Do we want side wall reflections?

[thcdru2k]

Do we want this specific reflection, at this delay, with this spectrum, from this speaker directivity, at this listening distance, for this playback goal? I think I can answer this in many contradictory ways

 

[digitalfrost]

I can only give anecdotal evidence, but it's relevant I think. My listening position is asymmetrical. Due to a sloping roof on the right side of the room, I have moved my listening spot all the way to the left. So on the left there is a wall very close by the left speaker, and on the right there is a gap which allows me to walk by and then the sloping roof.

I used to put basotect in the first reflection zone on the left and that worked, but also it made that side rather dead. I could live with it, but a friend of mine absolutely hated the basotect and even preferred the sound with a bare wall. I have since bought an acoustic panel that is a mixture of absorption and reflection and that works best.

So as with most things in life, the dose makes the poison.

 

[Duke]

View attachment 538073

If I'm reading the caption correctly, the reflective panels resulted in only a "slight increase" in the earliest reflected energy arriving 8 milliseconds after the direct sound, but a "marked increase" in the later reflected energy arriving 12 to 30 milliseconds after the direct sound.

So the reflective panels actually added MORE later-arriving reflection energy than early-arriving reflection energy!

It seems plausible to me that the rather surprising increase in (imo desirable) late-arriving reflections, apparently without much increase in the (imo undesirable) early reflections, played a role in preference.

 

[olieb]

So the reflective panels actually added MORE later-arriving reflection energy than early-arriving reflection energy!

I think this is a misunderstanding.
In the caption it is talked about the “temporary walls used to hang the variable acoustic treatment“ not about the specular reflecting panels themselves. In my view the information about “primary“, “secondary“ and “tertiary“ energy is about the change of the room from the experimental design (before applying the different wall treatments).

Assuming that the McGill room is shown and not the second mixing room, these walls are placed differently in the photo compared to the room plan. The plan would not make sense for investigating early lateral reflections as the walls would (partially) block these in the first place.

From the geometry a delay of 8ms is too long for the first reflections from these walls.
I drew the reflection paths for this geometry and the delay of the first reflections off these walls (the way it is shown in the photo) would be ≈4.6ms.
The blue “wall“ in the plan has a length of 1.2m .

The documentation of the experiment in the paper is not good. Many details are missing. There is hardly any information about the second room at all. ...

And modifying the statement of @thcdru2k:
“Do we want this specific reflection, at this delay, with this spectrum, from this speaker directivity, at this listening distance, for this playback goal“ in this room with these snippets of music?

The way I see it not much can be inferred from the results.

 

[Buckchester]

I’ve been telling people for weeks that they gotta get some cable lifts to improve their sound - night and day! They all think I’m kinda nuts though. Yeah-they thought the same thing about Einstein too! Look how that turned out. Nuff said.

 

[Duke]

I think this is a misunderstanding.
In the caption it is talked about the “temporary walls used to hang the variable acoustic treatment“ not about the specular reflecting panels themselves. In my view the information about “primary“, “secondary“ and “tertiary“ energy is about the change of the room from the experimental design (before applying the different wall treatments).

Assuming that the McGill room is shown and not the second mixing room, these walls are placed differently in the photo compared to the room plan. The plan would not make sense for investigating early lateral reflections as the walls would (partially) block these in the first place.
View attachment 538210

From the geometry a delay of 8ms is too long for the first reflections from these walls.
I drew the reflection paths for this geometry and the delay of the first reflections off these walls (the way it is shown in the photo) would be ≈4.6ms.

So are those "temporary walls" just curtains to conceal whatever the actual wall treatment is?

If so, it looks to me like the side walls, where the actual reflections occur, are deliberately angled in a way that would direct a specular reflection behind the listening position, which would be consistent with the late reflections increasing in energy moreso than the early reflections at the listening position. And that would also explain the 8 milliseconds arrival time for the first lateral reflection energy.

So if the "reflections" case is using those angled side walls, imo that is not representative of normal home listening rooms, wherein the first sidewall reflections are not angled away from the listening area.

Edited to add:

Eyeballing the temporary walls again, IF those are indeed reflecting panels, it looks to me like the first ipsilateral reflection would largely if not entirely pass BEHIND the mix position and therefore miss the listener entirely - BUT the first contralateral reflection would not! And that first contralateral reflection looks to me like it would clock in pretty close to that 8 milliseconds.

Imo this geometrical minimization of the first ipsilateral reflection would be big departure from what normally happens in home audio.

I need to get a copy of the paper so I can figure out what's really happening. I'm not sure that it's representative of reflection vs diffusion vs absorption in a typical home audio context.

 

[olieb]

So are those "temporary walls" just curtains to conceal whatever the actual wall treatment is?

From my understanding these are “four-foot by four-foot (approximately 1.22 m by 1.22 m) area of false side wall“ and are used to hang “three different acoustical treatments mounted on modular panels (Fig. 2).“ [the blue things]
And there are curtains as part of these false walls to hide the treatment in use.

And that would also explain the 8 milliseconds arrival time for the first lateral reflection energy.

Again, I think there is a reason for the use of the term “primary energy“ instead of first reflection.
The original room walls at the sides are mostly taken out of the equation for the first lateral reflection in this arrangement.
These happen (dry wall panels) or not (absorption) at the false walls.
So there is an increase of “primary energy“ around 8ms from the frame arrangement, but the lateral first reflection from the false walls has a delay of 4.6ms (if not absorbed or deflected).

And the whole experiment with rooms of RT60 measured as 175ms and 200ms is certainly “not representative of normal home listening rooms“.

 

[Duke]

So there is an increase of “primary energy“ around 8ms from the frame arrangement, but the lateral first reflection from the false walls has a delay of 4.6ms (if not absorbed or deflected).

So the "frame arrangement" and the "false walls" are two different things? Pardon me for being so dense.

 

[Somafunk]

Fully treated 42m3 room with peq for room modes under 180hz and RT 60 of .29 (ish), sounds perfect for my main 90%+ listening genre of electronic music, I prefer a direct dry sound, minimal reflections and I’ve took the room as far as was affordable for my budget.

I imagine if I listened to vocal led or acoustic recordings I may want reflections and a greater RT 60

 

[Duke]

No...

Amir, I'm not just trying to be a contrarian here. If you see a mistake in my analysis, PLEASE point it out!

See the diagram below, which is Figure 2 from the paper, but I've drawn in the first ipsilateral and first contralateral reflection paths. I tried to draw the angle of incidence equal to the angle of reflection, and hopefully got close enough:

Now here's the important part: The first ipsilateral reflection MISSES the listening position altogether! There are no early same-side-wall reflections in the McGill study! (Or at least not at wavelengths were the propagation can be approximated by rays.)

The first lateral reflection is the one off the opposite side wall, and it arrives about 8 milliseconds later than the direct sound, just as the caption says. Here is the caption:

"Fig. 2. Top view of the testing facility at McGill University, with the temporary walls used to hang the variable acoustic treatment (the solid black lines on either side of the listener). The spacing of the temporary walls promoted a slight increase in primary reflected energy circa 8 ms after the arrival of the direct sound and a marked increase in secondary and tertiary reflected energy arriving approximately 12 to 30 ms after the direct sound."

This is not at all similar to what normally happens in a home audio room. In a normal home audio room, there would be a strong same-side-wall reflection usually arriving earlier than 8 milliseconds after the direct sound. Instead, in the McGill room, in the "reflective" case, we get no early same-side-wall reflections; a "slight increase" in the opposite-side-wall reflections; and a "marked increase" in later-arriving reflected energy.

@amirm, do you think my analysis indicating differences between the McGill study's set-up and what normally happens in a home audio context is correct (or close enough)?

And if so, do you think these difference are of any significance?

Anyway, now that I've looked at the actual first sidewall reflection paths, the results of the McGill study are not surprising to me.

 

[EERecordist]

Another interesting study is "Preferences of Critical Listening Environments Among Sound Engineers - SAKARI TERVO, , PERTTU LAUKKANEN, TAPIO LOKKI, JUKKA P ATYNEN - Department of Media Technology, Aalto University School of Science, FI-00076 Aalto - 2014". This study acknowledges the findings of the study mentioned by the OP and the work done by F. Toole, but it tries to get a better understanding of which control room acoustics sound engineers prefer.

A few quotes relevant to this thread:

• "LEF (lateral energy fraction) correlates negatively at moderate levels (p<0.05) at high frequencies (1kHz and 2kHz octave bands). That is, the mixing engineers prefer that there is no energy from the sides besides the direct sound from the loudspeakers".
• "The correlations in Table 5 suggest that sound energy from sidewall reflections at high frequencies should be avoided".
• "Mastering engineers prefer more reverberant environments in general than mixing engineers and clarity is less significant for them".

The study confirms that just like audiophiles, also sound engineers can have preferences. It's the reason for 80's LEDE, RFZ and modern “non-environment” control room designs. (Note 'control room', not suggesting that this is what you need to do at home).

Side note: Regarding the frequency band related observations there's study "Lateral reflections are favorable in concert halls due to binaural loudness - Tapio Lokki, Jukka Pätynen - Aalto University - November 2011", where one of the core findings is that lateral reflections are perceived louder than frontal reflections due to the shape of the human head and ears:

• The outer ear, head and torso form a direction-dependent filter. It boosts high-mid frequencies (~2–5 kHz region) and attenuates others depending on the direction of arrival
• For lateral sound ≈90°, one ear receives stronger direct signal while the other is shadowed, which creates large interaural level differences (ILD) and interaural time differences (ITD)

In professional recording and mix control rooms with a large budget there will be still be reflections from the equipment, monitors, and glass surfaces such as windows between studios.

My experience was the non-environment Hidley rooms. They have fabric walls and ceilings hiding absorbers. I used soffit-mount speakers. Nearfields will still have reflections from the board and screens. In my view, the purpose of the non-environment room is to eliminate the room and focus on choice of microphone, microphone placement, and all the recording processing/EQ/etc. In the mix delay and echo may be added. I don't think it is expected the home listener will have a non-environment room.

I spent a lot of time listening live in vintage concert halls 450 and about 2000 seats, and recital halls. For halls and acoustic instruments, most people would expect a live hall often with sound absorbing seats and carpet. A dead floor side helps with audience noise such as coughing, handling programs, and shifting around. Today it is common for the seats to have the same acoustic characteristics filled or empty. That helps vastly with EQing the room.

Tracking rooms where the musicians are in recording studios tend to be live and have higher ceilings, I'm not experienced on design of those. It is also common to put gobos, movable acoustic walls, between musicians for isolation. Commonly the musicians will be listening through headphones.

Many vintage halls discovered by accident that all the plaster curlicues and gingerbread at the front, the sides, and even on the ceiling produced a pleasing complex reflected sound field.

At home in living rooms, as opposed to high-end home theater rooms, I think we just get used to all the reflections, with or without EQ.

The experience that an individual is used to will determine their preferences in controlled studies, in my opinion.

 

[Ype]

I am trying to understand this discussion, as I enjoy solving practical/technical problems.
Below is written with stereo playback in mind.

To quote my earlier post:

...what is wrong with starting with what would be best practices in stereo preferred by most people?

If I want a more direct sound field I simply listen in nearfield, in midfield or far field I have a better idea of what the average user in their room would hear.
One needs both approaches in mixing and especially mastering.

For playback one can do whatever they like, but after spending a lot of time and energy on treating first reflection points, how are you going to *compare this to how it was before?
I can compare both experiences simply by listening in near and mid field, how do you know what you prefer?

I presume you are aware of how thick your absorbers need to be not the color the sound and all of this is done well.
The preferences of how many reflections and how loud one wants to hear the reflections also differs between music and voice as well as from genre to genre from what I remember.
However, most people prefer louder beneficial reflections and/or higher delays then a room can give us, also depending on the recording, which one can do by up mixing and adding more speakers in front of the listener but at a different angle.

One however does not want to hear echo's, which is impacted by both loudness and delay.
This will never happen with just the reflections in a small listening room.

But one has to start somewhere, and that is with what we know.
As for me personally, side wall first reflection points would be one of the the last places I'd treat for stereo and if I wanted to treat it I'd try diffusors first, as they are seen as beneficial reflections.

Again, we have to start somewhere, so I start with what is pretty well understood, get the basics right, which is a great designed speaker.
If one wants to add room treatment, do not start with the side wall first reflection points, see it as an optional choice, but after treating other parts of the room that have not been shown to be beneficial reflections in stereo.

...

What would you say is wrong with this method of going about things?
We take what is already known as a guidelines, no need to try and re-invent the wheel.

When it comes to mixing and mastering, my viewpoint would be that the question what mastering and mixing engineers prefer is the wrong question.
The question should be what their audiences prefer and how they are consuming the content.

Besides, as far as I know hearing loss will also impact preference and this is a hazard in this field.
Listening in nearfield *or adding more absorption might lessen some side effects from hearing loss.

I am still waiting for a clear answer, or at least one that is clear to me, so I added some more thoughts of my own.
Feel free to tell me you disagree but explain why that is as I seem to have trouble finding the point of the argument.

 

[Thomas_A]

Amir, I'm not just trying to be a contrarian here. If you see a mistake in my analysis, PLEASE point it out!

See the diagram below, which is Figure 2 from the paper, but I've drawn in the first ipsilateral and first contralateral reflection paths. I tried to draw the angle of incidence equal to the angle of reflection, and hopefully got close enough:


View attachment 538244

Now here's the important part: The first ipsilateral reflection MISSES the listening position altogether! There are no early same-side-wall reflections in the McGill study! (Or at least not at wavelengths were the propagation can be approximated by rays.)

The first lateral reflection is the one off the opposite side wall, and it arrives about 8 milliseconds later than the direct sound, just as the caption says. Here is the caption:

"Fig. 2. Top view of the testing facility at McGill University, with the temporary walls used to hang the variable acoustic treatment (the solid black lines on either side of the listener). The spacing of the temporary walls promoted a slight increase in primary reflected energy circa 8 ms after the arrival of the direct sound and a marked increase in secondary and tertiary reflected energy arriving approximately 12 to 30 ms after the direct sound."

This is not at all similar to what normally happens in a home audio room. In a normal home audio room, there would be a strong same-side-wall reflection usually arriving earlier than 8 milliseconds after the direct sound. Instead, in the McGill room, in the "reflective" case, we get no early same-side-wall reflections; a "slight increase" in the opposite-side-wall reflections; and a "marked increase" in later-arriving reflected energy.

@amirm, do you think my analysis indicating differences between the McGill study's set-up and what normally happens in a home audio context is correct (or close enough)?

And if so, do you think these difference are of any significance?

Anyway, now that I've looked at the actual first sidewall reflection paths, the results of the McGill study are not surprising to me.

I saw that too...and a giant head and quite large speakers as well.
(As a side note a near 45° toe-in decreases the level of the nearest wall...discussed before...such speaker design can be done)

 

[Geert]

I think this is a misunderstanding.
In the caption it is talked about the “temporary walls used to hang the variable acoustic treatment“ not about the specular reflecting panels themselves.

There's a follow-up study where they build on the design of this study: "Interaction between critical listening environment acoustics and listener reverberation preference - Brett Leonard*, Richard L. King and Grzegorz Sikora - June 2013". This paper has better images of the room and the location of the reflective panels:

Now here's the important part: The first ipsilateral reflection MISSES the listening position altogether! There are no early same-side-wall reflections in the McGill study!

The study is designed like that, it's documented in the paper (which I don't have access to for the moment). The study focusses on lateral reflections, so they had to avoid early reflections interfering.

 

[Elgrosso]

I still don’t get it, but I’m just missing something they obviously knew what they were doing.
In the empty room they seem to already cancel early reflections (octagon shape + treatment).
So they added reflective panels to diverge some sound to create in some way some lateral late reflections.
But with this geometry, these side panels placements and angles, the only sound reflected goes to the back, and maybe bounce back to the head. So they are back reflections then, not lateral. (?)

 

[olieb]

There's a follow-up study where they build on the design of this study: "Interaction between critical listening environment acoustics and listener reverberation preference - Brett Leonard*, Richard L. King and Grzegorz Sikora - June 2013". This paper has better images of the room and the location of the reflective panels:

Hm, for me that is not better as information about panel placement at all, sorry.

If the plan is correct (it looks quite precise) and the side panels are placed this far back then the first reflections from the walls (in the original room) are not altered by the panels at all (red dotted lines).
And the (first) reflections from the panels do not reach the listener. (blue dotted lines)
So, whatever is supposed to be investigated, a change of first lateral reflections does not take place.
And as mentioned the photo shows a placement of the panels (whatever room that is) much more to the front.
To me the forward placement makes at least some sense in the context of investigating lateral earls reflections.
But who knows, in any case this is confusing and inconsistent.

 

[Elgrosso]

Looking at the last photo, the first section of the side wall (the one you drew the red reflection on) seems to be treated, probably to absorb.

 

[Bjorn]

@Bjorn thanks for the interesting comment.

Can you clarify on this point...

Do you mean only that it is undesireable to diffuse vertically or absorb in a study where the subject is in-plane lateral reflections; or that in all practical listening cases it is also undesirable?

Diffusion on side walls should be aiming the energy at the ears directly. A 2D diffuser will waste half the energy toward the ceiling and floor. So a 1D diffuser is what should be used there, and if there's a desire for increased spaciousness it needs to offer temporal diffusion/changing the phase.

A 2D diffuser is a product mainly used with a high ceiling and where there's need to distribute sound to several seats (cinema, home theater, etc.).

 

[Geert]

In the empty room they seem to already cancel early reflections (octagon shape + treatment).

Yes, it's kind of a studio control room with very short reverb time.

So they added reflective panels to diverge some sound to create in some way some lateral late reflections. But with this geometry, these side panels placements and angles, the only sound reflected goes to the back, and maybe bounce back to the head. So they are back reflections then, not lateral. (?)

It's explained earlier; the left panel is reflecting for the right speaker:

If the plan is correct (it looks quite precise) and the side panels are placed this far back then the first reflections from the walls (in the original room) are not altered by the panels at all (red dotted lines).

Given the angle of these reflections (also consider the toe-in of the speakers) and the acoustic treatment of the walls, these reflections are probably not very strong.

And the (first) reflections from the panels do not reach the listener. (blue dotted lines)

See image above.

 

[Chris A]

The LLD is much closer than the acoustic Critical Distance.

So what do you call the distant point at which reflections overwhelm direct arrivals from the loudspeaker in home hi-fi listening rooms?

I believe that's actually the root argument in this thread--and whether or not the listener has accommodated to that particular sound...the mixture of direct arrivals and room reflections that overwhelms the phase response at the listener's ears. Lots of confusion over what to call "fidelity" here.

For the purposes of this discussion, whatever you call that distance is the one that I was reporting as "critical distance" for my ears.

The concept of reverberant field is said to be only useful in intermediate and large rooms. But for some reason, there is an immediate dismissal of talking about the different distances at which there are a mixture of direct arrivals vs. reflections in small rooms--i.e., those rooms that are typically the size of home hi-fi listening rooms. That's the apparent source of the real confusion I believe...

...not being able to talk about the problems of loudspeakers not having good directivity control in-room and how to deal with their in-room nearfield reflections.

This is also an issue in recording studios, where the apparent obsession with "mastering translation" has prevented studio operators from using loudspeakers with much flatter directivity performance vs. frequency, and up until relatively recently, resulted in a historical resistance to EQing on-axis amplitude response of their studio monitors to flatten both direct arrival amplitude and phase response (i.e., minimum phase).

How do you EQ a loudspeaker that has unequal directivity, and where do you place such loudspeakers to get "translation"? That's the corner that they have painted themselves into with their mastering practices, thus flowing down to the consumers trying to figure out what to do in their listening rooms with their own loudspeakers having poor (and different) directivity response from the "translating" studio monitors. The "circle of confusion" is institutionalized here.

Chris