Hi all
It's time for a small updateBeside AES Convention paper, there are finally published paper from local Serbian Conference, free for download (english): Petrovic Bogic, Davidovic Zorica:
A new acoustical design of control room for multichannel production and reproduction, 18th Telecommunications Forum TELFOR 2010, Belgrade, Serbia.
Our (already known) white paper about MyRoom Design can be found
here or
here.
We get a couple of questions via email, and some responses can be good for a topic, and possibly interesting for other people.
About type of wideband absorption behind air transparent diffusers...We wouldn’t describe absorption realization in details, because we expect that reader (skilled acoustician) already has his own experience with “best” method. We always used Tom Hidley/Philip Newell principle (waveguides formed with hanged panels), because we believe this method is best fit to air transparent diffuser. But we don’t like to discourage others, with different experience, to use layers of mid density rockwoll or any other known method.
About depth of wideband absorption behind transparent diffusers...Depth vary from place to place… and to be sure,... we fill with absorption panels any cavity we found in room, with this, we formed one big “bass trap” around listener (half space above floor) . It’s substantial that we don’t have acoustically different “ends” of room… all surfaces except floor are highly absorptive. And yes, before you mount air transparent diffusers, room is completely “dead” and this was deliberately. We noted in our white paper, about volume of room we use for “bass trapping” or wideband absorption, and if you can’t figure out which depth is used… here is more precise information:
- depth vary from 30-80cm (12-32") in smaller room (Pressed Lizard studio), and from 10cm to 150cm (4-60") in bigger room (RES Media studios).
Be aware that in “sub-compact” rooms, with room volumes below 100m3, you can’t exceed some "maximum needed" absorption for low frequencies, because room modes energy are much denser in audible low frequencies region AND, in same time, you have much smaller space (volume) for absorption… because this, low end absorption won’t be “ideal” anyway, whatever absorption method you use and whichever volume you spent for it!
Good thing is that most loudspeakers are omnidirectional below 200-300Hz and deepest bass traps works in some way in overall treatment even if you have it only behind and possibly around (soffit mounted) speakers.
About windows, doors, racks, ventilation ducts…In our white paper we didn’t describe detailed plan of any particular studio, instead of this we described general design principles. This means that you can have windows to recording rooms, day light windows, doors for entrance, ventilation ducts, racks… etc. It’s only important to take care about symmetry, especially front side. Highly diffused rooms are very sensitive to non symmetrical obstacles. Or better, people are very sensitive to non-symmetrical arranged obstacles in highly diffused rooms.
For example, if you have window to your front-left side, but not have one to your front right side, symmetrical to left, that may be a problem. Try to make “blind” window to your right side with same dimensions at your left side (not needed to be real window, may be some picture or illustration behind glass). This helps uniform distribution of reflections to your ears, and not causes frequency dependent position distortion in your stereo image.
Rack, doors:
Air transparent Fractal Diffuser:
About measurements...All frequency response measurements are smoothed to 1/3 octave because International/European recommendations for control rooms ask this:
1. "
Multichannel surround sound systems and operations", AES Technical Council, Document AESTD1001.1.01-10, New York.
2. “
Methods for the subjective assessment of small impairments in audio systems including multichannel sound systems”, ITU-R Recommendation BS.1116 (rev. 1), ITU, Geneva, 1997.
3. W. Hoeg, L. Christensen, R. Walker, “
Subjective assessment of audio quality– the means and methods within the EBU”, EBU Technical Review Winter 1997, pp. 40-43.
4. "
Listening conditions for the assessment of sound programme material: monophonic and two–channel stereophonic", EBU Tech. 3276 – 2nd edition May 1998, European Broadcasting Union, Geneva, Switzerland
...
It's good to be aware that RT60 recommendation is even easier to reach, than frequency response in AESTD1001.1.01-10, because there may be, in RT60 recommended boundaries, “jumps” allowed below 125Hz. Instead of this, in frequency response boundaries we need “flat” response down to 40Hz. Peaks in measured RT60 are highly correlated with room modes, and because this, they are highly correlated with non-linearities in frequency response. This means that you need much better RT60 response if you like to have recommended frequency response. In older recommendations (ITU/EBU) there are wider boundaries at low frequencies, for frequency response, that was easier to reach, but nevermind...
To be honest, we didn’t really expect to build acoustics treatment in such small rooms and to get AES/ITU/EBU recommendations fully satisfied, especially for low frequencies below 125Hz. We are defined MyRoom method with consideration to listeners, mixing engineers, not to measurement microphones only, using all our knowledge and experience of acoustics and psychoacoustics, from books, papers and in the field. Nevertheless, we get a decent measurement results in regards to recommendations.
Basic idea was to get best low end absorption as possible in “sub-compact” room, in all directions, spending largest possible volume of room for this, but still remain practical, and to bring back liveness in room later, with air transparent diffusers. That is our approach to get small rooms treated as best as can be useful for music production.
Measurements came later, when we finished these rooms, and we publish it in our papers.
It’s true that room measurements results are not biggest quality of MyRoom treatment, because there are many, mostly psychoacoustic qualities that we can’t measure directly or objectively. Subjective impressions and mixing engineer experiences are one of our ways we used to define MyRoom method. And is also true that there are many different ways to reach recommended values for RT60 and frequency response, but all this methods don't sounds similar.
Origins...Fundamental acoustical behavior we used for MyRoom Design is ordinary living room, where we live most of our time from birth. That ordinary room is mostly homogenous/chaotic reflective and absorptive in all directions. We believe that this acoustical behavior is a better way to get good and natural environment for mixing engineer, especially for mix translation and time needed for adaptation.
This means, for control room acoustics, an “ordinary” room with highly controlled low frequencies and decent and homogenous liveness for high frequencies, from all directions.
After all, added diffusion is a logical step to bring more details to listeners ears, and make control room for professional, dedicated purposes.
For all diffuse surfaces, except floor, inspiration came from Massenburg Blackbird Room.
For low end treatment inspiration came from Tom Hidley/Philip Newell Non-Environment design.
Construction that link all this and make it possible was "Air Transparent Diffuser" (image below)
Cross section of air transparent diffuser with applied wideband absorber (behind):
About phase grating and amplitude grating diffusers used in MyRoom principle...Our air transparent diffusers are combined phase grating and amplitude grating construction.
Amplitude grating effect exists because space between (different depth) slats forms Helmholtz absorbers with different (random) working frequencies, and slats are already reflective. Because different working frequencies of slat absorber, there are amplitude grating effect (position dependent) for single frequency, in the working range.
Reflection from slat fronts (different depths/position) forming phase grating effect.
But, if someone don’t have a budget to build that type of diffusers, there is a way to build ordinary amplitude grating diffuser instead, binary MLS type, for example, used to bring back liveness in room after heavy and thick wideband absorption and maintain needed RT60. Even if amplitude+phase grating construction is better for diffusion, from our experience, bringing back liveness in a room is important, and when you do this with only binary diffusers (1D or 2D) you still may have nice and pleasing place for work, even without strong diffusion sound field.
To be continued...
Best regards
Bogic
Prispevek uredil/a: Bogic Petrovic, 02 marec 2011 - 02:07 .
