OK, this is from what was originally planned as an even longer post that I abandoned for multiple reasons I won't get into here. You can think of me as a magpie, no original ideas but rather collecting colorful bits of ideas. Some may accuse me as "You seem to be approaching ASR as if it a courtroom" or "copypasta" or otherwise unreadable because of spacing/formatting issues. Also, I originally framed the following as a private message to you, but I have been told elsewhere that "PM is not the place to migrate debate from thread." Anyway, I tried to edit down to the topic of early reflections from any direction, also possible concerns regarding localization or (frequency-dependent) spatial shift, as well as preference.
As I said in another thread, please feel free not to specifically quote/reference me. I was trying to argue in terms of individual preference, then as now. You win, I concede, you're right, I actually have little else to say,
Bech 1995
https://asa.scitation.org/doi/10.1121/1.413047
“Electroacoustic simulation of the right-hand loudspeaker of a stereophonic setup, positioned in a small room.”
“The results show that only the first-order ceiling and floor reflections are likely to contribute individually to the timbre of a speech signal. For a noise signal additional reflections, from the wall to the left of the listener, will individually contribute to the timbre. The threshold of detection for all reflections depends on the level of the reverberant field. If the reverberant field is removed, thresholds will decrease by 2-5 dB.”
“Care should be taken when generalizating from the results presented in this paper.”
Gerhard 1996
https://www.researchgate.net/public...ENT_FOR_OPTIMISED_PHANTOM_SOURCE_REPRODUCTION
https://www.researchgate.net/profil...hnical-Papers-on-Stereo-and-Spatial-Audio.pdf
The basis for classic Audio Physic speaker placement recommendations
“[M]ost reflections are frequency dependent so these spatial shifts are also frequency dependent, which implies sub 1 ms reflections can distort the outline of an image for all but the smallest bandwidth sounds.”
“Any loudspeaker layout optimised for best phantom image reproduction should minimize the number of early reflections below 5 ms and should maximise the level difference between the loudspeaker signal and these reflections.”
- “In a normally shaped listening room the loudspeakers are placed near the foci of an ellipse that just fits within the room boundaries.”
- “The listening position is located close to one of the longer walls of the room midway between the loudspeakers, the distance being typically 0.4 m to 1 m, depending on room size…the first of these reflections comes from the wall directly behind the listening position, where it is believed from experience that this reflection is not so objectionable for phantom image perception.”
- “To reduce those room modes which are maximally exited for loudspeakers placed at the midpoint between two walls, the loudspeakers are either moved a little in a direction towards the listener if the subtended angle between them and the listener is below 60 ° or alternatively, a little in the opposite direction. The resulting subtended angle should be in the 70 ° to 90 ° range. In a companion paper [10] it is shown that the centre image (considered to be the weakest part of a two loudspeaker stereo sound stage) in a 74°/low reflection- layout can be more stable than the two thirds off centre image of a 60°/high-reflection layout.”
Micro placement
- By ear…”until a firm centre image is reached at all frequencies”
- “By measurement…For this method a microphone is placed at the centre location normally occupied by the listener's head. Measuring the impulse response of both loudspeakers in parallel a difference in distance can be observed as a double pulse (see Figure 8). Next, the position of one speaker is changed until only one pulse is observable. Now one speaker is wired "out of phase" and the angle of this speaker is changed until the suppression of the pulse is maximized.”
Loudspeaker requirements
- “[Level matching within a range of ± 0.5 dB should be mandatory.
- The loudspeaker should also be designed for low cabinet edge diffraction, as diffraction also causes secondary wave radiation (see for instance [11]) which compromises stereo focus
- The design axis of the loudspeaker where frequency and impulse response are optimum should point in a direction towards the listener, a minimum requirement for any competent loudspeaker design, although not always met.
- Finally, the distance where the soundfields of all drivers merge should not be too far away from the loudspeaker.”
Bech 1996
https://asa.scitation.org/doi/10.1121/1.414952
“The results have confirmed the findings of the first report that the floor reflection will contribute on an individual basis to the timbre of a noise signal.”
Bech 1998
https://pubs.aip.org/asa/jasa/artic...-sound-in-small-rooms?redirectedFrom=fulltext
“Subjects can reliably distinguish between timbre and spatial aspect of the sound field, that the spectral energy above 2 kHz of the individual reflection determines the importance of the reflection for the spatial aspects, and that only the first order floor reflection will contribute to the spatial aspects”
Toole 2008
Sound Reproduction, 2nd edition
“Why do recording and mixing engineers prefer to listen with reduced lateral reflections (higher IACC)? Perhaps they need to hear things that recreational listeners don’t. This is a popular explanation, and it sounds reasonable, but experiments reported in Section 6.2 indicate that we humans have a remarkable ability to hear what is in a recording in spite of room reflections—lots of them. But there is an alternative explanation, based on the observation that some listeners can become sensitized to these sounds and hear them in an exaggerated form. Ando et al. (2000) found that musicians judge reflections to be about seven times greater than ordinary listeners, meaning that they derive a satisfying amount of spaciousness from reflections at a much lower sound level than ordinary folk: “Musicians prefer weaker amplitudes than listeners do.” It is logical to think that this might apply to recording professionals as well, perhaps even more so, because they create artificial reflections electronically and manipulate them at will while listening to the effects. There can be no better opportunity for training and/or adaptation. In fact, it is entirely reasonable to think that acousticians who spend much of their lives moving around in rooms while listening to revealing test signals can become sensitized to aspects of sound fields that ordinary listeners blithely ignore. This is a caution to all of us who work in the fi eld of audio and acoustics. Our preferences may reflect accumulated biases and therefore may not be the same as those of our customers.”
“The broadly accepted midfrequency (around 500 Hz) reverberation time of 0.3–0.5 s”
Linkwitz 2010
https://www.linkwitzlab.com/TMT-Leipzig'10/TMT-Hearing%20spatial%20detail.pdf
“Thus the distance of the phantom source is based on distance cues that the left and right loudspeakers provide themselves by their direct signals and room reflections. These cues place the phantom source slightly behind a line connecting the loudspeakers.”
“
Two loudspeakers can produce a perceptual event that has no precedence in nature and evolution. The brain adapts to the situation by comparing the novel auditory cues to familiar ones. Two identical signal streams arriving from symmetrically placed sources at both ears can only mean that there is a sound source half-way between the loudspeakers, even when we cannot detect direct signals coming from that direction.”
“We can distinguish, though, the phantom center source from a real source, a center loudspeaker. The phantom source is less focused and has a different tonality. The reason is acoustic cross-talk between left and right loudspeaker signals at the ears and secondly, spectral coloration due to a 30 degree angle of sound incidence versus 0 degree for the frontal source.”
Discussed a little here:
http://www.moultonlabs.com/more/principles_of_multitrack_mixing_the_phantom_image/P0/
Linkwitz 2013
https://www.linkwitzlab.com/Sound_field/Field_control.htm
“First order room reflections must be delayed by >6 ms compared to the direct sound reaching the listener…the loudspeaker and listener equilateral triangle is preferably set up symmetrical to room boundaries or large reflecting surfaces…the direct-to-reverberant sound level ratio for sounds above the Schroeder frequency should be greater than –6 dB at the listening position.”
Evans 2013
https://openresearch.surrey.ac.uk/e...ty-upon/99511687202346?institution=44SUR_INST
Thesis with extremely useful summary of research to date, auralisation study
“The magnitude of effects caused by different loudspeaker directivities is reduced with the increased presence
of reflections.”
“Changes in loudspeaker directivity, in combination with changes in absorption, mostly affect perceived width, loudness and reverberence”
“A narrowing in on-axis directivity is associated with a perceived reduction in width, brightness, closeness and spaciousness and an increase in absorption associated with reduced loudness and reverberence”
Nakahara 2013
https://www.audiosciencereview.com/...ased-speaker-designs.6441/page-49#post-382097
“Absorption of the First Reflections Area on the side walls: “Width of sound image” will be narrower and
“Envelopment” will be lower. Absorption of the First Reflections Area on the front wall and the ceiling: “Width of sound image” will be narrower and “Clarity” will increase. Absorption of the First Reflections Area on the back
wall: “Width of sound image” will be narrower, “Envelopment” will be lower, and “Clarity” increases. Regardless of the absorption of the First Reflection Area, listening impression will change according to absorption of the other areas.”
Laukkanen 2014
https://users.aalto.fi/~ktlokki/Publs/mst_laukkanen.pdf
“The results of the preference tests clearly showed that mixing engineers prefer quite dry rooms (T60 of 0.15 - 0.20 s) and interviews confirm that the stereo image and the amount of room reverberation are the most important factors for them. In contrast, mastering engineers seemed to prefer more lively rooms (T60 of 0.30 - 0.40 s) and the frequency balance was the most important factor for
them. It was also noticed that the preference rating varied between different music samples, especially among mixing engineers.”
“The preference rating seemed also to vary between different music samples…on the basis of the listening tests, it seemed that different music genres need different kind of treatment in control room.”
Toole 2015
https://www.aes.org/e-lib/browse.cfm?elib=17839
“From about 200 Hz to around 600–1000 Hz the energetic sound events happen within about the first 50 ms—listeners are exposed to direct plus a few early-reflected sounds. Above this, for the top three octaves or more, the direct sound is the dominant factor.”
“In Fig. 14 the author has modified the original data to separately show the result of evaluations by trained and untrained listeners…More data would be enlightening, but this amount is sufficient to indicate that a single target curve is not likely to satisfy all listeners. Add to this the program variations created by the “circle of confusion” and there is a strong argument for incorporating easily accessible bass and treble tone controls in playback equipment. The first task for such controls would be to allow users to optimize the spectral balance of their loudspeakers in their rooms, and, on an ongoing basis, to compensate for spectral imbalances as they appear in movies and music.”
“The attenuated high frequencies preferred by the trained listeners stands in contrast to the preferences exhibited by those same listeners in numerous double-blind multiple-comparison loudspeaker evaluations…Is this a consequence of the different experimental methods: the different listener tasks? In one, listeners adjusted the bass and/or treble balance in a single loudspeaker model; in the other they rated spectral balances and other attributes in randomized comparisons of different products. It is a subtle but important difference awaiting an explanation.”
Laitinen, Pulkki 2015
https://pubs.aip.org/asa/jasa/artic...rolling-the-perceived-distance-of-an-auditory
“We assume that the D/R-ratio is the dominant distance cue with typical sound reproduction setups in domestic environments. Based on listening tests, it is shown that manipulation of the directivity pattern along with a single distance-dependent gain proves to be effective and able to render sources closer than the distance of the loudspeaker”
Toole 2016
https://www.audioholics.com/room-acoustics/room-reflections-human-adaptation
“There is no single “right” way to do things. About 37 years ago, when I was setting up the NRC listening room, I ran a drapery track down the front portion of the side walls and across the wall of the room behind the loudspeakers, hanging 4-foot sections of densely-folded heavy drapes. The track was about 6 inches from the wall for good broadband absorption. These could be moved around, and in the case of the sidewall reflections, we quickly found that things sounded better if they were pushed back for more "spacious" classical music, and pulled out for "in your face" rock/pop stuff. I knew a couple of stereo enthusiasts who copied the idea at home. I concluded that, in terms of loudspeaker/room combinations, one size does not fit all.”
Geddes 2016
Relevant information at 2:19 and 5:03 suggests possible critical range of frequency response, as well as one derivation for 10 ms as threshold for early reflections
Heddle 2016
https://acoustics.asn.au/conference_proceedings/AASNZ2016/papers/p143.pdf
Goals: “absence of competing sound…non- fatiguing. In short, this means the minimization of cognitive load, not overly acoustically dead or reverberant…attenuation of room mode resonances…both the sound sources and the room should be symmetrical about the central front/back axis…attenuate early median plane reflections sufficiently, via absorption, acoustic shielding or redirection, for there to be minimal changes relative to the source spectrum…An early lateral reflection should ideally be a mimic of the direct sound with the exceptions of overall level, delay and polarity...Early reflection absorption treatment is as frequency independent as can be feasibly achieved and is localized to the most effective treatment zones on the reflective surface…Adequate signal to noise and direct to reverberant levels are mandated”
“It can be seen that achieving this design requirement [“a 6 msec time window…minimum delay between the direct sound and the arrival of the first reflection”] with minimum room dimensions, for a typical listening triangle, results in a room wider than it is deep.”
Sauro 2017
http://nwaalabs.ipower.com/Files/NWAA Labs/Diffusion, When phase and energy becomes more important than directivity in the perception of space 2017 NOLA.pdf
“One ear hears a certain phase and the other hears a different phase and the brain interprets this as “space” The larger the differences, the larger the apparent space..“
“The presence control on certain amps was used to give the effect of increasing the space of a certain sound. This was done by increasing the level of high frequency components at around 16KHz. This was also accomplished in PA work by the use of an “air” control.”
“A small phase difference at high frequencies translates into the feeling of being in a small room“
“A larger phase difference at high frequencies translates into the perception of being in a larger room.”
Toole
Sound Reproduction, 3rd edition
“A recent listing test proved its worth when it revealed that a loudspeaker having excellent looking spinorama data (Section 5.3), which normally is sufficient to describe sound quality, was not rated highly as expected. The problem was found to be intermodulation distortion, an extremely rare event, associated with the way sounds from a woofer and tweeter combined in a concentric arrangement—so constant vigilance and listening are essential” (p97-98)
“the thresholds for the side wall and the ceiling reflections is almost identical…the dominant effect of the lateral reflection was spaciousness (the result of intraaural differences) and that of the vertical reflection was time change (spectral differences) (p207-208)
“The absorbers on the front and rear walls avoid reflections within the angular ranges that contribute little to envelopment…envelopment is most influenced by sounds in the 100 Hz to 1 kHz frequency, the lower frequencies (p423)
Useful supplementary materials:
https://routledgetextbooks.com/textbooks/9781138921368/home-theatre.php
Bech and Lokki 2019
https://users.aalto.fi/~ktlokki/Publs/JASMAN_vol_146_iss_5_3562_1.pdf
Sound field reproduction using spherical loudspeaker array in anechoic chamber, listeners told ““Imagine that you are in a typical residential room, listening to a 2-ch stereophonic reproduction over loudspeakers.”
Four perceptual constructs comprising attribute clusters
- Reverberance: relates to the later energy [of the sound field], “excellent relation” to RT30 and early decay time
- Width and envelopment: relate to the earlier energy of the sound field
- Bass
- Proximity, negatively correlates to width and envelopment, “strong correlation” with clarity index 50 (C50) and direct to reverb ratio (DRR)
“Assessors systematically preferred the sound fields with lower RT. In our study, the most preferred acoustical conditions presented fields that evoked the sense of being less reverberant and less wide and enveloping. The sources were perceived as closer to the listener, exhibiting high levels of proximity. It is also important to note that the current results suggested that a negative preference is apparent for acoustical conditions with RT higher than 0.4 s”
“One could attempt to alter the DRR within a field by means of directivity control in the loudspeakers, aiming to evoke certain perceptual aspects that would otherwise be dominated by the room’s natural acoustical field.”
Toole 2020
https://gearspace.com/board/showpost.php?p=15187387&postcount=61
“This requires reverberation times under 0.5 second.”
“Music with lots of decorrelated sounds, classical for example, is sometimes enhanced by reflections, although coincident-mic recordings may benefit from a lack of reflections - letting the direct sounds be more dominant (the Blumlein stereo effects work best in an almost anechoic situation).
Pan-potted recordings (the majority of pop) end up delivering essentially monophonic sounds from left and right loudspeakers, and these may well benefit from a bit of spatial enhancement. Otherwise we are left with what really annoys me about stereo: a relatively spatial set of phantom images created by both loudspeakers, and two "anchor" images created by the left and right loudspeakers playing solo. “
“I too think that the "Schroeder" diffuser is not a universal solution, with uniquely advantageous properties. The "time smear" argument might have merit in specific applications but it has yet to be demonstrated in any scientific way. I think the needs of large venue "live" performances are quite different from small room multichannel reproduction. The fact that curved surfaces deliver relatively strong single reflections over wide angular ranges can, in fact, be highly advantageous in some situations.”
Sauro 2020
http://nwaalabs.ipower.com/Files/NWAA Labs/AES PNW Old Problems, New Solutions, Architectural Acoustics in Flux2.ZIP
“We cannot measure absorption directly. We measure the differences in the reverberation time in a reverberation room and use that to determine the amount of absorption needed in the room to effect that change. “
“Absorption is controlled by…the ratio of perimeter length to the area …the spacing of absorption is also controlling factor. “
“Baffles measured have an average of 1.2-1.3 times more absorption when hung in a room “
“The low frequency absorption [of wall mounted panels] only increases when the spacing from the wall does not exceed 1 inch. The effect disappears after 1 inch. This only applies to unimpeded airspace. “
Kantamaa 2020
https://aaltodoc.aalto.fi/bitstream...Kantamaa_Olli_2020.pdf?sequence=2&isAllowed=y
“Increase of the low–middle frequency [80-600 HZ] directivity improved clarity, reduced sound colouration, improved virtual sound image definition and transient reproduction compared to the conventional loudspeaker directivity.”
Lokki 2021
https://acris.aalto.fi/ws/portalfiles/portal/56976814/Riionheimo_Lokki_Movie_Sound_Part1.pdf
“The results from both listening tests show that differences in the sense of space, brightness, timbre, width, and clarity as well as in the distance are the most important when comparing cinemas and mixing rooms.”
Lokki 2022
https://acris.aalto.fi/ws/portalfiles/portal/56976875/Riionheimo_Lokki_Movie_Sound_Part2.pdf
“The perceived sense of space matches better with the measured C50 (scale inverted) in the middle frequencies than the measured reverberation time T30.”
“Perceived and measured clarity match well.”
“The width was difficult attribute to evaluate, indicating the word itself is ambiguous, especially with surround sound. The width of the soundscape is affected by the angle of the left and right screen speakers, the volume of the surround speakers, and the envelopment caused by the room reverberation.”
“The ratings for brightness match well with the level of the high frequencies above 4 kHz of the electroacoustical responses.”
“The perceptual distance matches better with measured clarity C50 in the middle frequencies than the actual listening distance.”
“The assessors preferred somewhat clear and dry sound over reverberant and distant; however the room should not be totally dead nor too bright.”
NWAA labs Stereophile article 2022
https://www.stereophile.com/content/nwaa-labs-measurement-beyond-atomic-level
“Each frequency range has a different driver set and a different far-field point. This distance, called
critical distance, is where the
direct sound from the speaker is equal in magnitude to the sound of the
reverberant field.”
"If you're within the critical distance from the speaker, you will hear the direct sound of the speaker. If you're beyond the critical distance, you will hear the reverberant field, aka the soundfield that is developed by the room.”
"Devices that are based on diffraction scramble the phase. When you scramble the phase, you lose localization…Most are restricted to frequencies around about 4kHz and 16kHz. They do absolutely nothing below 4kHz.”
https://www.diyaudio.com/community/...iway-loudspeaker-defects.392356/#post-7178418
Lokki 2024
https://research.aalto.fi/en/public...-exploring-the-impact-of-listening-room-on-pe
https://research.aalto.fi/en/public...-exploring-the-impact-of-listening-room-on-so
Riionheimo 2025
Hearing as intended: How differences in listening conditions affect sound translation
http://urn.fi/URN:ISBN:978-952-64-2852-9
My takeaway in this case is the system is complex enough that without constraining some of the variables, our ability to draw general conclusions is pretty limited.
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