RealTraps - How does that sound look?

A Beginner's Guide to Room Analysis

..Room Measurement Basics..

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Figure 1: This FuzzMeasure waterfall plot shows decay time as well as amplitude versus frequency. Click the image to see it full size.
By Dan FitzGerald

I have noticed that quite a number of people are beginning to use Software and Measurement Microphones to analyse their listening spaces. The immediate results can be confusing and disappointing. To get the best from a Room Analysis Tool, some skill and understanding is essential. This is intended as a simple primer on how to do it, and how to view the results.

WHAT TO EXPECT

No miracles! It is unlikely that you will get beautifully flat frequency response curves. I find the software is best at showing changes rather than describing 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. There are other powerful ones also. Particular frequencies or bands of frequencies which ring on longer than their neighbours will cause particular notes or tones to stick out, blurring music or speech. This behaviour can be seen very graphically on the waterfall plot in Figure 1 at left which has three axes. Top

 

 

 

 

"I recommend an omni studio recording microphone, or a Sound Level Meter (SLM) with a line level audio output socket."

 

 

 

 

 

 

 

 

 

 

 

 

 

"A sound absorbent panel, temporarily held in place can nuke these problems, also showing exactly where to put treatment."

 

 

 

 

 

 

 

 

 

 

 

 

 

"I tie a thread with a small weight to the microphone. Positioning the dangling weight over the marked floor spot guarantees repeatable location and height."

The third axis describes time passing starting from zero at the back. Consider the spectrum at zero 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. This 3-dimensional picture is worth a thousand words to the experienced eye.

Reverb Time (aka RT60, EDT, T20, T30) can be displayed in octave or third-octave bands as simple bar graphs. 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

I believe the type of Microphone does not particularly matter for our purposes. I have used the built-in mic on Laptops and iMacs. Omni is best but not vital. Rather than buy a "measurement microphone" I recommend an omni studio recording microphone, or a Sound Level Meter (SLM) with a line level audio output socket. This socket has a pre-amplified output from the SLM's in-built microphone. I would go for a modern SLM with a large screen rather than the Radio Shack model. Sound level meters have many secondary uses and benefits. Also see the article Measuring Microphones.

MEASUREMENT AND EVALUATION TOOLS

ETF 5 is a 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 use several simultaneously to achieve a holistic evaluation of the room.

Here are some other tools that I find invaluable:

An audio CD of reference tracks

LabMeter, a frequency meter for Mac computers

A tape measure and Room Mode Calculator

The free MiniRator and Test Tone CD

A signal generator plug-in for your DAW

Our body can generate tones of varied pitch and duration. Uniquely, this human tone generator can move about the room, tuning in and stimulating hot spots. Humming, barking, or grunting at varied pitches can provoke the booms, honks, and rings, and identify exactly where they are. Labmeter linked above will show the frequency. You might want to be alone when doing this!

A sound absorbent panel, such as a RealTraps MiniTrap or MicroTrap, temporarily held in place can nuke these problems, also showing exactly where to put treatment.

Lastly, a caveat - some software graphs allows clicking on, say a peak, to identify it's frequency. These seemingly exact frequencies are not always correct due to internal math resolution. I wouldn't use them to set a Room EQ for instance. Top

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 such as the engineer's seat, the producer's seat, and the rear 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. Eight 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 remember to use fully descriptive labels.

I tie a thread with a small weight to the microphone. Positioning the dangling weight over a spot marked on the floor guarantees repeatable location and height. I point the microphone directly at the tweeter. 45 degrees or straight up 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 meter away from the microphone. When measuring, 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 response and C weighting. Label each measurement and move on. Top

 
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Figure 2: The horizontal axis is frequency, and vertical is sound level, or amplitude. Click the image to see it full size.

 

 

 

 

 

 

"ETF and FuzzMeasure will not tell you simply what is wrong, how to treat your room, where to put what. They can, however, bestow great certainty when making comparative choices."

 

 

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, plus a confusing array of peaks and dips up through the spectrum. Let's take a real world example, shown in Figure 2 at left. 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.

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 musically crucial 100 Hz zone. Here we find a 6 dB improvement. Now, consider if you were to EQ a full mix with such a broad 6 dB boost. This is a big and welcome change. A poll of 7 professional sound engineers was done in this room. All aspects averaged, the room 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 100 Hz zone. Try listening at spots in the room where the graph is flattest. Use top quality headphones to judge kick drum and bass relationships. Top

The waterfall plots 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 of reverb time measurements in such a small dead room. I will point out, however, that once again, zooming in to an area of particular interest clearly illustrates a spectacular change. The Before waterfall showed a 1.3 second long boom at 37 Hz. After, it was reduced to 0.7 Seconds. 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

ETF and FuzzMeasure will not tell you simply what is wrong, how to treat your room, or where to put treatment. They can, however, bestow great certainty when making comparative choices. Acoustics can be a fascinating and useful study. I find knowledge of how sound behaves extremely helpful in recording. There is a wealth of knowledge all over the RealTraps site and elsewhere. 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, plus side-wall and ceiling reflection points. This is not voodoo and it doesn't change from room to room. Treat the room first, then use software and other tools to make informed choices - best speaker position, best seating positions, best speaker EQ settings, and so forth. Room treatment will yield spectacular results, no doubt. The use of measurements to decide on positional and other tweaks is the icing on the cake, not the dough. Top


Sound Engineer "DanDan" FitzGerald began by mixing at music gigs. This was followed by some international fame for his "hi-fi" studio and location recordings. In the early 1990s his recording of No Frontiers by Mary Black took over from Ricky Lee Jones as the test CD in magazines and at trade shows. DD currently records and mixes Other Voices, a live music TV series. This year Other Voices was broadcast in HD with 5.1 by RaveHD globally. Recorded in a tiny church in Dingle at the western edge of Europe, the series recently featured Ryan Adams and Amy Winehouse. The entire series can be viewed online at Other Voices. Dan is now developing an acoustics and noise control consultancy SoundSound Acoustics.

 

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