Basic acoustic measurement primer v3.0

[DanDan]

Acoustic Analysis Software Primer V3 May 2012

NOTE- This is a Work In Progress, it will be altered frequently so try the Refresh on your Browser.

Room Analysis Software of decent quality has become very affordable. This has led to many attempting to use it to evaluate their studio or other listening room. The immediate results can be confusing and disappointing. To get the best from any tool, some understanding of the principles and some operating skills are needed. This article is intended as an introduction to the topic and a simple primer on how to get started measuring. We will then move on to viewing and ‘interrogating’ the Measurements.

IR Measurement
A Simply put the plan is to Play back an ideal sound Stimulus and Record what the room (and speaker) does to it. Play and Record, what could possibly go wrong?
A Balloon bursting generates a pretty good Impulse. The expanding wavefront contains the leading edge of a large span of frequencies. All of which started simultaneously. A starter pistol is good too. Electronic stimulus is obviously more convenient and controllable. So we Play a Sine Sweep from 10Hz to 20KHz.
All frequencies of interest slowly stated over a time span. The Software is capable of transforming this back into All Frequencies stated instantaneously, i.e. an Impulse. This is Played loudly, well clear of background noise. We Record this sound in the Room. This is a combination of the Stimulus, i.e. all Frequencies of Interest, and the Room’s Response to them.
Magically we now remove the initial stimulus. This leaves us with only the Room’s (including the Speaker) Response. We have a map of what happens to any sound in that room. These Impulse Responses are the same as those used in Convolution Reverbs such as Altiverb. Indeed Arjan explains the IR really well in the first two minutes of this video Audio Ease Altiverb. The original, professional convolution reverb.


Let’s Roll
Decent full range Mic preferably Omni. Phantom Power on. Connect to Interface or Preamp/Line In, say Channel 1, Left. Boot the software. Select Input from Ch 1. Find a Level Meter in the Software. Shout at the mic from a foot or so. Adjust the mic pre so that the Meter hits 0dB Full Scale. If this is not possible, something in the computer is blocking or boosting. Look into Sound Preferences or Drivers in your OS. Set any/all available Input Level faders to 0dB, Nominal, or Full On.

We have established Input for Recording. An everday task.
Now to Playback. We wish to use the other Channel to avoid confusion or acoustic feedback.
Select Output Ch2 and connect the physical Out to the Right Speaker.
Set the Sweep 10-20,000Hz over say a 10 Second period.
Start with Volume/Monitor Control very low. Hit Measure and listen for the Sweep.
Try a series of sweeps with increasing levels until it becomes uncomfortably loud.
Then use earplugs or sealed cans to protect your ears and push on upwards carefully. Watch for clip indicators on your amp or active monitor.
If you have a Sound Level Meter we are hoping for around 90dB C Slow.
Keep an eye on the Software’s Input Meter, tweaking the mic pre gain if necessary to get somewhere close to Full Scale.
The Frequency Response or SPL graph should read peaks at say -6dB or so.
Exactly as we do in everyday Recording.

That was it. It’s done. We have a Measurement. Now we can view it using all the Tools at hand. The IR itself nor subsequent sweeps do not get changed by these Views. Play with the obvious ones, Zooming, Dragging, Smoothing, all of it. RTM on and off. The Scroll Wheel can be a bit wild in REW. Set Graph Limits by hand instead.


What to expect
Horrible looking graphs! It takes an experienced brain to evaluate how a room sounds or errs, and what to do about it. Software is not the only measuring tool. Sine Waves, Tone Bursts, Vocalising, Handclaps, Starting Pistols, Pillows, Stopwatches, Balloons, are all useful in the experienced hand. Let’s not expect a health check and a treatment prescription from Software. We see many graphs posted with the question, ‘how does my room look’. The answer is ‘wrong question’ or ‘squiggly’. These are tools not room doctors.

Viewing the Graphs
The software can have strange default figures for Graph Extents. Let’s bypass those.
For Y axis levels lets try 95dB or 0dBFS for the top to correspond somewhat to actual SPL of the Speaker Playback. Let’s say 40dB for the bottom to correspond to ambient room/machine noise.
Smoothing, Zooming, and Frequency Span are all interactive on the X axis.
A Full Range Frequency Response graph will look crazy at the top end with no smoothing. Use 1/3 Octave or whatever you fancy to get a sense of perspective.
Many of us favour a slope downwards to HF.
When Zoomed In to smaller spans, e.g. the typical 20-300Hz LF, or some particular anomaly up higher, no smoothing obviously best revels the details.
Again, these Viewing controls are like those on a Microscope. They do not affect the sample.

Frequency Response graphs hardly need an introduction here.
However, this is a static view of a moment in time.
Sound decays at different rates at different frequencies. Thus the spectrum changes with time. There are various graphical ways to show this. Decay in REW is literally snapshots at intervals. Cumulative Spectral Decay or Waterfall is pretty similar, showing energy accumulated in the gaps between slices. There is also the Sonogram. All show the varying spectra of decay originating from a hypothetical perfect sound. An Impulse. Remember, all those frequencies of interest, starting simultaneously?
The Waterfall appears like a series of Frequency Response snapshots taken at short intervals. Like a cartoon, these snaps portray action over time. This vividly shows how modal resonance rings on way beyond the initial stimulus. Such modes cause individual Bass notes to pop up loudly, while others without this boost become inaudible. Similarly a null caused by a destructive reflection can make a bass note inaudible. Such short term reflection nulls do not change over time and will appear as a void in the spectrum. These boosts and nulls make mix decisions regarding LF, where most of the energy in music resides, very unreliable. Good Headphones can help.
The ETC shows reflections from the room boundaries, the desk, or Diffraction from the edge of a speaker box. The level and arrival time of reflections show if they are welcome or not. Very broadly speaking it would be nice not to see any above -20dB during the first 20mS. As the reflections are directly visible over time, Decay Rate and ratio of Direct Sound to Reflections and the later room noise are readable.
Room Decay graphs such as EDT, T20, T30 etc. are easily ruined in small rooms by modes. A single mode may be only a few Hz wide, but linger for a whole second or more leading to wild Decay figures. REW has a Topt feature which tries to address these difficulties. It’s readings seem to be generally close to T30.
There are other ways of viewing the information gathered at Measurement. Given time and experimentation, one learns how to interpret these different views of the room sound. More importantly, Graph changes caused by moving a speaker or the listener position are very easily evaluated.

The Microphone
Any multi-pattern condenser switched to omni will work fine. The frequency response bumps of say an 87 or 414 are tiny compared to the 30dB anomalies we find in real rooms. I recommend jumping in. Get a demo of the software and go for it with whatever mic you have at hand. In time, if you develop a liking for measurement, 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 and stable frequency response. Unfortunately this is often at the expense of very poor noise figures. There are a couple which are quiet enough for recording. DPA and EarthWorks. At the cheap end I favour DIY XXX or ones that come with a Calibration file. Dayton.
Sound Level Meters are very useful in the studio and elsewhere. It is good to get a feel for 60dBA SPL or 90dBC Slow.
However I absolutely do not recommend using a cheap Sound Level Meter as a measurement microphone. They have caused problems for many here.

Acoustic Analysis Software
ETF and it’s current variations was one of the earliest programs. The Acoustisoft website has great walk-through tutorials on actual uses.
FuzzMeasure Pro is a similar product for the Mac. This one is very easy to use. It has sophisticated averaging of measurements taken at different positions.
REW is very comprehensive, which results in a learning curve. It costs a nice round figure, 0, and it has a very educational manual. REW runs on Java. Apple do not implement Java correctly so there are problems with external interfaces.
But the onboard Mac Line In/Out work fine.
ARTA is highly regarded. Smaart is widely used in the Live Sound Industry.
There are integrated packages of Mic, Interface, Software. OmniMic. XTZ.
Some of these packages extend to include Digital Room Correction.

[jhbrandt]

Excellent, Dan.
Cheers!

[avare]

+1! Great! Even better than before! Thanks!

!,
Andre

[Glenn Kuras]

Outstanding Dan!!!

[Weasel9992]

Fabulous Dan!

Frank

[jd_]

Thank you!

I am just entering the devilish world of room acoustics and this will help me a lot.

[Ethan Winer]

Originally Posted by DanDan

Room Analysis Software Primer V2
Substantially Revised Feb 2010

Sticky! Sticky! Sticky! Sticky!

[Jeffrey Hedback]

Bump...Sticky!

[DanDan]

Thank you gentlemen for the kind words. The work was all in V1. However one learns, so this one is hopefully a lot more useful. You will of course recognise the substantial revisions. They are direct results of our discussions here, which kinda says a lot. Hopefully this is now more of a consensus with the aim of minimising confusion amongst the target audience. There is no 'right' way to do this, but we are gradually establishing one.
Best Regards, DD

[jhbrandt]

BUMP!

I just wanted to get the back to the top.

[Heavyfunker]

You rock dude! great work and thank you for this thread!

[rosevision]

+1

[tonio]

DD,
Nice primer.

Question on driving both speakers for waterfall / decay measurements. How do you do that in Fuzzmeasure? You can only set up 1 channel at a time.
Do you just take 2 readings (L & R) and average them?

How to set up for ETC ? A full sweep or a short chirp, @ what frequency?

Thanks!

[DanDan]

Hi Tonio, thanks. Chris has good reasons why he didn't include two speaker drive in FM. You can live without it just fine. Two speakers should excite the modes in the room a bit more evenly. One could be in a null, but two is less likely. More even mode stimulation should give better Waterfalls.
Two drive is not exactly the same thing as two individual responses averaged. Two speakers driven will fight somewhat, just like in real life with music!
I harbour the opinion that Room Measurements should ideally be done with two speakers running BUT with a Dummy Head Binaural Microphone recording in Stereo. FM can do this. This should give results more relevant to how and what we hear. I currently simulate this by moving the mic about half a head distance to the Left and Right of my measure spot, and repointing it directly at each tweeter. Ears.

Finally to answer your question :-) I use the Mono switch on my speaker controller to drive both speakers. If you don't have that facility, make a Y cable. For ETC drive one speaker. Full range sweep. For all measurements it can be worth trying longer sweeps, say 10 Seconds or/and synchronous averaging. Better S/N ratio, important for EDT.

DD

[tonio]

Thanks DD! Suppose I missed the mono switch thumbsup Makes sense now. Does it matter on the smoothing algo in Fuzzmeasure? I've read its best to use 1/12 octave in FM? If using 1/6 octave (as in your example) it really smooths it out, but if switch to 1/12 octive the peak/nulls get deeper.

Also, (in FM v3) Chris has the impulse response on the bottom of the GUI. Is it recommended to move the start point to the initial amplitude reading? Moving the starting amplitude changes the frequency response (somewhat).
Its like a science in just reading/setting it up.

T

[DanDan]

tonio,
No single smoothing choice will suit all purposes. Thus the choices.
I regard smoothing as similar to zoom tools in a DAW or Sketchup. Zoom out for perspective, to see an overall picture, zoom in to ferret out some little anomaly. At LF it is necessary to use little or no smoothing in order to fully see the peaks and dips. At HF, the same setting results in an almost unviewable mess with hundreds of dangerous looking black lines. Not realistic. Use whatever smoothing delivers the most informative view. Ditto the frequency extents. There is little point in viewing modal activity over say 300Hz on a Waterfall.

A perspective comes to mind, how does this picture relate to the sound, the tonality, how does that sound look, or how does that picture sound.
The ear has a finite resolution. Like pixels in a picture. Nothing smaller than a pixel can be distinguished. Subject to ongoing review, but historically and with good reason, this has been regarded as one third of an octave. I use 1/3 view to get a sense of the tonality of the room. Flat would be quite harsh. A nice downward slope towards HF works well in a small dead non diffuse room. Without smoothing these trends are not easily visible.
Also, without smoothing we can see all sorts of spectacular anomalies, e.g. vicious looking comb filter dips. These often have extremely narrow bandwidth, rendering them relatively insignificant. In that case I regard a smoothed view as closer to audible reality.
But obviously if I were chasing the source of the combing I would turn off the smoothing and use graphical zooming to completely over emphasise the anomaly I am working on. A magnifying glass.
DD

[DanDan]

tonio, I kept this deliberately separate. I recommend that you address FM questions directly to Chris. He is very interested and responsive. John of REW is similarly extremely helpful.
In FM you can Create a Minimum Phase Copy of your Measurement.
As far as I can make out this is always a good idea. It moves the IR and ETC to zero. It also improves the clarity of Waterfalls. Perhaps other benefits.
When doing quick comparative tests I just shoot and view, move along.
When digging in, I always use that MP function.
Perhaps it should be the default behaviour.
DD

[tonio]

Dan,
Thanks for all the insight! I am sure myself and fellow GS' can use all the information to reach audio nirvana.

You are an asset to this community

T

[Rod Gervais]

Dan,

First time I ever saw this - a fantastic job........

Rod

[DanDan]

Thanks Rod. And Heavy and Rose.
DD Blushing but chuffed.

[miner]

I just started treating my room ... lots of jigsaw figures on my screen ...

This post is great, it's a ray of hope

Many thanks DanDan

[Jens Eklund]

Top effort DanDan!

I would like to include the mention of perforated panels as a very effective solution (if applied correctly) for the trickiest bass problems (but also the midrange when you don’t want to absorb the highs).

Sincerely Jens Eklund

[Brian Cares]

Hey guys,

I just posted a thread about this:

http://www.bbc.co.uk/rd/pubs/archive...icpractice.pdf


But i thought it also deserved a link in this sticky.
The document contains the bundled knowledge of the BBC and includes detailed plans and every information you need to build and treat a studio to perfection.

Pure gold

[dave999z]

first, very helpful stuff. thanks a lot for doing this!

second, my only mics are a studio projects C1, an SM57, and an SM58. i presume none of these would make a good measurement mic. are there any affordable omni small diaphragm condensers worth getting (i.e., that would serve their initial brief purpose for room measurement but then actually be useful for recordings down the road)? perhaps in the price range of an avantone ck-1 (approx. $150)? if anyone has thoughts on a good option, that'd be great!

[DanDan]

The Avantone specs look very good. Remarkable value. Take note of the HF boost in omni when viewing measurements. OR, write a correction file from the published frequency response.
Assuming this is made in China (or maybe Burma ) I personally avoid buying such products as much as possible. If everyone continues to support this, i.e. direct copying with no R and D, there will be no new inventions or genuine products. How much is that concept worth?

DD

[kidcarbon]

Thank you for that piece Dan.

I am having trouble using REW.

I have an old Terratec EWS88mt sound card that produced this wonderful looking flat response during calibration.


I then set up a CAD e70 (omni) > focusrite ISA One > EWS88mt > Room EQ Wizard. This chain yielded these graphs from one sweep, at one listening position.


I am having a really hard time interpreting this stuff. I can see the +8db peak at 50hz as well as the -8db null at 60hz. Everything past 300hz seems like nonsense.

I understand it would probably be best, just throw up as many bass traps as money can buy, and then things will be 90% ok (maybe). I have been following the teachings of Winer, Kuras, Brandt, Andre, and now Yourself, but i just can't seem to wrap my head around what to do with this info.

Bottom line is i would like any suggestions on a remedial route to sweep/impulse interpretation. I would like to look at these graphs and say "hell yeah this room is awful" and then do my best to fix it with appropriate treatment. After treatment, i would like to look at this data and say 'ahhh, that's what that fiberglass is doing."

Before i start exploring all the 8+ possible listening positions for peaks and nulls i would like to truly understand what i am looking at in one position.

Thank you,
-Nick

P.S. The Master Handbook by Everest is light years over my head.

[kidcarbon]

im unsure how to get those fancy images that most people have posted. here are the links to the three images on my previous post.

EWS88mt Calibration

Sweap Measurment

Waterfall Graph

Thanks again,

-Nick

[DanDan]

kid, you have illustrated my main point perfectly.
i.e. These Softwares do not answer broad questions like

How bad (or good) is my room?
What treatment do I need and where to place it?

Having said that, I reckon that waterfall is not at all scary. The full range graphs always have too much information. Use third octave smoothing if you want to see the tonal slope or trends. Use full resolution and zooming to examine particular details. If you drive both speakers there is no point in viewing full range. All you will see is the result of a battle fought with sharp comb filters.

The room treatment and setup articles at RealTraps and GIK will get you there.

DD

[kidcarbon]

Hello All,

First off, thank you to Lupo and DanDan for the pointers and references.

After 3 days of reading hundreds of posts and using REW, i think i finally got over the first foothill. (only 10,000 more feet to go)

The room dimensions are as followed: 24' x 20'. Although this a basement room, the walls are covered in raw ceder planks. the floor has one small area rug at the listening position while the rest of the floor is concrete. the ceiling is a standard grid style drop ceiling. No acoustic treatment has been employed.

The mix desk is centered along the 20' front wall. The monitors sit about a 1' off the wall. The listening position is 5' from the wall

The graphs included in this post were all done with one channel (left). The SPL "c weighting" box was un-ticked. and the loop correction for the sound card calibration was completed.

I did not calibrate the CAD e70 mic that i used because i had no .cal file. from what i have read, calibration is an insignificant gesture, when compared to the modal and spectral anomalies of the room.

I will now make a feeble attempt to extrapolate data from these graphs. i did the best i could to massage the "windowing" to meet the requests of so many senior posters.

THE FREQUENCY DOMAIN

The Frequency Response graph and Waterfall graph are telling me that there is a huge peak in the room at 60hz and 160hz. Looking further into the waterfall i can see that the modal ringing at 60hz is lasting for about 1 second. I think this is a significant finding. The numbers tell me to start adding bass traps to the corners. (everywhere???)

The second really obvious assessment, is the null at ~105hz and 120hz. From most of what i am reading, except for Ethan's posts, I can do nothing about this except move the listening position forward or backward, an inch or two, and try again.

THE TIME DOMAIN

The ETC graph is telling me, via the measuring tool, that the first significant reflection had to travel 2.7 additional feet in order to reach the microphone. This means that a reflection point 1.35ft away from the microphone, is causing this reflection.

Well i got out the trusty measuring tape, and the closest surface i could find at 1.35ft was the desk the monitors are perched on. (it was actually 2 feet away)

The 7ft ceiling was three feet away. I am assuming that the ceiling is the cause for the next reflection seen at about 3.75msec. (going by the math i would expect the ceiling to be ~2 feet away if the total extra distance traveled for the second reflection was 4.21ft.)

Questions:

Is there any ideas on on my modal issues? Peaks and Nulls.

Am i approaching the ETC interpretation correctly?

Am i in the ballpark for locating reflection points or is my math and logic wrong?

Thank you,
-Nick

[elan]

whoa a room with a 12 db or more dip is used professionally everyday? Interesting! So my room is better? :D

Would be interesting to see a not averaged to 1/3 frequency response graph, I would like to see the comb filtering :P

And also a L - R measurement to have an idea of the symmetry

I'm looking for pro studio freq response all the time!! Just to have an idea of what is a good result

Thank you