Tips & Techniques:Room Analysis Primer

How does that sound look?

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 skill and understanding is essential. This is intended as a simple primer on how to do it, and how to view it.

What to expect.
No miracles! It is unlikely that you will get beautifully flat frequency response curves. I find that Software is best at showing changes rather than evaluating a static situation. It excels at comparative jobs, such as finding the best speaker and listener positions by trial and measurement. Frequency response is the most common and easily understood graph, but there are other, perhaps more powerful ones. In most rooms, there will be particular frequencies or bands of frequencies which ring on longer than their neighbours. This causes particular notes or tones to stick out, blurring music or speech. This behaviour can be seen very graphically on a Waterfall plot. These graphs have three Axes:
Horizontal is Frequency, in this case restricted to 20- 600Hz.
Vertical is Sound Level.
The third axis describes time starting from zero at the back. Consider the spectrum at 0 as the instant when the noise is suddenly turned off. The ‘slices’ coming towards the viewer are spectra taken at later instants. An ideal room would have very even decay, longer at low frequencies, gradually shortening towards the highs.

Room Analysis Primer-waterfall.jpg

Given time and experimentation, one learns how to interpret these different ways of visualising the room sound. Changes however, can always be immediately seen and appreciated.

The Microphone.
You may well have a suitable mic in your closet already. A 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. Measurement mics are omnidirectional Small Diaphragm Condensers. These are optimised to have a very flat frequency response, unfortunately this is often at the expense of very poor noise figures. I recommend spending a little more on an omni SDC which is also quiet enough for recording. DPA and EarthWorks spring to mind here. I have found the DPA 4060 lavalier omni to be the single most useful microphone in my studo. Most Sound Level Meters have an AC OUT socket. This socket has a pre-amplified line level output from the SLM’s inbuilt microphone. This output is useable for comparitive measurements. You could even borrow/hire a perfectly flat mic and use the software to generate a calibration curve to correct this output to flat. The RadioShack SLM is popular historically, but there are many options these days with similar low prices. I use an Az one with a very large display. I strongly recommend picking up an SLM. You will discover all sorts of uses for them, plus they are fun and eductational.

Measurement and Evaluation Tools.
ETF5 is a very affordable PC program which most directly addresses our needs.
FuzzMeasure Pro is a similar product for the Mac. There are many other analysis tools, from free to extremely expensive. I strongly encourage supporting the very reasonably priced professional offerings mentioned here.
I use several tools and helpers simultaneously to achieve a holistic evaluation of the room. Here are some that I find invaluable.
A CD of reference Tracks. I bring mine with me everywhere.
LabMeter, a Mac Frequency meter from rustykat.com
A Room Mode calculator such as this one http://www.bobgolds.com/Mode/RoomModes.htm
The free MiniRator and Test tones at RealTraps - Home
The SoundCheck CDs by Alan Parsons.
The Signal Generator plug-in in your DAW or even better a real one.
Oneself- the body can generate tones of varied pitch and duration. Uniquely this human tone generator can move about the room, tuning in to, and stimulating hot spots. Humming barking or grunting at varied pitches can provoke the booms honks and rings, plus identify exactly where they are. Labmeter can show the frequency. A sound absorbent panel, temporarily held in place by a friend or mic stand, can nuke these problems, also showing exactly where to put treatment. You might want to be alone when doing this! This last process may seem humorous, it is, but it is very powerful. Lastly, a caveat; some software graphs allows clicking on, say a peak, to identify it’s frequency. The resulting decimal point readout suggests great accuracy. I have found them to be way out, for example P-z showed a 10 percent error at 100Hz. I wouldn’t use these readouts while mixing or to create a Room Eq. for instance.

How to do it.
I am assuming a rectangular room, with speakers at the narrow front wall. Identify the zones at 3/8 (38%) of room length from the front wall and ditto from the back wall. These zones mathematically have the best balance of room modes and should sound best. Another rule of thumb suggests there is little Bass at the room centre. These are useful, often correct, guidelines, however Measurement always trumps Theory. Using masking tape, label the floor at all significant listening spots, e.g. Engineers seat, Producers seat, Couch. Use descriptive names and numbers for your chosen spots. My software uses one speaker at a time during Measurement sweeps, so I use names like, L38FC (Left Speaker Front Centre), L38BL (Left Speaker Back Left) and so on. Establish your own system and stick to it. 8 or so spots seems appropriate in a small room. Mount the Microphone or SLM on a stand or tripod. Seated ear height is good. If you use a Mixing Desk and like to prowl around, then use standing ear height. Mix and match heights if you like, but do remember to use fully descriptive labels. I tie a thread with a small weight to the microphone. The dangling weight over the marked floor spot guarantees repeatable location and height.
I point the mic directly at the tweeter. 45 degrees upward is more usual Stateside. Whichever you chose, do keep it consistent.
A human body close to the mic causes strong midrange anomalies. So stay at least a metre away from the mic. The software generates very loud noises. Wear earplugs or closed headphones. Start at low volume, try a couple of measurements, increasing the volume until you feel the room is well driven. Watch out for overload lights on active speakers, particularly at high frequencies. If you have an SLM I recommend 85-90dB SPL with Slow and C weightings. Label each measurement and move on.


How to view it.
Don’t panic! The curves almost always look awful. Frequency response graphs shows scary peaks and dips. Waterfall plots will often show one very low extremely long decay accompanied by a confusing array of peaks and dips up through the spectrum. Let’s take a real world example. This room is from hell. It has mostly concrete surfaces, it is asymmetrical, and worst of all it has alcoves. It is a tuned indoor swimming pool.

Room Analysis Primer-fresp.png

The Green Before curve shows the room with elementary treatment; the Red After curve is with much more considered treatment, of considerable quantity and quality. We did the lot; four corners, alcove corners, ceiling cloud, RFZ. Sadly, the After curve has a very similar shape to the Before. Disappointing. Let’s look closer, with focus on the one big issue; the musical crucial 100 Hz zone. Here we find a 6dB improvement. Now, consider if you were to eq a full mix with such a broad 6dB boost….. This is a big and welcome change. A poll of 7 professional sound engineers was done in this room. All aspects averaged, it scored 9 out of 10. Before it was a 6. Perhaps the curves don’t do justice to the great sound and the great change. The Frequency plot does deliver some clear and solid advice though: Review the speaker positions. Be careful of Bass decisions in the 100Hz zone. Try listening at spots in the room where the graph is flattest. Use top quality headphones to help judge Kick and Bass relationships.

The Waterfall plot told another story. On a cursory glance the After again looked very similar to Before in shape, just generally shorter. Octave Reverb Time Measurements varied a lot with position, some were zero. I am somewhat doubtful as to the accuracy or validity of such Reverb Time measurements in small rooms. Again I would only use them for comparisons.
In this roomt he Before Waterfall showed a 1.3 Second long boom at 37Hz. After, it was reduced to 0.7 S. Sonically, this changed a Kick drum from a chest massage to a nice ‘subby’ thump. Note there is almost no sign of this huge anomaly in the averaged Frequency response graph.


Conclusions.
FM and ETF will not tell you simply what is wrong, or how to treat your room, or where to put what. They can however bestow great certainty when making comparative choices. There is a wealth of knowledge at John Sayers Productions, RealTraps - Home, GIK Acoustics., studiotips - tips on studio design, acoustics, and wiring, and here. There is simple clear advice as to where to put treatment and why. All the advice agrees on the basics; Broadband or Bass Traps in the corners, a Ceiling Cloud, Side Reflection points. This is not voodoo and it doesn’t change from room to room. Treat the Room first, then use Software to make comparative choices- best speaker position, best seating positions, best speaker eq settings, etc. Room Treatment will yield spectacular results, absolutely no doubt. The use of Measurements to decide on Positional and other tweaks is the Icing on the cake, not the Dough.

Dan FitzGerald AMIOA

Sound Sound - Homepage
Contributors: mw, Jules
Created by DanDan, 13th November 2008 at 10:04 PM
Last edited by mw, 27th March 2012 at 03:05 PM
0 Comments, 12,129 Views



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