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  • 1/10/12 Gearslut].com - View Single Post - Basic acoustic measurement primer v2.1


    WhUead: Basic acoustic measurement primer v2.1VieZ Single PoVW

    25th Februar\ 2010 #

    DanDanLives for gear

    Join Date: Aug 2003Location: Cork IrelandPosts: 5,685ThUead SWaUWeU

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    Room Anal\ViV SofWZaUe PUimeU V2.1ReYiVed AXgXVW 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.

    WhaW Wo e[pecWThere 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. Lets not expect a health check and a treatmentprescription from these programs. They are tools not room doctors.

    The GUaphVI 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.

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    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 itsneighbours 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.

    SoXnd LeYel MeWerSLMs, 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

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    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 Anal\sis 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 its 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 its verysophisticated averaging of measurements. e.g. Take measurementsat several different listening positions, then average them to get abroader sense of the rooms 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.

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

    The SoundCheck CDs by Alan Parsons.

    LabMeter, a Mac Frequency meter from

    A Room Mode calculator such as this one 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 its 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 wouldnt use

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