Acoustic Analysis Software Primer V3 May 2012
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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.