From PRW forum:
http://prorecordingworkshop.lefora.com/2011/11/03/front-to-back-acoustic-technique/
Hope this will help. They do not feel dead. Some of the smaller rooms are a bit on the dark side, but not dead. They are very quiet though. This can be mistaken for dead sometimes.
"The short story for now:
These are directional rooms (Front-to-Back).
The general idea is to be able to maintain both a very controlled environment for the speakers (so maintain a quasi free-field response speaker to room) AND a natural environment for the engineer.
Which may at first seem to be in complete opposition. But they are both very important factors, that are met by the FTB rooms.
So the main question wasn't really about how to achieve a "flat response" in a control room, the techniques to achieve that being out there for decades, but to define what is a natural environment for humans. And this is central to that design.
The reason for this question is that the auditory system is constantly probing our environment and just like our eyes, adapts to it constantly, judges distance, "colors", movement etc. And the information gathered by the auditory system is constantly put in relation with the visual information.
Where things get a bit more complicated is that the visual information has in a way priority over the auditory system. So when the visual info doesn't match the auditory info, the brain will go to great length to make the two fit by enhancing and 'bending' our auditory system's response. Our auditory system has a lot more room for distortion of info than our eyes have. A lot of studies have shown that.
Furthermore, the audio information is treated in different parts of the brain depending on whether they are Environmental info or Semantic info.
Environmental info are all what has a relationship with the space around us. This is treated by a part of the brain that is running mainly on instinct. So it's reflex based. A bit like your eye's pupils. You only have so much control over your pupil's mechanical behavior.
Semantic info are all the info collected that have a social meaning. Speech in particular. To use the eyes analogy again, it's a bit like the part of your brains that tells you "this is a car" when you see one. It's about the content / metadata. Your eyes in themselves don't know what they see, they just see and adapt to the light etc. Same for the ears.
On a side note, music is sort of in between - to make things easier. It's treated as both a semantic and environmental info constantly - does not oscillate like other events. Argh.
So, when we are in environments that are particular, like a very dead control room, the environmental information (interaction with the surfaces) is somehow missing or distorted - and often the missing information are localization cues (and background noise in very quiet rooms). Even though we are conscious of the deadness / quietness of the room, our auditory system will keep on focusing on some specific areas because of the missing info - it's looking (hearing) for it. Because the environmental info management is based on a reflex routine and not a conscious routine. And keeps on going constantly in a probing loop.
And when it does so, it does not influence a bit our perception, if influences it a lot.
It's virtually like putting a massive multiband EQ in our ears, and more. Especially since our hearing isn't working in a continuous manner (even if we feel it is) but rather will concentrate of what is of interest to us at this very moment.
What came out of all this is that a natural environment is one where we satisfy to all the cues the auditory system needs to function in a 'relaxed mode'. So where all the info gathered are correlating to the visual environment and are clearly defined. The auditory system is then "relaxed" and therefore in a "neutral" mode. It does not enhance certain frequencies or perception methods. You give it what it needs. It's response is... "flat".
And if you manage to do that in a highly acoustically controlled environment then you get the best of both world:
A flat room response with a flat / relaxed hearing system response. So all is set to neutral. Source and receptor.
On the practical side, FTB Rooms work like that:
- Speaker to room response is quasi free field / quasi anechoic. Speakers are flush mount if possible.
- Engineer to room response has to follow a very particular protocol to send the brain information: localization cues. All this without interfering with the room response. Tricky.
For that we use what is called "self noises" interactions.
This is where one of the big differences is between LEDE and FTB btw. I don't believe you need any interaction between speaker and room. I think this is only detrimental. The 'Haas kicker' serves no real purpose to me, only makes thing difficult for the brain. The Haas gap / ISD gap is only adding confusion. Some of you already know that, but I should state it so it doesn't come as a surprise: I'm not a fan of LEDE. It just doesn't really make sense to me as a system. Anyway.
So it's not the music/speakers that are interacting with the systems used to send us localization cues, it's the noises we produce ourselves while working in the room, talking, moving. The "self-noises".
The cues are roughly the following:
Reflective front wall gives distance (wall boundary) + lateralization cues (L/R).
Ceiling diffusors give Azimuth cues and (Ceiling) Boundary cue.
Back wall diffusors give Depth cue / Boundary cue.
The diffusors and systems used are very particular, and their placement is obviously critical, so is the % of the surface covered and to a lesser amount the distances to cues. The design is very rigid in these areas.
Seen from the speaker perspective, the front wall is unseen (FTB uses flush mount). But this requires a very strict room geometry. Ceiling diffusors are recessed in treatment and follow a precise placement that makes them almost unnoticeable in an ETC mesurement and room response, but still interact a lot with the engineer - which is what we want.
The most difficult one is the back wall diffusor that is obviously in the way of the speaker response. Argh.
So again, size, placement, surface covered are essential to achieve the desired effect. Although here, there is a clear compromise, we still manage to have their feedback at sweet spot to be around 5 to 10% of the incident energy, depending on room size etc. So influence on speaker to room response is very limited. Hmmm... Did I just give super-secret data here?
All the rest is broadband and LF treatment.
If time allows, I will go deeper into all that.
When I read my post, I realize it's not clear at all. Sorry guys. It's a complex subject which is hard to explain in a few lines.
I've never said that much about FTB on the Internut though."
