by Pat Brown
Pat Brown shows the process of collecting room measurements in Sunrise Auditorium.
The Convention Center at AES-Brazil included a 2300-seat auditorium. It is an architecturally interesting space constructed in the 1970s. The fluted ceiling structure is concrete and the room is subjectively very live. Local consultant Jose Giner offered to meet with us before the seminar one morning to capture some room measurements. He provided a dodec/amplifier to use as the sound source. I provided the microphone and recorder. This panorama helps convey what the room looks like. It was shot with center stage as “on-axis” (click to enlarge).
Measurement Condition Challenges
Peter Mapp’s “First Law of Hoovers” states that the room will be the noisiest at the very time you wish to measure. While there were no “Hoovers” here, there were plenty of other noise sources.
The first was the background noise level. Continuous noise emanated from the HVAC system. There was no option to turn it off. Sporadic noise came from the adjacent show floor. The early morning measurement time helped, but there was no way to have a quiet room for our tests. The second obstacle was the low output level of the dodec source. It was designed for studies of small, quiet studios. This was a large noisy auditorium. We drove the amplifier to just short of clip with a swept sine wave, which produced the maximum possible energy into the room for the testing.
With room testing you must play the cards you are dealt. Content that we had the best available scenario, we chose three measurement positions and recorded a 14-second sine sweep at each. The total measurement time was less than 7 minutes. The Total setup time was about 15 minutes. So, in less than 30 minutes we had good, directional RIRs for this space at three test positions.
We could have achieved a higher SNR by several means:
1. Use a higher output dodec.
2. Use a higher output directional loudspeaker
3. Measure at 2 am.
Wallace Sabine used Option 3. One must weigh the benefits against the effort and expense. The measurement process is always guided by “Why am I measuring?” and in every case “less than perfect” data is a much better option than “no data.”
The Recording Rig
The recording rig was a TetraMic and 4-channel WAV recorder. The 4-channel sweeps would be post-processed into the desired format later. I travel with a photographic tripod that doubles as a mic stand. I fabricated a mic holder from bar stock that extends the microphone well-above the tripod. There is a magnetic mount at the top that fits several mic clips (TetraMic and standard measurement mic). The other end fits the quick-release tripod head, allowing me to quickly switch between microphone, camera, etc.
This is the measurement kit that travels with me everywhere. I can find everything in it with the lights off. It is simple, compact, and has never failed to make it through airport security (although I have had to break it out for inspection a few times).
I post-processed the files by convolving each recorded sweep with an inverse version to create the RIRs, using MltVlvWCP – a multi-channel convolver used here as a deconvolver. I then used ReflPhinder to analyze the B-Format files. ReflPhinder and MltVlvWCP can be found as a package at www.catt.se. The beauty of working with WAV files is that you can open the them in any mainstream measurement program. If you take this route, use the XY-Stereo versions, as the B-Format versions require some special processing to yield meaningful results (which is what ReflPhinder does). Since the deconvolution process does not obtain time-of-flight (TOF) information, I added it manually by using a laser range finder to:
1. measure the physical distance to the source,
2. convert to time in milliseconds
3. correct the TOF in a wave editor (I used Wavosaur).
All three test positions were on the main floor, as I felt that the signal-to-noise ratio (SNR) would be inadequate in the balcony. TP1 and TP2 are in the center aisle with dodec center stage. The distance is indicated in the file name. TP3 was 45-degree off-axis and at 65 feet.
The B-Format files are especially powerful in ReflPhinder as they allow the the investigator to produce and listen to the RIR from any angle around the microphone (hence the name of the program). Both B-Format and XY Stereo RIR WAV files can be downloaded from the SynAudCon IR Exchange – Project #47. You can have a listen with your favorite test tracks using GratisVolver.
The RIRs produced from the sweeps are quite usable, in spite of the poor signal-to-noise ratio. Both the direct field, early reflections and late reflections are well out of the noise floor. The dynamic range of the RIRs approached 60 dB – not bad for some quick sweeps from a low-level omni source in a noisy, cavernous room. This is more than adequate for both Reverberation Time and Early-Decay Time measures. I like to have higher signal-to-noise for convolutions, as there is an audible low-level “birdie” in the late decaying tail when the WAVs are convolved with speech. There are some signal processing tricks that can be used to remove this, but it is easier to just recognize it as an artifact and ignore it.
So why did we measure this room? There is something be learned from every space that we enter. They are all unique. We enter, we listen, we walk away, not retaining what we heard in the space. With these measurements, I can re-enter the space at any time and listen some more. I can compare it to other spaces for which I have done the same thing. Any discussions as to the cause of “what it sounds like” can be investigated. Like the photographs, measurements are a way to preserve the experience of being in the space.
Just as athletes and musicians must “keep up their chops” to be able to deliver on demand, it is necessary for audio practitioners to measure often in order to measure efficiently. The process makes us better, and it makes the tools that we use better. The discussion moves from “subjective” to “objective” when we have data. I will close with some of my favorite quotes from Lord Kelvin. pb
“To measure is to know.”
“If you can not measure it, you can not improve it.”
“In physical science the first essential step in the direction of learning any subject is to find principles of numerical reckoning and practicable methods for measuring some quality connected with it. I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely in your thoughts advanced to the state of Science, whatever the matter may be.” [PLA, vol. 1, “Electrical Units of Measurement”, 1883-05-03]