When we consider side wall reflections or other reflections in a small room, I believe we need distinguish between accuracy and personal preferences. Accuracy is something we can look at a more from a scientific viewpoint. That was the approach with the development of LEDE. The goal was to hear the recorded signal as accurately as possible. Whether you want that at home, is an entirely personal choice. Personally, I don’t believe there will be a scientific result for “preferences”. Preferences here will vary depending on the music genre, the rooms geometry, the acoustics of the room, speaker directivity, and probably the mood of the day!
But let’s go back to accuracy and what happened during the development of LEDE. The name LEDE (live and dead end) is unfortunately quite misleading of what the design turned into. The reason for that is that design developed over many years with the introduction of ETC measurements that came along later and with diffusers, that also came along later. So LEDE was eventually not a design where you made one end completely dead and the other live. It only started that way.
Instead, treating specular reflections with surgical precision and not absorb at unnecessary surfaces became very important. A quality LEDE room would therefore not be dead or dry, but combine accuracy with a lot of energy.
In the development of LEDE listening tests were done over many years. Some sighted and some blind. The quantity of listeners varied. All of this was shared in news letters, long before the internet. Was there enough participants to make it statistical? I don’t know if that was the case, but I assume for the most part it wasn't. From some of the newsletter I have, I can see that the numbers varied. But let’s keep in mind here that we’re not really discussing something that is audible or not.
What was found out in these researchers was the following:
All specular energy had a negative impact on accuracy or hearing the recorded signal detailed as possible. The earlier more vs late arrival ones. Some more than others depending on the angle they arrived from. The earlier they arrived, the more detrimental effect did they have on localization, tonality, localization and imaging. Discrete reflections would also become audible if other were suppressed.
The result of this was they they created a reflective free zone. Since this was only for one listening position, it didn’t require large surface area to be treated. It did involve at least side wall reflections, ceiling reflections, and if baffle step from speakers was high; an area on the front wall also needed some treatment.
Absorption was however, not the only treatment used here. Angled panels or angled walls and sometimes ceiling were also used to create what was considered to be an anechoic listening area. Reflections needed to be attenuated a minimum of 25 dB to be considered inaudible or anechoic. The treatment needed to be effective down to at least the Schroeder frequency, something thin and small absorption panels don’t achieve.
Using slanted walls and ceiling had a clear benefit vs absorption panels, because this eventually led to more diffuse energy. The rear wall was treated with, diffusion, when the diffusers entered the scene. The first QRD diffusers were incredible deep (60 cm I believe) and made out of concrete. The first commercial ones were 23 cm deep.
The idea with diffusion in the rear of the room was to achieve a closer result to the best concert halls, where the late arrival side wall reflections created development and spaciousness,. But also accuracy by removing late arriving high gain specular energy without making the room dead. The diffused energy should arrive 2-5 ms later than the time arrival of the recorded signal. This implied that the diffuse energy would normally arrive at 20 ms or later in most LEDE rooms. Which also happens to be in the area of the arrival of lateral contribution of the best concert halls at that time.
Later the studies over Toole/Olive came and seemed to indicate something else to some degree. We have the reaearch called “Perception of reflections in typical rooms”.
It was conducted in an anechoic chamber and in what’s called a normal room. The “normal room” isn’t very well described but it says:
The room was first used in its most "live" form, with the movable curtains compressed into the corners. The mid-frequency reverberation time was about 0.4 s. Then the early reflections from adjacent room boundaries were reduced in amplitude by careful positioning of the curtains and the addition of some absorbing materials.
Since we don’t know what absorption was used here and how much area they covered, we can’t really know how valid the research was. A curtain with some addition of some absorbing material sounds very much like very bandlimited treatment that mainly effect the highs, but that’s speculation on my part.
Whether an anechoic chamber is good place to conduct such a study is also to something to discuss. Some other studies like Barron (1971) and Barron & Marshall (1981) were also conducted in an anechoic chamber.
Toole/Olive addressed the LEDE concept but they seem to have been made a mistake which has to large degree cost them respect in many acoustic circles. They didn’t address the latest LEDE design but a former design, when it was under development and before diffusers were used. Before the use of QRD diffusers, hard panels were used in specific places in the rear of the room to create what was called Haas kicker or trigger. It was an experiment to use something else than only absorption in the rear (to avoid dead rooms), and it also had certain psychoacoustical effects. However, the Haas kicker died away as soon as diffusers came along. Toole/Olive criticized the outdated older design.
Later comes the notion from Olive/Toole that lateral reflections wasn’t as bad but could actually be preferable. However, this was not looked up from a place of accuracy but was more about preferences. Something that was also discovered in the development of LEDE. Especially late arriving lateral reflections could be pleasing and preferable with some types of music. But preferences and accuracy are not the same! The LEDE concept was about hearing the recorded signal as well as possible. Or as an acoustician called Russel Berger said:
“Olive/Toole’s tests bear out what we already know, that strong lateral reflections can sound "better" to test subject listeners. Unfortunately for him and his argument, is that reading the results from his own test data, presents a completely different picture, when it is viewed and analyzed through the lens that is is ones intent to create a room that accurately references source material.
We found this out early on with the experimental LEDE rooms that employed what we then mistakenly called "Haas Kickers" or panels that provided strong reflections of the direct sound from behind, but delayed through time-of-flight placement to arrive in a manner to enhance the precedence effect. The problem was that material played back in the presence of these strong reflections "sounded better" than the original. That is not the goal. We want our monitoring environment to reveal flaws and inaccuracies in our micing techniques, recordings, and mixing choices.
To get to an accurate listening environment, the short reflected energy must be sufficiently controlled enough to remove the effects of combining it with the direct signal and convoluting the listeners experience. The part of how much ambience and at what delayed interval energy is reintroduced into the listening position is probably more a matter of taste and experience. “
We need distuingish between what is accuracy and what personal pleasing. The first is something we can study and find objective results for to a large degree. But for the latter, it will depend one several aspects as already discussed. I might come back to this later as well, but this post is getting too long now!
