Do we want side wall reflections?

[Bjorn]

In the thread "The ASR "objectivists tribe" has to read the Keynotes at the AES160th by Lars Risbo" there was a discussion about side wall reflections I think it's better suited in a separated thread.

I want to start with adressing a study Amir pointed to. The study is called "The Practical Effects of Lateral Energy in Critical Listening Environments" and can be found here:

https://www.researchgate.net/publication/262414254_The_Practical_Effects_of_Lateral_Energy_in_Critical_Listening_Environments

Amir used this study to show that lateral reflections are preferable. But let's look a bit deeper into the study.

1.The result was as the following:
8 listeners preferred diffusion (RPG Skyline)
7 listeners prefered absorption (6" absorber/15 cm)
11 listenerd prefered reflection

Clearly this does not show a statistical preference for reflection. We could actually turn this around and say 15 vs 11 preferred something else than a reflection. But there's no statistical preference here for either.

2. As I stated in the thread "Keynotes at the AES160th by Lars Risbo", we also need to consider the time for arrival and the speaker direcitivty when we consider side wall reflections. Very early reflections are considerably more damaging to areas like clarity/details, localization or you may simply call it imaging or pin pointing. Later ones are less detrimental.

In the study "The Practical Effects of Lateral Energy in Critical Listening Environments" the reflection that tested arrived as late as 8 ms after the direct signal and also effected reflections after 12 ms and up to 30 ms.

With an arrival as late as 8 ms and later, we're not looking here the normal side wall reflection you have from the nearest side wall to each speaker. In most homes and setups, the neares side wall reflection arrive much earlier.

8 ms would be more the contralateral reflection, meaning the reflection you have from the opposite side wall from eack speaker. That's the reflection you are seeing going to the left in the illustration below and it arrives much later in time vs the one to the right.

With an arrival time of 8 ms, the result will be very different from vs a side wall reflection that arrives only after a few ms or for example within 5 ms. In order for a reflection to have a good effect on spaciousness, it generally needs to arrive later than 7 ms. And that's excatly what it does here.

3. I also mentioned that we need to consider the type of diffusion being used. Diffusers can greatly vary in effect. A true diffuser in the original term should alter the phase. That's why they were called phase graters. A Skyline, which was used in this study, does not really do that.

The RPG Skyline is also very absorptive due to it's material. And it's 2D diffuser, meaning it diffuses in both planes. Something we psycoacoustically don't want for a diffuser on the side wall! That implies we are sending half the energy towards the ceiling and floor, something we know doesn't effect spaciosuness and development like lateral contribution.

What they should have used in the study was 1D diffuser with deeper wells and also with smaller wells within the bigger wells to achieve sufficient brodband diffusion. A standard QRD diffuser would simply reflect the energy above 4-5 kHz and is exactly don't want when testing diffusion in a small room with closer proximity.

4. Another aspect to consider when looking at side wall reflections is the type of music being uses. Toole also talks about this for side wall reflections and indicate that for classical music, lateral arriving reflections seem to be preferable but not always for certain other music genres like pop and rock.

Let's see what music they used in this study:

"In a departure from previous work, this testing was limited to a single musical genre. All of the musical material was taken from a single commercial release of soprano voice and orchestra. The excerpts used for testing were 30 second clips of three Richard Strauss songs, embodying a range of dynamics and musical textures. As the musical excerpts were taken from a commercial release, a nominal 0 dB level represents the artist-approved balance."

So they tested ony one musical genre and this happened to be classical, which is exactly the type of music genres which seems to work best with a more spacious contribution.
Thus we have another major weakness with this study.

So to sum up. There's no clear science here showing that we generally prefer side wall reflections. They have only studied fairly late arriving reflections, used one music genre (classical), haven't used the correct and psycoacoustcially better diffuser, and there's no a statisctial result either.

They type of room and how it was treated would also contribute to the result. There's isn't much clear details on how to the room was treated. The paper wrongly focuses on something that doesn't exist in such a small room, namely reverberation time. I will address that topic later, but there are indication that the study has been done in a room with already a lot of acoustic treatment. They point that the room was designed for criticial mixing, mastering and training. It's possible that the room was overly dead and they also used Skyline diffusers in the rear of the room. But we don't really know.

To be continued.

 

[MaxwellsEq]

Some studios use soffit-mounted main monitors (and use quality smaller speakers in the nearfield some distance from walls)

What do we think about soffit-mounting?

 

[Trdat]

Great analysis on the study. I hope the thread catches momentum.

I got two sound systems one a standard 3 way bookshelf speaker the second a 90 by 50 horn crossed over at 800hz to a 15inch. With material that has enough reverberation in the recording the horn sounds better(of course subjective) while if the recording is dry, studio or a standard pop track it sounds better on 3 way as it adds spaciousness due to the wider dispersion characteristics and more reflections the room provides, while the horn's dry output suits reverb type content mainly live recordings.

My point is that perhaps, like you mentioned the late arriving reflections that add spaciousness is needed according to the dispersion characteristics of the speaker and to the music style people listen to and then acoustically tuned to taste.

Many of the experts on Gearspace never recommend specular reflections however it is still debated on the type of diffusion or scattering one can have on side walls. Keen to hear more.

 

[-Matt-]

@Bjorn thanks for the interesting comment.

Can you clarify on this point...

The RPG Skyline is also very absorptive due to it's material. And it's 2D diffuser, meaning it diffuses in both planes. Something we psycoacoustically don't want for a diffuser on the side wall!

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?

 

[Elgrosso]

I’m even surprised they got some side reflections at all based on the geometry, the added side panels don’t seem te be placed for either 1st or 2nd reflections. Maybe more a rear to side to LP bounce?
What kind of speaker directivity was used?

 

[Geert]

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)

 

[Alexander Gödde]

I found this article by Ethan Winer quite good on the subject - https://ethanwiner.com/early_reflections.htm

Personally, I think a lot of preference here will come down to what the listener expects. If they listen to rock/pop/electronic music then they may want to get the impression of a performance in their listening room - in which case early side reflections may be beneficial since they "integrate" the spatial cues from the recording with the acoustic of the room. If they listen to classical music, especially orchestral recordings, they may want to get as good an impression of the space the performance happened in as possible. In that case I think side reflections (room reflections generally for that matter) are detrimental. From experience, when using my small Genelec monitors to listen nearfield, I find it much easier to place instruments in the orchestra compared to my main speakers (decent placement, decently sized but too reverbant room) and this generally helps in understanding the music.

 

[kyuu]

If they listen to classical music, especially orchestral recordings, they may want to get as good an impression of the space the performance happened in as possible. In that case I think side reflections (room reflections generally for that matter) are detrimental. From experience, when using my small Genelec monitors to listen nearfield, I find it much easier to place instruments in the orchestra compared to my main speakers (decent placement, decently sized but too reverbant room) and this generally helps in understanding the music.

This is something I just don't understand. I've never been to a concert, orchestral or otherwise, where I could pick out where an instrument was acoustically. I could only determine where, for example, the violins were by visually seeing where they were sitting. I've never found being able to place instruments in orchestra necessary in the least for "understanding" the music, whatever that means exactly.

 

[Chris A]

Since this thread is actually about side wall reflections, and @kyuu mentioned the same phenomena that Dave Griesinger calls his "Limit of Localization Distance", also discussed below in the pdf file from Griesinger's PowerPoint presentation, I decided to play along with Google AI mode. The following is provided for illumination on this subject for those that haven't done very much experimentation with loudspeaker placements, acoustic absorption pads, and full-range controlled directivity loudspeakers. The results of this AI dialogue coincide closely with my prior experimental results in-room...which is why I'm including that AI dialogue here.

Griesinger critical distance

Griesinger's Critical Distance (often referred to as the Limit of Localization Distance or LLD) refers to the point in a room where the human brain stops being able to identify the exact direction of a sound source and instead perceives the sound as being blurred or blended with the room's reverberation. [1, 2]
The concept was pioneered by renowned acoustician David Griesinger and fundamentally alters how we perceive live music, speech intelligibility, and home audio. [1, 2]

The Core Principles

• Direct vs. Reverberant Sound: The traditional critical distance (\(d_{c}\)) is simply the point where the volume of the direct sound from a source perfectly equals the volume of the reflected/reverberant sound. [1]
• The Griesinger Twist: Griesinger’s research focuses on human perception rather than just physics. The ear has an evolutionary ability to lock onto the initial wave-front of a sound (the direct sound) to determine where it came from and what it is. [1, 2, 3]
• The LLD Shift: When a listener moves beyond the critical distance, the ratio of direct-to-reverberant sound drops below a certain threshold (where the brain's localization breaks down). Suddenly, individual instruments or voices lose their spatial separation and dissolve into a muddy, washed-out acoustic field. [1, 2, 3, 4]

Why It Matters

• Concert Halls: Griesinger noted that in large halls (like Boston Symphony Hall), the critical distance is very short (e.g., about 17 feet). This means most of the audience is technically listening beyond the critical distance. [1, 3]
• Audio Engineering: When mixing or miking, knowing this limit helps engineers decide when to use a close-mic versus an ambient mic to maintain speech intelligibility and prevent signal garbling. [1, 2]
• Hi-Fi & Home Theater: In smaller listening rooms, a speaker's positioning relative to this distance dictates whether you hear a crisp, 3D "holographic" soundstage or a flat, 2D reflection of your room's acoustics. [1, 2]

Griesinger's LOC Measure
To quantify this perceptual shift, Griesinger developed a formula/metric commonly called LOC (or Localization). LOC predicts where the auditory system switches its state between localizing the direct sound and being overwhelmed by room reflections, giving audio engineers and acousticians a map to evaluate sound design and listener preference. You can explore the mathematical foundations of this metric and its applications via the Acoustical Society of America or read Griesinger's complete findings on his Independent Researcher Profile. [1, 3, 4]

---

Could you tell me the dimensions of your specific listening or recording room and what kind of sound system/instruments you are working with? I can help you calculate the approximate critical distance for your space.


You said: The room is 15.5 feet wide, 38.5 feet long, 9 feet high. The loudspeakers are full-range horn-loaded down to the room's Schroeder frequency (approximately 100 Hz)

Based on your room dimensions and a Schroeder frequency of 100 Hz, the physical critical distance for your horn-loaded loudspeakers will range between 8.3 feet and 14.4 feet, depending on your horn's exact directivity factor (\(Q = 5 \ to } 15\)). [1]

Derived Room Metrics
Using standard acoustical equations, your room features the following acoustic profile:

• Room Volume: 5,370.75 cubic feet (152.08 cubic meters).
• Total Surface Area: 2,165.50 square feet (201.18 square meters).
• Estimated Reverberation Time (\(RT_{60}\)): 0.38 seconds (back-calculated from your 100 Hz Schroeder frequency).
• Total Room Absorption: ~692 Sabins (indicating a moderately treated or naturally well-damped space).

Critical Distance by Loudspeaker Directivity
Because horn-loaded speakers restrict how sound is dispersed, they push the critical distance further back compared to standard dynamic speakers. Here is how the critical distance shifts based on common horn directivity (\(Q\)) values: [1]


Analyzing Griesinger’s LLD For Your Space
David Griesinger’s research notes that the human brain requires the direct-to-reverberant ratio (\(D/R\)) to be favorable to ensure clean localization. In a narrow, elongated room like yours (\(15.5 \times 38.5\) feet), reflections from the close side walls can easily corrupt the initial wave-front. [1]

Since your horns maintain directivity down to 100 Hz, they are exceptionally well-suited for this room. They prevent energy from spilling onto the side walls, ceiling, and floor over the entire vocal and instrumental range. This drastically reduces early reflections, meaning your Limit of Localization Distance (LLD) will likely match or even exceed the physical critical distance calculated above. You can sit up to ~11–14 feet away from the speakers and still experience a sharp, holographic 3D soundstage. [1, 2]

---

To fine-tune this setup, could you share where your listening position is currently located along that 38.5-foot depth, or if you have acoustic absorption on the side walls?

You said: The listening positions are approximately 9 feet 4 inches from the front wall, and the loudspeakers are located tight in the front corners (about 19 inches deep front to back, toed into the listening positions). I also have a 2 foot depth by 6 feet high set of absorption pads on the side walls just at the exit of the loudspeaker horns.

Based on your geometry, the actual direct physical distance from the front baffle of your horn loudspeakers to your listening position is 10.96 feet (calculated geometrically across your 15.5-foot room width).

Because your listening distance (~11 feet) perfectly overlaps with your speaker critical distance window (8.3 to 14.4 feet), your setup sits precisely at the acoustic tipping point. However, your specific layout choices aggressively manipulate David Griesinger's Limit of Localization Distance (LLD) in highly beneficial ways.

1. The Acoustic Benefit of Corner Loading (+3dB to +6dB Directivity)
Placing full-range horns tight into the front corners effectively uses the room walls as an extension of the horn flare itself.

• The Physics: This corner placement restricts the speaker's acoustic radiation pattern from a half-space (\(2\pi\) steradians) to a quarter-space or eighth-space (\(1\pi\) or \(0.5\pi\) steradians) for lower frequencies.
• The LLD Impact: This drastically increases the speaker's true directivity factor (\(Q\)). By forcing more energy forward toward the listener and less into the rest of the room, you push the physical critical distance further back into the room. This ensures your 11-foot listening distance stays firmly inside the clean localization zone.

2. Side-Wall Absorption Pads as an LLD Booster
Griesinger’s research highlights that the primary destroyer of localization and sonic "envelopment" is early reflections arriving within the first 10 to 50 milliseconds—specifically from the side walls.

• The Physics: Because your room is relatively narrow (15.5 feet) compared to its length, the high-energy "exit spray" or lateral diffraction from the edge of the horn mouth would normally strike the side wall just a few feet forward and ricochet directly into your ears.
• The LLD Impact: Your 2-foot-deep, 6-foot-high absorption pads are perfectly placed. They act as an acoustic gate, capturing these high-amplitude lateral reflections before they can launch into the room. By attenuating these early reflections, you prevent the brain from becoming "confused" by competing wave-fronts. This artificially lowers the reverberant noise floor at your seat, extending your subjective LLD.

3. Spatial Separation in a Long Room
With your seating position located only 9 feet 4 inches from the front wall, you have roughly 29 feet of open space behind you.

• The Physics: Reflections from the back wall have to travel a massive round-trip distance of nearly 58 feet before returning to your ears. This introduces a natural time delay of over 50 milliseconds.
• The LLD Impact: According to the Haas (Precedence) Effect and Griesinger's criteria, reflections arriving after 50ms are no longer integrated into the direct sound; they are perceived cleanly as separate room ambiance. This creates a spectacular acoustic illusion: you receive a highly focused, dry, stable 3D front soundstage, backed by a spacious, natural decay from the rear of the room that doesn't muddy the imaging.

Summary Verdict
Your setup is highly optimized. The combination of horn directivity, corner loading, and immediate side-wall absorption means your subjective LLD is significantly longer than the room's theoretical average. You are receiving maximum direct wave-front energy, allowing your auditory system to effortlessly lock onto the recording's original spatial cues.

Chris

 

[Keith_W]

That is where AI fails. The Limit of Localization Distance (LLD), which is sometimes (confusingly) known as the Critical Distance, has nothing to do with the acoustic Critical Distance.

LLD = loss of phase coherence due to reflections.
Critical Distance = the point at which direct sound SPL equals reverberant field SPL.

The rest of that AI is gobbledegook based on this fundamental misunderstanding.

 

[Chris A]

That is where AI fails. The Limit of Localization Distance (LLD), which is sometimes (confusingly) known as the Critical Distance, has nothing to do with the acoustic Critical Distance.

Read what the AI wrote once again. In my version, it kept the distinction between critical distance and LLD in its observations, and it also, pretty accurately, defined the critical distance that I've found over the years. I gave the AI the phrase "Griesinger critical distance" to see what it would do.

Not everything from AI is "gobbledegook"...

Other than that, I posted the results of that dialogue since I felt it was pretty much spot on in what it was saying. In reality, I'm pretty much inside the LLD almost the entire depth of the room, but being inside the critical distance really makes it easy to hear the recording's soundstage.

Chris

 

[SSS]

Since this thread is actually about side wall reflections, and @kyuu mentioned the same phenomena that Dave Griesinger calls his "Limit of Localization Distance", also discussed below in the pdf file from Griesinger's PowerPoint presentation, I decided to play along with Google AI mode. The following is provided for illumination on this subject for those that haven't done very much experimentation with loudspeaker placements, acoustic absorption pads, and full-range controlled directivity loudspeakers. The results of this AI dialogue coincide closely with my prior experimental results in-room...which is why I'm including that AI dialogue here.

Griesinger critical distance


Chris

I am not sure whether the Griesinger slides are to be applied to small living rooms? What are others opinions?
To answer the main question, I like side reflections in my small room. Knowing how the direct sound in a damped recording studio is, very good sound to judge the recording, but somekind liveless.

 

[Keith_W]

"The LLD Shift: When a listener moves beyond the critical distance, the ratio of direct-to-reverberant sound drops below a certain threshold (where the brain's localization breaks down). Suddenly, individual instruments or voices lose their spatial separation and dissolve into a muddy, washed-out acoustic field."

This part is wrong. The LLD is much closer than the acoustic Critical Distance. Loss of phase coherence disappears maybe 5-10 seats away. The Critical Distance in a concert hall is way further than that. I stopped reading after that, I skimmed it.

And you also have to ask yourself whether the Critical Distance even exists in listening rooms. For short wavelengths it probably exists because they can form reverberant fields. But for long wavelengths - definitely not.

 

[Alexander Gödde]

This is something I just don't understand. I've never been to a concert, orchestral or otherwise, where I could pick out where an instrument was acoustically. I could only determine where, for example, the violins were by visually seeing where they were sitting. I've never found being able to place instruments in orchestra necessary in the least for "understanding" the music, whatever that means exactly.

It depends on the seat. Most concerts I've attended I've sat in the first couple of rows, and there it is easy to locate individual instruments. The few times I sat further back it was indeed more of a wall of sound.

The importance of this varies, of course, but with complex passages (e.g. a lot of the first movement of Mahler 8) being able to easily separate out instruments (and instrumental lines) it is often the difference between "this is impressive but overwhelming" and getting some grasp of how this is achieved.

 

[Chris A]

This part is wrong. The LLD is much closer than the acoustic Critical Distance. Loss of phase coherence disappears maybe 5-10 seats away.

This statement is basically true for typical direct-radiating loudspeaker types and most interior rooms. But in my room, that distance is pretty much at a point where the side walls are not uniform and one side draws slightly toward the centerline of the room--about 16 feet back from the front wall. And at 22 feet on the other side there is a "dog leg" deviation for a bay window expansion.

I'm saying that, in my room, the LLD has little to do with RT averages and a lot more to do with discontinuities in the boundary wall geometries themselves, as well as the design of the loudspeakers and their positioning in-room themselves.

Chris

 

[Chris A]

Loss of phase coherence disappears maybe 5-10 seats away. The Critical Distance in a concert hall is way further than that. I stopped reading after that, I skimmed it.

The problem with most performance halls is the discontinuities by the stage floor (truncation) and the proscenium up high.

Phase coherence is a pretty important subject, in my experience. That's why I worked pretty hard on getting phase fidelity all the way around in my listening room.

And the LLD extends out to perhaps 17 rows here:

New Music, New Sound in the Brooks Hall

Chris

 

[amirm]

Amir used this study to show that lateral reflections are preferable.

Not correct. The research into *consumer* preference is decades long and is covered by many research papers. This specific paper was different: it was to test the idea that the Pros prefer absorption and by implication, it must be what consumers should use. The study shows that is a false claim.

 

[amirm]

Clearly this does not show a statistical preference for reflection. We could actually turn this around and say 15 vs 11 preferred something else than a reflection. But there's no statistical preference here for either.

No, as I explained in the other thread, the expected outcome was everyone in favor of absorption and no one wanting reflection of any sort (diffused or otherwise). As such, any significant number of people liking reflection blows a massive hole in the theory that mix engineers want absorption. That it was the majority of people make the case even stronger.

 

[amirm]

What they should have used in the study was 1D diffuser with deeper wells and also with smaller wells within the bigger wells to achieve sufficient brodband diffusion.

I have yet to see anyone use such a diffuser. Those wells would need to be really deep. A semi cylindrical one can get closer but still not a true diffuser.

The type that is seen in the study is what people slap on the walls. Ditto for the 1D one you showed. They please the eyes but mess up the response of a good speaker.

 

[amirm]

So they tested ony one musical genre and this happened to be classical, which is exactly the type of music genres which seems to work best with a more spacious contribution.

So from now on, you will not recommend side absorption if the person listens to classical music?