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25th February 2010 #�

DanDanLives for gear

Join Date: Aug 2003Location: Cork IrelandPosts: 5,685Thread Starter

Basic acoustic measurement primer v2.1

Room Analysis Software Primer V2.1Revised August 2010

Room Analysis Software of decent quality has become veryaffordable. This has led to many attempting to use it to evaluatetheir studio or other listening room. The immediate results can beconfusing and disappointing. To get the best from any tool, someunderstanding of the principles and some operating skills areneeded. More importantly, let's back up a bit, I recommend thatthe basic room treatments be done before any attempt atmeasurement. Corner Bass Traps, RFZ, including Cloud. When thesebasics are in place let's then measure and tweak positions to getthe absolute best from the space. This article is intended as asimple primer on how to do the measurements, and how to viewthem. As of Feb 2010 it has been substantially revised toaccommodate new wisdoms, the results of practical experimentsand discussions on the fora. Please remember it is a Primer,intended to help you get going quickly and easily. I have simplifieda few issues in order to keep it tight.

What to expectThere is no silver bullet here unfortunately. Expect to see horriblelooking graphs. Software can excel at comparative tasks, such asfinding the best speaker and listener positions. However it takes anexperienced brain to evaluate how a room sounds or errs, and whatto do about it. Let’s not expect a health check and a treatmentprescription from these programs. They are tools not room doctors.

The GraphsI will deal with only a few of the easiest understood graphs. Forthe rest, RTFM. Frequency response is the most common and is easily understood.The Waterfall is a perhaps more powerful one. Most of our roomsare dominated by strong modes. Tuned by the dimensions of theroom, these modal frequencies ring on longer than their neighbours.Conversely there are modal nulls which lead to missing information.These modes are powerful and they wreak havoc. They causeindividual Bass notes to stick out or to vanish. This makes mixdecisions regarding the bottom end, where most of the energy inmusic resides, very unreliable. The Waterfall plot displays thisbehaviour so graphically that it hardly needs explanation. It is like aseries of Frequency response snapshots taken at short intervals.Like a cartoon, these snaps portray action over time.



Decay graphs such as EDT, T20,30 are to be regarded carefully.Even in Labs, decay measurements under 125Hz or so are quiteuncertain. LF modes dominate in small rooms, so the Decay here isnot at all random and diffuse like Reverb in a large space. The concept of RT60 cannot even exist in small rooms. REW has a Topt feature which tries to address these difficultieswith some success.FM3 has also improved greatly at Decay and now includes thirdoctaves. The BBC used third octave decays as an indicator of room quality.No third octave decay length was permitted to differ from it’sneighbours by more than 10%. Note that this test will show upproblematic modes. Any room passing this test would be a very fineone, IMHO.

Given time and experimentation, one learns how to interpret thesedifferent views of the room sound. Graph changes caused by say achange of speaker or listener position, are however very easilyevaluated as an improvement or not.

The Microphone You may well have a useable mic in your closet already. A multi-pattern condenser switched to omni will work fine. The frequencyresponse bumps of say an 87 or 414 are tiny compared to the 30dBanomalies we find in real rooms. I recommend jumping in. Get ademo of the software and go for it with whatever mic you have athand. Be aware that directional mics may have poor LF responsecompared to an OMNI. They can also underestimate some modesdue to position and direction. With that caveat, most mics are OKfor comparative tests. In time, if you develop a liking formeasurement, you might want a more suitable mic or software.Measurement mics are omnidirectional Small Diaphragm Condensers.The best of these are optimised to have a very accurate andstable frequency response, unfortunately this is often at theexpense of very poor noise figures. I recommend only the oneswhich are also quiet enough for recording. DPA and EarthWorksspring to mind here.

Sound Level MeterSLMs, even cheap ones, are very useful for many purposes. Theyare well worth having around. There are many recent cheap unitsway better than the venerable Broadcasting Shed one. Many SLM'scan output their Mic signal via the AC OUT socket. This is fine withdecent units. Recent experience has shown it to be unreliable with



cheap ones. Some have very skewed frequency response. Evenworse, the AC OUT can have electronic noise or DC on it.Not recommended.

CalibrationIt is not necessary to calibrate your rig to standard SPL's. It is however, wise to optimise the S/N ratio. Adjust your speaker volume to drive the room very well. You maywish to block your ears with headphones or earplugs. Watch outfor overload lights on powered monitors, particularly at HF.This puts our source signal well above background noise, computerfans, etc. Longer sweeps or multiple sweeps averaged can also beused to increase S/N ratio. With a decent sweep level established, now adjust your mic pregain using trial measurements and/or the meters on your interface.Get a decent level, graph peaks should end up within say -6dBFS to OdBFS. This is really just normal digital recording level setting.

CorrectionAs one becomes familiar with the Software, one may want to availof some little sophisticated extras. It is possible to input amicrophone correction curve to adjust a known mic response toflat.e.g. If your mic comes with a printed frequency response curveshowing -2dB at 100Hz, you can apply an automatic +2dBcorrection in the software. Similarly there is a loop back correctionfacility to automatically correct for sound card or interfaceanomalies including delay. FuzzMeasure has a Create Time DomainAverage facility. It is essential to use the loop back correction forthis to function properly.

Acoustic Analysis Software I will only mention the free or cheap packages that I know. Thereare many out there, from Freeware to extremely expensive Labstandard. However this little group have democratised the processby being both competent and affordable. ETF and it’s current variations was one of the earliest programs.The Acoustisoft website has great walk-through tutorials on actualuses. FuzzMeasure Pro is a similar product for the Mac. This one is veryeasy to use, but is fully competent. Another strength is it’s verysophisticated averaging of measurements. e.g. Take measurementsat several different listening positions, then average them to get abroader sense of the room’s response. REW now works with Mac and PC. It is very comprehensive, whichresults in a learning curve. It costs a nice round figure, 0. Themanual deserves special mention, superb writing. ARTA is PC only. It appears to be very comprehensive andreasonably priced. The manual is pretty much a text book, itexplains the measurement techniques in hand to the very deepestlevels.

Other Tools and Helpers I use a collection of must haves together to achieve a holisticevaluation of the room.



A CD of reference Tracks. I bring mine with me everywhere.

The SoundCheck CDs by Alan Parsons.

LabMeter, a Mac Frequency meter from rustykat.com

A Room Mode calculator such as this onehttp://www.bobgolds.com/Mode/RoomModes.htmThere are many RM calcs, with different strengths. Test drive afew.

The free MiniRator and Test tones at RealTraps - HomeI use sine waves to drive the modes crazy. SignalSuite is nice andsmooth. Try the Signal Generator plug-in in your DAW, sometimesthe frequency jumps make this difficult. REW has a unique Sweepfollows Graph feature on it's Signal Generator. This has a wonderfulsmooth action. I use an ancient steam driven analogue one with a real sweep dial. Oneself- the body can generate tones of varied pitch and duration,and most importantly can easily alter the source (and listening)location. This human moving tone generator uniquely reacts to theresults of its own output. No loudspeaker has this ability to moveabout, provoking and reacting to room response. Short pitchedtonal bursts such as grunts, barks, or vowels, can very stronglystimulate the booms honks and rings. Because of the variablestimulus location, we can identify exactly where they live.Labmeter can show the frequency. Handclaps are excellent forfinding Flutter echo, or single slap echoes, again by moving into theactual paths and hot spots.You might want to be alone when doing all of this! A sound absorbent panel, temporarily held in place by a friend ormic stand, can categorically prove the paths of these flutters andsuch. This will coincidentally show exactly where to put treatment.Lastly, a caveat; some software graphs allow clicking on, say apeak, to identify it’s frequency. The resulting decimal point readoutsuggests great accuracy. I have found them to be way out, theyare not meant to be accurate. For example P-z shows a 10 percenterror at 100Hz. I wouldn’t use these readouts while mixing or tocreate a Room Eq. for instance.

How to do it I am assuming a rectangular room, with speakers at the narrowfront wall. Identify the zones at 3/8 (38%) of room length from thefront wall and ditto from the back wall. These zones mathematicallyshould have the best balance of room modal activity. Another ruleof thumb suggests there is little Low Bass at the room centre.These are useful, often but not always correct, guidelines.However Measurement always trumps Theory. Use these guidesonly as suggested starting points. Prove by measuring. Be ready forsurprises. Using masking tape, label the floor at all significantlistening spots, e.g. Engineers seat, Producers seat, Couch. Usedescriptive names and numbers for your chosen spots. I use nameslike L38FC (Left Speaker Front Centre), L38BL (Left Speaker BackLeft) and so on. Establish your own system and stick to it. 8 or sospots seems appropriate in a small room. Don’t be afraid to changeyour spots in response to the graphs. Mount the Microphone or



SLM on a stand or tripod. Seated ear height is good. If you use aMixing Desk and like to prowl around, then include standing earheight. Mix and match heights if you like, but do remember to usefully descriptive labels. I tie a thread with a small weight to themicrophone or tripod. The dangling weight over the marked floorspot guarantees repeatable location and height.

Controversy There are conflicting views on the following, which gives rise toconfusion.This will all clear up shortly as my world domination plan takeseffect.

I point the mic directly at the tweeter. This will give the best resultin the vast majority of situations. You may see mics pointed at theceiling, particularly in the USA. The issues behind this are quitecomplex so I have dealt with them herehttp://www.gearslutz.com/board/7257133-post1.htmlSimply put, if you have a normal Free Field measurement orrecording mic, point it at the tweeter. If you are using the internalmic of a USA ANSI standard Sound Level Meter, or any otherDiffuse Field Mic, point it at the ceiling. For Frequency Response and ETC drive one speaker only. (Or 1.1with the Sub)For Waterfalls and Decay measurements drive both or all speakers.This is more likely to get all the modes going. A Y lead may beneeded, I use a Mono switch on my controller. You could drive themodes even better by placing the speakers in the floor tricorners. Feel free to use the graph tools. I like to use smoothing to viewtrends, e.g. the slope of a curve, the biggest anomalies in aWaterfall. Without smoothing HF on graphs can be an unhelpfulmess. Later on, when searching forensically for individualanomalies, use little or no smoothing, in combination with zooming. On Waterfalls, limit the Frequency Range to 300 Hz to see themodes clearly.

TipsA human body close to the mic causes strong midrange errors. Upto 6dB around 400Hz. So stay at least a metre away from the mic.Similarly keep the mic at least 1 metre from surfaces. If you see a suspected problem, e.g. a dip or peak perhaps causedby a reflection from your desk or floor. Try temporarily covering thereflecting surface with an absorbent or diffusing panel. If theproblem vanishes, your suspicions were correct.

Some of the default view settings can be a bit odd so I recommendover-riding them. Adjust the Window setting to the same length as your Sweep andGraph.For Waterfalls set the duration to say 1000mS for an untreatedroom, 500mS for treated. Tweak to fill the screen nicely. Thelowest visible level threshold interacts with this. Set it to say-50dB. Again tweak to fill the screen nicely while ensuring that theend tails of the Modes submerge nicely down into the noise floor. Be aware that many mix rooms are quite noisy due to fans andsuch. 45dBA is common, so use that lower limit control as you



would a noise gate.

How to view itOptimistically! Take a look at the Frequency Response, third octavesmoothing. Ideally this should be a nice even slope from say +3dB around100Hz to perhaps –3dB around 10K or so. Next restrict the viewing frequency range to say below 300Hz.Lighten thesmoothing or turn it off entirely. Observe the Peaks and Nulls. Remember 40/80Hz is around the low E on electric Bass and electricGuitar. Kick Drum and Bass action is hot from 40-120. Now let’s expand the spectrum again, say up to 18K. Look for signs of comb filtering. This will have a repeating series ofdips and spikes, thus the comb. Experiment, use the view changing tools ofSmoothing and Zooming, to see right into areas of interest.

Don’t panicThe curves almost always look awful. Frequency response graphsshow scary peaks and dips. Waterfall plots will often show one verylow extremely long decay accompanied by a confusing array ofpeaks and dips up through the spectrum. For some perspectivelet’s take a real world example.

This room was from hell. It has mostly concrete surfaces, it isasymmetrical, and worst of all it has alcoves. It is a tuned indoorswimming pool. The Green Before curve shows the room with elementarytreatment; the Red After curve is with much more consideredtreatment, of considerable quantity and quality. We did the lot;four corners, alcove corners, Ceiling Cloud, RFZ. Sadly, the Aftercurve has a very similar shape to the Before. Disappointing andvery common. Let’s look closer focussing on the musical crucial 100Hz zone. Here we find a broad 6dB improvement. Now, consider ifyou were to eq a full mix with such a broad 6dB boost….. This is abig and welcome change. This room is used every dayprofessionally, with great success. Perhaps the curves generatedby software are not great at fully describing a sonic experience. Asurvey of Engineers gave this a 9. The graphs however deliver veryclear warnings. E.g. Be careful of Bass decisions in the 100Hz zone.Try to find workarounds. Find the listening spot spots in the roomwhere the graph is flattest. Perhaps refer to top qualityheadphones to hear the mix without any room or speaker.

Octave Reverb Time Measurements varied a lot with position, some



were zero. The modes were so clearly so overpowering that theseDecay views were of little use.

The Waterfall plot told another story. On a cursory glance theAfter again looked very similar to Before in shape, just generallyshorter. Not very impressive. However, with focus we find the good story. Before, the Waterfallshowed a 1.3 Second long boom at 37Hz. After, it was reduced to0.7 S. Go SuperChunks. Sonically, this changed a Kick drum from achest massage to a nice ‘subby’ thump. Note there is almost nosign of this huge anomaly in the averaged Frequency Responsegraph. Waterfalls are IMHO the most useful of all the graphs.

ConclusionsSoftware alone cannot evaluate a listening space. It does howeverbestow great certainty when making comparative choices. Whentreating a room there is simple clear advice on the usual websitesas to where to put treatment and why. They all agree on thebasics; Broadband or Bass Traps in the corners, a Ceiling Cloud,Side Reflection points. This is not voodoo and it doesn’t changefrom room to room. Treat the Room first, then use Software tomake comparative choices such as best speaker position, bestseating positions, best speaker eq settings, etc.

Dan FitzGerald AMIOA MAES

Irish AcousticsSound Sound - Homepage

Last edited by DanDan; 24th August 2010 at 02:30 PM.. Reason: Little updates

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