0.1 Hz…that’s 1 cycle every 10 seconds! -3dB points are 0.5Hz and26Hz. Mains hum is not a problem at these frequencies!34 Silicon Chipmillivolts not volts). Any large voltageswill usually result in clipping andconsequently the spectrum analyserwill show multiple peaks from the oddharmonics.The Jaycar multimeter is an option ifyou wish to have a hand held detectorfor quick infrasound detection withouthaving to set up a computer and adjustthe software.It is fed from the Pre-CHAMP output via the 10k preset pot. You canset the maximum output from theFatpigdog spectrum of a 15-inch speaker fed with 200W.The resolution is set at 1Hz and the sweep time is onesecond. You could actually feel the sound – and it was notnice!siliconchip.com.auThe Jaycar QM1327 Multimeter works fine as a frequencymeter and also an AC voltmeter. It’s simply held in placeon the back of the Infrasound Detector with self-adhesivehook’n’loop tape (usually sold under the “Velcro” brand).“CHAMP” by setting the preset fullyanti-clockwise.The only other modification is theaddition of a 68Ω “dummy load” resistor which prevents the output capacitorfrom building up a DC charge, whichwould otherwise result in false readings. You could attach a loudspeakerinstead but you won’t hear much below25Hz (and it will drain the battery morequickly).By changing the parameters on theanalyser – such as sweep time, startand stop frequencies and resolutionbandwidth, you can save and print yourspectra for further analysis.Furthermore, by setting the spectrumanalyser to “max hold” you will be ableto observe any infrasound which occursduring an extended period of time.Using the virtual spectrum analyserrequires some practice and patience(just like a real benchtop spectrum analyser) but if you experiment, you willlearn to master it all fairly quickly. We’llhave much more to say on this later.This spectrum shows the maximum sound level forsuburban Pacific Highway traffic. The microphone isa good 5-10 metres away from vehicles and there issignificant noise at 2Hz! Note also the peak at around20Hz – probably from engines.siliconchip.com.auConstructionThe “hardware” is built into analuminium diecast box (to minimisenoise) measuring 119 x 93.5 x 34mm(eg, Jaycar HB5067). Inside this are thePCBs for the modified PreCHAMP (andCHAMP if you wish to use it) and a 9Vbattery in suitable holder.Layout is not particularly criticalbut given the very high amplificationof the PreCHAMP/CHAMP combina-You can set up the detector and leave it running for up toan hour. We caught a distant thunder clap at 5Hz and acalculated 84dB. The resolution was set at 1.0Hz and thesweep time was one second. The maximum hold functionruns continuously and updates every second.March 2013 359V BATTERY1 001fF120pS2CON1ELECTRETMICTO SPECTRUMANALYSERPRE-CHAMP9V BATTERYHOLDEROUTPUT+VIN14970110CSGNDGNDNEW 39k RESISTORSOLDERED UNDERPCBVR114920110INPUT4700001fF 16V220001fFOUTGNDCSS1S1'CHAMP' AMPLIFIER(MOUNTED SIDE-ON)12V LED(MOUNTEDON CASE)+VGND68001eOUTPUT TO FREQ COUNTERSPST switch to switch the larger capacitor in and out of circuit but thearrangement shown (using a DPDTswitch) allows easy mounting of thetwo external capacitors: they are simplywired across the outside terminals andthe wires back to the PCB are wired tothe centre terminals.Assuming you want to include the“CHAMP” power amplifier, to providesufficient voltage to the Jaycar Frequency meter (multimeter), constructit as per the kit instructions (or refer toSILICON CHIP February 1994).The modifications we have madeto give a flatter frequency responseinvolve changing two capacitors. Youwill find that the 4,700µF capacitor islarge but fits neatly on the PCB. However, it is a little too tall and the finishedamplifier will have to be put on its sideso it can easily fit in the diecast box.Now you can drill and mount allthe hardware on the diecast box usingthe picture as a template and solder allthe wires up according to the diagram.Fitting a tripod adaptorKATo enable easy use in the field, wewanted to be able to attach the unitto a photographic tripod. So we fittedFig.2 (above): component layout is notcritical but this diagram should giveyou a guide. Both the PreCHAMP andCHAMP PCBs are held in place withdouble-sided foam pads. The photo atright shows the same internal view,together with the T-nut tripod adaptorwe fitted to the end of the diecast case.tion (about 4000 times), outputs shouldbe kept relatively clear of inputs, as isnormal practice for an amplifier.Start by constructing the Pre-CHAMPpre-amplifier as per the instructionsgiven with a kit (or refer to SILICONCHIP July 1994).See Figs.1&2 for the modificationsrequired – you will only need to changethe values of three capacitors and thesewill easily fit on the PCB.The 39kΩ resistor should be solderedto the underside of the board input oracross the input pins.To the two holes on the board marked“1n5” solder two leads and connectthese leads to the two central pins ofDPDT switch S2.Then solder the 1uFcapacitor to one side and the 120pfcapacitor to the other (see photo).Then run leads to CON3 and VR2 asshown in the wiring diagram.We could have used just a simple36 Silicon Chipsiliconchip.com.auAs it has a 1/4-inch Whitworthinternal thread (same as most tripods)we used one of these furniture T-nutsfrom a hardware store, flattened outthe points, and drilled the box to suit.Then we glued it in place with some2-part epoxy.our box with a 1/4-inch threaded bush(Whitworth thread; standard on mosttripods/cameras/etc).In fact, we used a “T-nut” fittingintended for furniture and shelf hardware (pictured) which has an internal1/4-inch Whitworth thread. It had fourpunched points intended to help it griptimber – we simply flattened these outwith a hammer, then glued it in placewith epoxy inside and out, making sureno epoxy got inside the thread.T-Nuts are available from most hardware stores and they are really cheap!However, you need to ensure you doget 1/4 Whitworth – it appears that5/16 and 3/8 are much more common.If you must use 3/8-inch, 3/8 to 1/4inch adaptors are available from betterphotographic stores.Finishing offIt will be easier to solder the wires tothe boards first, then solder the wiresto all the switches and sockets beforemounting them inside the box.Because the circuit boards are tinyand sometimes have no provision fornormal screw mounts, you will haveto use some good quality, thick doublesided foam pads. Cut it to cover thebottom of the “pre-CHAMP” board thenHere’s how we mountedthe electret microphone,using an old dynamic micwindshield as the base. Theinsert is held in place withan adhesive foam tab.press it firmly in place, allowing plentyof room for everything to clear.Then fit the “CHAMP” amplifier byputting double sided tape to the side4,700µF capacitor and the side of theboard and then pressing it all into placeas shown.Check again to see if any wires havecome loose then mount the battery inits holder and switch on. The currentdrain should be about 8mA or so. If allis OK, put the lid on and plug in yourcomputer, set up the software and starttesting.The microphoneFor the microphone assembly, drill ahole large enough for the electret in thebase of the box, solder a short lengthof shielded cable to the microphonewith the shielding to earth (the sideconnected to the outer case of the electret) and the other end to the the inputterminals of the PreCHAMP.We are looking at frequencies below30Hz on the G-weighting setting sohum should not be a problem until youswitch to C-weightingWe cut the top off an old dynamicmic and mounted it on the box, thenattached the electret to the side withdouble sided tape as shown.We maintained the original micthread to allow us to attach a windshield and also to calibrate our setupand to make quick changes to testvarious microphones without havingto unscrew the box all the time.But this is not critical and you canjust stick the electret to the inside ofthe box with double-sided tape or evensolder it directly to the input pins andjust have an appropriate hole in thediecast box.Whatever you do, you should beable to access the electret to enableParts List –Infrasonic Detector1 PreCHAMP Kit1 CHAMP Kit [optional - see text]1 Diecast box (eg, Jaycar HB5067)1 frequency-reading multimeter (eg,Jaycar QM1327) [optional - seetext]1 SPST miniature toggle switch (S1)1 DPDT minature toggle switch (S2)1 3.5mm mono socket, panel-mounting1 banana socket - red1 banana socket - black1 short red wire fitted with bananaplugs each end1 short black wire fitted with bananaplugs each end1 electret microphone insert (eg Jaycar AM-4011) [see text]1 microphone1 1/4-in Whitworth T-nut for tripodmount [see text]1 9V battery1 U-shaped 9V battery holder1 3.5mm to 3.5mm shielded audiocable (to connect to sound card)Short lengths hookup wire andshielded audio cableDouble-sided adhesive foam padsSelf-adhesive hook & loop tape, etcEpoxy glue (for tripod adaptor)1 Fatpigdog Virtual Analyzer program(download from www.fatpigdog.com [approx. $30]).Semiconductors1 LED, panel mounting 12V typeCapacitors1 4700µF 16V electrolytic1 1000µF 25V electroyltic2 470µF 16V electrolytic1 1µF MKT1 120pF ceramicResistors1 39kΩ1 8.2kΩ1 68Ω1 100kΩ (or 50kΩ) log pot1 knob to suit potquick changes because there is significant variation between electretsas the graphs will show and having itmounted on shielded cable makes iteasier to solder and unsolder.Checking it outOnce everything is done connect theoutput from the pre-CHAMP to yourcomputer Mic input making sure yoursiliconchip.com.auMarch 2013 37FrequencyADD dB to (Hz)measurement0.5.................................. 411.................................... 292.................................... 173.................................... 114...................................... 85...................................... 56...................................... 47...................................... 38...................................... 29...................................... 110..................................... 011..................................... 012..................................... 013..................................... 014..................................... 015..................................... 016..................................... 017..................................... 018..................................... 019..................................... 020.................................. 0.521..................................... 122..................................... 123.................................. 1.524.................................. 1.825.................................. 2.526..................................... 327.................................. 3.228.................................. 3.329.................................. 3.430.................................. 3.5Table 1: Correction table for a JaycarAM-4011 electret mic insert.sound card mixer is set flat; ie, no bassor treble boost.Check to see if the microphone isworking by switching to C-weightingand then talking or whistling. Measurethe output with a DMM set on AC orplug the output into an amplifier oroscilloscope.Alternatively, you might like to plugthe output of the Pre-CHAMP into themic socket of your computer soundcard and view your “whistle” on thespectrum analyzer. Your whistle shouldgive you a peak at around 1-2kHz, plusharmonics at 2 and 3kHz.Once all your checks are done switchit to G-weighting and observe the LED(assuming you have added the CHAMP)It should flash in time with the signaland you can open and shut a door totest it (a car door opening is approx 0.52Hz). If all goes well you will finally be38 Silicon Chipready to fine tune it all and try someinfrasound testing.Plug in the Jaycar multimeter, switchit to the Hz range and read off thefrequency. On C-weighting you willprobably see something in kHz but onthe G-weighting setting you should seefrequencies below 20Hz.The frequencies will probably jumparound a bit and you can vary the gaincontrol to stabilise the readings.In the SILICON CHIP office, we saw 8Hzcoming up consistently on the counterand also on the spectrum analyser.It disappeared when we switched offour air conditioner but it was a hot dayso we put up with the 8Hz (although itwas less than 75dB [SPL]).CalibrationAs we mentioned before, calibrationis only really needed if you want toestablish sound pressure level .Frequency calibration is alreadyinbuilt in the software and multimeterand is not required for our purposes.It is fairly straightforward but it willhelp if you already have a sound levelmeter (like the Jaycar QM1591) and anaudio oscillator but if you don’t havethese items and you don’t calibrate, youwill still get a pretty good idea fromthe relative dB levels indicated in thespectrum analyser.For example our leaf blower is ratedat SPL 70dB at one metre. We set thedetector to C-weighting and foundthat the fatpigdog analyser indicated-15dB at 35Hz at 1 metre, so switching to G-weighting will mean that anyinfrasound frequency BELOW 26Hzwill also be 70dB, if you see -15dB onthe analyser.Sure, it’s a rough measurement butthere are many devices which have adB rating on their label such as mowers,snippers, saws etc and you can checkthese out.For a more accurate calibration,feed a tone (say 1kHz) through anamplifier and loudspeaker and checkyour C-weighted result against yourC- weighted sound level meter. Try various levels, incrementing them by 5dB.Most sound level meters have absolutely no response below 35Hz so thereis no point checking the G-weightedsetting.If you don’t use the fatpigdog software, don’t worry because you canswitch the Jaycar multimeter to “ACvolts”, making sure the gain controlis fully advanced and just take note ofthe reading at various sound pressurelevels. Our setup showed approx 0.9vAC at 94dB.For frequencies below 7Hz the accuracy falls off somewhat but if youare looking at 0.5Hz, just switch it toDC volts and watch the rise and fall!Other unique applications –vibration anaysisThis instrument is very useful inchecking out vibration problems as wefound with our 8Hz air conditioning.Sometimes these problems go undetected for years and some have claimedthat they may be responsible for nausea,headaches, sleep problems or just ageneral sense of unpleasantness.Additionally, traditional methodsof sound level monitoring have onlyfocussed on the audible spectrum andhave not even considered infrasoundeffects on the human (or animal) bodyand the access to infrasound measuring devices has been both difficult andexpensive.Any vibrating device will give offsound and our setup will detect itand/or datalog it. Not only that but fora few dollars it could be used in justabout any industrial situation wherevibrations may be destructive – suchas engines, chassis, suspensions evenbuildings and bridges!Data logging withwaterfall analysisThe software will also enable you todo waterfall analysis and this is reallya way of viewing a spectrum analysisas it varies over time.It can be used as a datalogger forinfrasound and audio signals. The vertical scale shows the frequencies of thevarious harmonics while the horizontalscale is time so the whole chart is arecord of a few minutes.TO SETUP FOR WATERFALL CHARTSThe wiring setup is virtually the same asfor 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 andresolution bandwidths (Res. Bw…..).And try different colour schemes byclicking on “jet”Press BMP to save the image you want.siliconchip.com.auSetting up and operating the Virtual AnalyzerWe assume you have downloaded the spectrum analyzersoftware from www.fatpigdog.com/spectrumanalyzer (orupdated if you’re using an older version).The originator, Spyro Gumas, is very communicativeand can assist if you have any problems.To start, open and run the program. We used WindowsXP but check the website first for compatibility with Vista,Windows 7, 8, etc.Initially, you will see the black and white MS-DOS screenappear. 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 themodified preCHAMP: the top line is C weighted andis 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 at10Hz.Calibration can be carried out at 1kHz on the Csetting and then is the same when the unit is switchedto G, up to a max of 20Hz!NOTE: Our Audio Precision analyser cannot gobelow 10Hz.The analyser is now ready to do a ten-second sweep ofyour sound source from 0.5Hz to 100Hz with a resolution of0.5Hz and will continuously update itself with the maximumsignal. For example we set it going during a thunderstormto record the sound over a period of 20 minutesYou 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 difficultybecause some computers have different delay arrangementswith the soundcard and you may need to compensate thiswith “tstupid”.When you are happy with a particular trace, you might liketo activate the marker to examine point of interest.Click on “marker” then “ON” and then click “peak”. Themarker will then indicate the dominant frequencyYou will see a red dot appear on the trace, then move themarker 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 “Mrk2.558Hz, -86.2dB”.Once you have measurements of the points you are interested in, go to Table 1 and add or subtract the dB value atthe frequency of interest. For example if you measured -10dB at5Hz from the chart you have to ADD 5dB, ie -10+5=-5Now during calibration, for our setup we found that -15dB onour spectrum analyser was 74dB SPL so we have to add 10dB(because -5dB is 10dB louder than -15dB).So SPL=74+10SPL=84dBAccuracyThe figures quoted in this article are those achieved ona PC fitted with a generic sound card (ie, nothing special!)so we have every reason to believe that you should achievesimilar results. However, no guarantees can be given! SCOnce the virtual instrument pops up, plug the output from theprechamp into your soundcard mic input, switch to G weightingthen set up as follows.You can attach the multimeter to theCHAMP 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 frequencyClick on start (F2) and type in “0.5” <enter>.Click on stop key (F3) and type “100” <enter> (This sets therange to 0.5Hz-100Hz)Click on Lin/Log key (F4) so you see lin/(log) – now thefrequency range is set to a logarithmic scale.Then:Click on bandwidthClick 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 indicatethe number of averages at the top of the page.siliconchip.com.auThis shows the actual frequency response of the finishedsetup using the Jaycar electret and is usable down to1 Hz! You need to allow for falloff by using the tableprovided. Eg, for 1Hz you need to add 29dB to your basefigure to obtain the correct SPL.March 2013 39