The No.1 Website for Pro Audio
 All  This Thread  Reviews  Gear Database  Gear for sale     Latest  Trending
Which treatment is more important for CR: absorptio or reflection (ISD)
Old 4 weeks ago
  #1
Here for the gear
Which treatment is more important for CR: absorption or reflection (ITDG)

Hi,

we have an acoustic treated CR L/W/H 3/6,26/3,10m
The waterfall diagramm looks ok. There are no major dips. But it is difficult for me to hear ER or the character / size of the recording room.
How important is the ISD gap?
Is there a possibility to get a stronger reflection, or is it a matter of room design?

Thanx
Old 4 weeks ago
  #2
Lives for gear
 
avare's Avatar
 

S
What is your qiestion?

The clarity of the first. signal is important.
Old 4 weeks ago
  #3
Here for the gear
Quote:
Originally Posted by avare View Post
S
What is your qiestion?

The clarity of the first. signal is important.
Thank you for the reply ☺. You mean the first signal in the ETC?
In my measurements I am not shure there is an ISD gap or not.
Attached Thumbnails
Which treatment is more important for CR: absorptio or reflection (ISD)-etc.jpg  
Old 4 weeks ago
  #4
Here for the gear
In a RFZ concept, should there be a strong first reflection from the back after 20ms?
I have a lot of absorption in my room, maybe there sholud be more reflective areas?
Would this improove the ability to hear the room dimension in the mix, the depth of the mix?
Old 4 weeks ago
  #5
Lives for gear
Quote:
Originally Posted by Amadeus45 View Post
In a RFZ concept, should there be a strong first reflection from the back after 20ms?
LEDE or RFZ+LEDE is the design criteria that has the Haas kicker. It needs to be diffused, so not just a specular reflection, but a short ambience starting around 20ms. This typically requires some shaping of the front wall as well to create longer flight paths that can hit the rear wall diffusers.

Your room dimensions are 3m long, 6.26m wide and 3.1m high? Seems a little odd...normally you want the speakers firing down the longest dimension of the room. In order to have a reflection at 20ms, you need ~3.3 meters behind you to the back wall. It looks like you have some reflections starting around that time, but your ETC is too zoomed out, can you post the mdat? You do have the one strong reflection that looks like its around 15ms and then I guess the floor bounce there before that.

Another approach that you see in rooms that are dead is diffusers or slats etc that are intended to reflect the listeners self noises back to himself, but not to add any early reflections in the speaker listener interaction. This is what you see with NE and Northward rooms and seems to have emerged as the promenet *modern* design criteria family...The idea is to create a natural environment for the listener, but also to have minimal speaker to room interaction. This is probably your best bet to start some experimenting with slats etc...put them where the speakers won't hit thim (ceiling, or on panels to the sides of the speakers etc).
Old 4 weeks ago
  #6
Here for the gear
Thank you Ryan for your advice.
Sorry about the dimensions, yes, the legth is 6,26.

The ETC graph was from an old measurement, so I will take another one the next time.

The speakers are on stands almost touching the front wall. Around the speaker are mounted absorbers (RFZ) Adding sluts around the speaker should add reflection to the back wall. Is that right?

What width should the slats be?

So the reflexion must travel from the speaker to the listener approximately 6,85m to reach the listener after 20ms?
Old 4 weeks ago
  #7
Lives for gear
 

Quote:
Around the speaker are mounted absorbers (RFZ)
Absorbers do not create a reflection free zone: only reflection can do that. RFZ is a well-defined concept, where the speakers are flush mounted in angled sections of the front wall, with very solid, thick, heavy "baffles" around them, along with other angled surfaces that re-direct the reflections towards the back of the room. At the back of the room, there's usually a combination of absorption and diffusion that produces the "termination" of the ITDG. It isn't possible to create an RFZ room with absorption only.

Quote:
Adding sluts around the speaker should add reflection to the back wall. Is that right?
The back wall is the one behind you as you are sitting at the mix position, facing the speakers. The front wall is the one in front of you, where the speakers are. Putting slats around the speakers on the front wall might help to improve your own sense of the room, but it won't help much with the overall acoustic response. If you need an increase in the termination after the ITDG, that will have to come from the rear of the room (behind you), not the front. And then you will have to be careful to not produce specular reflections (sharp, intense reflections): you need diffuse reflections, at a level about 20 dB below the direct sound (although some people prefer it to be a bit higher than that... while others don't want any ITDG at all...!)

Quote:
So the reflexion must travel from the speaker to the listener approximately 6,85m to reach the listener after 20ms?
Not really: The ITDG refers to the time after the direct sound has just gone past your ears as it heads towards the rear wall (behind your head), and the time when the first diffuse reflections arrive back form the rear wall at your ears. That's the distance you need to be looking at: the distance from your head to the back wall, then back to your head. That "there-and-back" distance needs to be at least 6.1 meters to provide the 20ms delay... in other words, the rear wall would have to be at least 3m behind your head for that (because it takes 10ms for sound to travel 3m, so 20ms for the return trip). If the distance is larger, then that's OK. It's not a problem, as you just get a longer ITDG, which is OK. Or if you wanted to get exactly 20ms, you could put treatment on the side walls that start returning diffuse reflections earlier than you can get them from the rear wall.

In other words, its the distance between your head and the first partly reflective surface behind you, that defines the termination of the ITDG. The distance to the speaker is not relevant here: This is all about how the sound travels after it went past your head, not in how it traveled to arrive at your head.

There are many ways to achieve the result, but it is all accomplished with careful placement and angling of reflective surfaces (mostly at the front of the room), absorption (mostly on the ceiling, sides, and rear of the room), and diffusion (mostly at the rear of the room). That's what true RFZ is all about. However, there are many variations on that same basic them!

- Stuart -
Old 4 weeks ago
  #8
Here for the gear
Thank you very much, Stuart,

for the detailed information which is very helpfull.
I have obviously not understood the concept of RFZ so far.
Having absorption in the front and the first reflection points like walls / ceiling and diffusors with absorption in the back part of the room should then be a concept of LEDE?

What is the advantage of ITDG termination, and why other people dont like to have one? Is it a matter of acoustic design?
Old 4 weeks ago
  #9
Lives for gear
Slats will reflect the frequency that's wavelength is the same size or smaller than the width of the slat. So if your slat was 50cm wide it would reflect ~700hz and up. If you can re-measure your room with REW that would be the best place to start. Also some pics would be helpful.

One of the things that would be good to look at in the measurement is decay times. Because absorbers are progressively less effective as you go down in frequency, it's typical that a 'dead room' has much shorter decay times in HF vs LF, this is where slats can be useful in only reflecting higher frequencies back into the room.

That said the problem with only rear wall slats is there isn't a lot of other places for the sound to bounce off of, so it's tricky to develop a more dense reflection field. This is an interesting thread showing the use of various diffusers and reflective panels to create a Haas kicker with a diffuse field-

My Listening Room
Old 4 weeks ago
  #10
Lives for gear
Quote:
Originally Posted by Soundman2020 View Post
Absorbers do not create a reflection free zone: only reflection can do that. RFZ is a well-defined concept, where the speakers are flush mounted in angled sections of the front wall, with very solid, thick, heavy "baffles" around them, along with other angled surfaces that re-direct the reflections towards the back of the room. At the back of the room, there's usually a combination of absorption and diffusion that produces the "termination" of the ITDG. It isn't possible to create an RFZ room with absorption only.
"An RFZ design strives to minimize the influence of the room acoustic on the sound reproduction and so provides a neutral two-channel critical listening room. The design creates a spatial and temporal RFZ surrounding the primary mixing or listen- ing position(s). The zone is spatial, because it only exists within a certain area in the room; and it is temporal, because the interfering reflections are only controlled over a certain time window, between the arrival of the direct sound, and prior to reflec- tions arriving from the rest of the room.
It is well established that early reflections affect the characteristics of the sound at the listening position [9, 10]. One solution is to use absorption to control first-order reflections between the source and the listener and so remove early arriving high level reflections, which produce colouration and image shift."
p.33 Trevor J. Cox and Peter D'Antonio 'Acoustic absorbers and diffusers'

Bolded by me.


As described RZF is a design room, not a "how to" to achieve it. So absorption is one solution, reflection is another.

Last edited by JayPee; 4 weeks ago at 09:11 PM..
Old 4 weeks ago
  #11
Lives for gear
 

Quote:
Originally Posted by Amadeus45 View Post
Thank you very much, Stuart,

for the detailed information which is very helpfull.
I have obviously not understood the concept of RFZ so far.
Having absorption in the front and the first reflection points like walls / ceiling and diffusors with absorption in the back part of the room should then be a concept of LEDE?

What is the advantage of ITDG termination, and why other people dont like to have one? Is it a matter of acoustic design?
This might get a little involved, but I'll try to keep it short some I'm not accused of writing 3,000 word essays to help people! (inside joke...).

OK, until the 1970's, control room "design" was not really a "thing" as it is today: the principles weren't understood all that well, stereo was still a pretty new concept, and rooms were often just built the same way they had been for mono mixing... with pretty lousy results. Some smart folks, such as the famous Tom Hidley, started making better rooms by using acoustic concepts and trying out various approaches, some more successful than others.

Things got more serious throughout the 70's, with improvements in speaker design, acoustic research, and control room design. One of those was LEDE. LEDE appeared in the late 70's and early 80's, and the concept was simple: The front end of the room was all "dead" with very thick highly absorptive surfaces everywhere, and the entire rear end was "live", with reflective surfaces. It did what it was supposed to do, and many good mixes were done in LEDE rooms. But it had disadvantages too. One major one was that the response was very different as you moved around the room, so the engineer at the mix position would hear the mix one way, while the producer or musicians seated on the couch at the back would hear it very differently, and someone standing elsewhere in the room would here things in yet another way. Some people also found true LEDE rooms to be fatiguing to work in for long periods. LEDE is actually a trademark, and to be able to call your room "LEDE" it would have to meet the specs, and it would have to be certified as such. Thus, there were a lot of "LEDE-style" rooms around, but fewer true LEDE rooms. (I'm not sure if the trademark owners still do certification, or if they still even exist! Originally it was owned by Syn-Aud-Con, but I haven't followed up on where it is now).

Anyway, since true LEDE wasn't so great, several variations came along, attempting to deal with the downside while keeping the good part. One of those is RFZ. It is an EXTENSION of LEDE, and achieves the same result differently. Just like LEDE, there is a tight specification for how it MUST be done, if it is to be considered RFZ. Quote from the original RFZ paper, published by D'Antonion way back in 1984:

"The RFZ/RPG Approach to Control Room Monitoring. A design for implementing a LEDE control room is proposed. The dead end is achieved by creating an RFZ by flush mounting the monitors as close to a trihedral corner as is physically possible and splaying the absorbent side walls and ceiling to minimize interfering reflections at the mix position. The live end is achieved by positioning RPG diffusors in the rear wall in such a manner as to reintroduce the energy passing the mix position, after an initial time delay, temporarily and spatially diffused. The approach leads to accurate monitoring, with a stereo image which is maintained across the entire width of the console and in the rear of the room."

So that's it. Since D'Antonio et. al. invented the concept, and defined it, and created the specifications for it, he gets to say what it is, and what it isn't. What he says it is, is this: "The dead end is achieved by creating an RFZ by flush mounting the monitors". Therefore, if your monitors are NOT flush mounted in solid, hard, rigid walls, then your room is not RFZ! It's that simple. Because that is the very definition of RFZ, as stated by the guy who created the concept. If your speakers are surrounded by absorption, then you do not have RFZ. Period. End of story.

The next pat of true RFZ is that the side walls and ceiling are splayed and absorbent "to minimize interfering reflections at the mix position". That is also part of the RFZ spec. So if you do not have splayed, absorbent side walls and ceiling, then you do not have RFZ. Period. End of story. Because that's what the spec says.

The final part of the RFZ picture is the rear wall, which is what your question regarding ITDG is actually about. The RFZ paper defines it this way:

"The live end is achieved by positioning RPG diffusors in the rear wall in such a manner as to reintroduce the energy passing the mix position, after an initial time delay, temporarily and spatially diffused."So, if your don't have RPG (phase grating) diffusers on the rear wall, and they don't produce an ITDG followed by a diffuse reflection (diffuse in both time and space), then you do not have an RFZ room. Period. End of story. Because that's the way the spec defines it.

So that's what the actual specification states. It defines all the technical parameters very clearly in the body of the paper, along with the reasoning, the math, and the testing, but the basis is right there, in the abstract at the very beginning of the paper.

This is where a LOT of people get confused! They want to talk about a room where there are no reflections around the mix position, or reduced reflections, and they want to call that "RFZ", but it simply is not, unless it meets the specs for RFZ. Some of the rooms that I design are based in many ways on the RFZ spec, but with variations, and I don't call them RFZ because they are not RFZ. Maybe "RFZ-style" or "RFZ-inspired" or something like that. But not RFZ, because ONLY a room that meets the RFZ spec can be called an RFZ room.

This is sort of like the car enthusiasts that want to build their own Lamborghini, (or Porshe, or whatever), and they take a chassis from a Chevy, an engine from a Ford, transmission from a Mercedes, suspension from a BMW, put it all together inside a shell that is shaped just like a real Lamborghini, and then say that they have made a Lamborghini: they did not! Because it doesn't meet the specs for being a true Lamborghini, as defined by the designer, and it does not perform like a true Lamborghini: it only looks like one, but what's "under the hood" is nothing like a real one.

It's the same with LEDE and RFZ and NER and CID and the various other room concepts out there: either your room meets the exact specs created by the original designer(s), or it does not. If it just looks like an RFZ or LEDE, but does not meet the specs, then it is NOT a true RFZ or LEDE.

That seems to be the source of much confusion on the internet, with many places that should know better saying that you can "Build a LEDE room like this, cheap with mattresses and plywood", or "Build an RFZ room with speakers on stands and lots of absorption around them". No, you can't, because that's not what the definition of LEDE or RFZ actually says.

If you want the real definition and complete specs of each, here are links to the original papers:

LEDE:
http://www.aes.org/e-lib/browse.cfm?elib=2852


RFZ:
http://www.aes.org/e-lib/browse.cfm?elib=11598

It's interesting to note that the title of the RFZ paper is all about how to achieve the LEDE concept, without actually doing LEDE...


Now, back to your questions:
Quote:
Having absorption in the front and the first reflection points like walls / ceiling and diffusors with absorption in the back part of the room should then be a concept of LEDE?
Well, sort of, kind of, a little bit, but not really! For the same reasons as above. LEDE does not specify diffusion on the side walls or rear, nor absorption at the back. RFZ does specify some of that, but not all of it, and it also specifies that the speakers must be flush mounted (are yours flush mounted?), as well as splayed side walls and ceiling that are absorptive... note that it does not say that you can just put absorption on the side walls and ceiling of any old room, but rather that the walls and ceiling themselves must be splayed AND absorptive. So unless your walls are splayed, then you don't have RFZ either!

But don't worry: just because you are not following the RFZ spec exactly does not mean that your room is going to be bad! As I mentioned above, many of the rooms I design are "RFZ-based", but not true RFZ. I use some of the concepts and goals of true RFZ, but not others. ITDG, for example: In small rooms, it is impossible to achieve the actual design spec for RFZ, simple because there is not enough distance between the listening position and the rear wall. So I have my own methods for dealing with that to still produce a good listening environment with an area around the mix position that is free from early reflections. (If I recall correctly, a few years ago someone here on GZ coined a term something like "ZWER" for "Zone Without Early Reflections", or something like that, to clarify the difference between true RFZ rooms and "RFZ-like" rooms.)

So, on to ITDG: What is it? In true rooms, ITDG is the Initial Time Delay between the moment the sound from the speakers passes by your ears, to the moment that the first reflection arrives back at your ears. During that time, there should be no high-level reflections of that initial sound at all, arriving at your ears. The "termination" is supposed to be a significant return of diffuse energy arriving "suddenly" at your ears, after the time delay has expired, but it must be at a much lower level (-20 dB), and it must occur after a certain time delay (20ms). The reasons for this are based on psycho-acoustics, which is the science of how we humans perceive sound with our ears and brains, rather than how it actually is, as measured by a mic. It turns out that if you hear the direct sound followed by strong reflections of that same sound that are delayed by LESS than about 20 ms, that messes up the ability of your ears to accurately determine the true direction that the sound came from, and also messes with your ability to accurately determine the true frequency response of the sound. Your ears and brain do not hear that early reflection as an "echo", but rather as being part of the original direct sound, sort of smeared out over time, and direction, and frequency. There are also other possible effects, such as a strong reflection "masking" some of the original direct sound (so you don't even hear it at all), etc. So, if your room design eliminates early reflections in the first 20 ms (including diffuse reflections!), then that gives your ears and brain a much, much better chance at hearing ONLY the direct sound from the speakers, without that being messed up.

Now, here's the thing: early reflections sound nice! They make the music sound more "spacious" and "airy" and "enveloping", and even "pleasant" (also for psycho-acoustic reasons). So that's fine, wonderful, excellent and very desirable .... for a home theater or audiophile listening room, or club, or performance hall, where the goal is just enjoyable listening... but it is NOT fine for a control room, where you are trying to hear and track the real sound of the instruments and musicians in the LIVE room, and trying to mix the actual sounds in those tracks you recorded. You do not want to hear a "nice pleasant" representation of those sounds with some added ambience from the room! Nope. You just want to hear the exact, original sounds in a control room. So the goal for a "pleasure listening room" and "critical listening room" are different. Very different. A control room MUST suppress those early reflections, so that ALL you hear is the clean, clear, neutral, natural sound from the speakers. Nothing else. Nothing that would "color" that sound by adding to it, or subtracting from it. That's the goal for a control room, and it is different from the the goal for a home theater or other room for "recreational listening". Some people seem to be very confused about this, and assume that whatever is good for a control room is also good for their home theater, or vice versa, but that is not true. You do not want a home theater to be clinically accurate with no ambient sound... that would not be nice... and y also do NOT want the control room to make the music "sound nice", or make it "sound more spacious" or give it "more air". You just want it to tell the truth about your mix: the plain, old ugly, nasty, truth, warts and all, without making it sound "better".

Quote:
What is the advantage of ITDG termination,
I mentioned that above, but basically, for a true RFZ room, it is the first "bunch" of diffuse reflections that arrive at your ears after the initial "silence". It is supposed to be a well-defined "sudden" increase in level, but it is still very low level, and it must be diffuse in both space and time (ie, it cannot be specular). It "terminates" the initial period where there are no early reflections at all of the direct sound, and it is followed by smooth diffuse decay across the spectrum.

Quote:
and why other people dont like to have one? Is it a matter of acoustic design?
Right! Different design concepts have different specifications. ITDG is specified in the RFZ design, but other concepts have other criteria. For example, when the very same D'Antonio (who invented the RFZ design and set the specs for it) collaborated with Massenberg to do Studio C at Blackbird studios, he came up with an entirely different concept and specification where there is no ITDG as specified in RFZ. They called this new design "ambechoic", and it is very, very different from both LEDE and RFZ: it produces a smooth, even, diffuse field all around, right from the start, with smooth, even decay and no pronounced ITDG or termination. Its a different concept, with different principles... by the very same guy who also invented RFZ! Which sort of proves that the ITDG is not necessary IN OTHER DESIGNS to produce the final goal. It IS necessary in the RFZ design, because that's what the design says. But you can still meet the necessary phsyco-acoustic goal in other ways, if you are not strictly following the RFZ concept. Which is what I do, most of the time... get to the same result in a different way.

Hopefully that answered your questions, ... in a very long and convoluted way!

( And it was only 2,500 words, so I hope I'm within my limits for the day! )


- Stuart -

Last edited by Soundman2020; 4 weeks ago at 06:57 PM..
Old 4 weeks ago
  #12
Moderator
 
Northward's Avatar
Quote:
Originally Posted by Soundman2020 View Post
This might get a little involved, but I'll try to keep it short some I'm not accused of writing 3,000 word essays to help people! (inside joke...

[...]

( And it was only 2,500 words, so I hope I'm within my limits for the day! )

- Stuart -
Only 2500? Didn't read it but it sure looked like 4000.

Do you have the audio book version?
Old 4 weeks ago
  #13
Gear Addict
 

2412 words, don't brag!
Old 4 weeks ago
  #14
Lives for gear
 
Jason Foi's Avatar
 

In the OP, he said he's having trouble hearing the character/size of the recording room. Isn't this problem a function of the size relationship between CR/LR? I thought this is why it's advised to have at least 3x the volume of the CR for a LR?

On a side note, I really do think Stuart should just write a book on acoustics.
Old 4 weeks ago
  #15
Gear Addict
 

Quote:
Originally Posted by Northward View Post
Only 2500? Didn't read it but it sure looked like 4000.

Do you have the audio book version?
He kicks your ass, only the marketing part should be enhanced :-)
Old 4 weeks ago
  #16
Moderator
 
Northward's Avatar
Quote:
Originally Posted by bert stoltenborg View Post
He kicks your ass, only the marketing part should be enhanced :-)
In terms of forum posting, hands down.

Joke aside, glad someone takes the time to help forum users this way.
Old 4 weeks ago
  #17
Lives for gear
 
avare's Avatar
 

[QUOTE=Soundman2020;14220846
( And it was only 2,500 words, so I hope I'm within my limits for the day! [/QUOTE]
An excellent post and not even close to 2,500 words.
Post Reply

Welcome to the Gearslutz Pro Audio Community!

Registration benefits include:
  • The ability to reply to and create new discussions
  • Access to members-only giveaways & competitions
  • Interact with VIP industry experts in our guest Q&As
  • Access to members-only sub forum discussions
  • Access to members-only Chat Room
  • Get INSTANT ACCESS to the world's best private pro audio Classifieds for only USD $20/year
  • Promote your eBay auctions and Reverb.com listings for free
  • Remove this message!
You need an account to post a reply. Create a username and password below and an account will be created and your post entered.


 
 
Slide to join now Processing…
Thread Tools
Search this Thread
Search this Thread:

Advanced Search
Forum Jump
Forum Jump