Early reflections make cancellation dips and summation humps in response. Late ones don't, and get mixed with other pathways, total effect is wider soundstage.
Ok, many hifists don't like the "dipole sound" but prefer nearfield like sharp imaging and snappy transients.
The-Magic-in-2-Channel-Sound
2.1 Room Reflections
While anechoic conditions are useful for studying directional hearing, one must be careful to translate the findings directly to situations where multiple reflections of a signal occur. Again, this is a situation that is predominant in natural hearing and where evolution has optimized the signal processing between the ears for survival purposes. For example, it is important not to be distracted by reflections in finding the direction from which a sound is coming. Psychoacoustic research has shown that a first reflection11, which occurs shortly after the direct signal (within <25ms), must be stronger than the direct signal before it shifts the direction of the first arriving signal. A second12 reflection from a different direction has to be even stronger than the first reflection to shift direction. But later reflections (>30 ms) draw increasingly more attention unless their amplitude decreases with longer delays. This makes sense because late reflections could actually be coming from a second source.
A loudspeaker in a room produces a large number of reflections13 and perceptual issues become difficult to study in detail because of the large number of signal streams that arrive at each ear of a listener. Figure 3. Matters become even more complicated when the loudspeaker changes polar characteristics with emitted frequency; speaker L is typical for the vast majority of box loudspeakers. These speakers radiate omni-directional at low frequencies and become increasingly more forward directional with higher frequencies while maintaining a flat on-axis response. Consequently such box loudspeaker illuminates the room quite differently from a constant directivity dipole - for example speaker R. The dipole's reflections produce different superimposed sound streams at the ears of a listener even when they arrive from the same directions as those of a box loudspeaker in the dipole's location.
Siegfried Linkwitz Talks Loudspeakers - YouTube
Ok, many hifists don't like the "dipole sound" but prefer nearfield like sharp imaging and snappy transients.
The-Magic-in-2-Channel-Sound
2.1 Room Reflections
While anechoic conditions are useful for studying directional hearing, one must be careful to translate the findings directly to situations where multiple reflections of a signal occur. Again, this is a situation that is predominant in natural hearing and where evolution has optimized the signal processing between the ears for survival purposes. For example, it is important not to be distracted by reflections in finding the direction from which a sound is coming. Psychoacoustic research has shown that a first reflection11, which occurs shortly after the direct signal (within <25ms), must be stronger than the direct signal before it shifts the direction of the first arriving signal. A second12 reflection from a different direction has to be even stronger than the first reflection to shift direction. But later reflections (>30 ms) draw increasingly more attention unless their amplitude decreases with longer delays. This makes sense because late reflections could actually be coming from a second source.
A loudspeaker in a room produces a large number of reflections13 and perceptual issues become difficult to study in detail because of the large number of signal streams that arrive at each ear of a listener. Figure 3. Matters become even more complicated when the loudspeaker changes polar characteristics with emitted frequency; speaker L is typical for the vast majority of box loudspeakers. These speakers radiate omni-directional at low frequencies and become increasingly more forward directional with higher frequencies while maintaining a flat on-axis response. Consequently such box loudspeaker illuminates the room quite differently from a constant directivity dipole - for example speaker R. The dipole's reflections produce different superimposed sound streams at the ears of a listener even when they arrive from the same directions as those of a box loudspeaker in the dipole's location.
Siegfried Linkwitz Talks Loudspeakers - YouTube
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From what I have read, Juhazi, above about 30-50msec you perceive reflections as echoes. Under that threshold the brain will combine new sounds with the first arrival regardless of which direction they come from. This is sometimes called "suppression" or "echo suppression". That is all that I can recall reading about the subject, and never anything that mentions some earlier threshold (e.g. 6msec) even though the 1m minimum front wall distance is something that I adhere to myself.
It's not really been an issue for me, nor have I been able to explore it, because I have not had a long but narrow listening room. Linkwitz's room in Corte Madera was such a shape, so he probably had a lot of time to consider this issue.
Can any one point to the source of the 6msec (or 1m) rule?
It's not really been an issue for me, nor have I been able to explore it, because I have not had a long but narrow listening room. Linkwitz's room in Corte Madera was such a shape, so he probably had a lot of time to consider this issue.
Can any one point to the source of the 6msec (or 1m) rule?
Which part, the part about less reflections or the part about early reflections being bad?
The sentiment that early (e.g. sidewall) reflections are "bad". Where did that come from originally? I suppose it is from the "keep the system out away from the front wall by at least 1m" rule, but I cant' recall where that came from either. What is the origin of these ROTs?
Many of these "rules" that people say and read they could simply test for themselves.
I can't remember the origin of 6ms "rule", but it comes from sound perception in brains of humans. Anyway first reflections do change timbre (frequency distribution) heavily.
A nice way to listen to monopole vs. dipole/omni difference is this - Play pink noise with one speaker, sit in spot, move your head and then walk around the room. A full-range dipole with also tweeter radiating backwards will retain timbre much better. Dipoles on the long wall and wide separation do give very good stero imaging and focusing too - late reflections do't mess that but just add "space".
I listened to typical monopole stereo and 5.1 speakers for 40 years and didn't like classical at home. For classical, choir etc. music dipoles in stereo are even better (for me, SL and many friends who co to concerts - more natural) than a regular 5.1 channel recording played in 5.1 - and easier to set up in a family living room!
David Griesinger is a real authority about soundfields and perception
http://www.sengpielaudio.com/Pitch-Timbre-SourceSeparation.ppt
http://www.davidgriesinger.com/Acoustics_Today/AES_preprint_2012_2.pdf
A nice way to listen to monopole vs. dipole/omni difference is this - Play pink noise with one speaker, sit in spot, move your head and then walk around the room. A full-range dipole with also tweeter radiating backwards will retain timbre much better. Dipoles on the long wall and wide separation do give very good stero imaging and focusing too - late reflections do't mess that but just add "space".
I listened to typical monopole stereo and 5.1 speakers for 40 years and didn't like classical at home. For classical, choir etc. music dipoles in stereo are even better (for me, SL and many friends who co to concerts - more natural) than a regular 5.1 channel recording played in 5.1 - and easier to set up in a family living room!
David Griesinger is a real authority about soundfields and perception
http://www.sengpielaudio.com/Pitch-Timbre-SourceSeparation.ppt
http://www.davidgriesinger.com/Acoustics_Today/AES_preprint_2012_2.pdf
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It's at least in one of Geddes's early whitepapers. I've seen it in other acoustics papers as well, that the brain smears sources that arrive within ~10 ms of each other.
Let me see what I can dig up...
Maybe the word you are looking for is fuses sources rather than smears sources, meaning that they are perceived as a single sound.
Keith
Thanks 454Casull. From the intro (with my highlights in blue):
Below you can find a link to an online pdf of SL's AES paper on the subject. It seems that on page 3 he uses "relevant observations from designing and evaluating loudspeakers" to assign the 6ms threshold, that is to say his own opinions and feelings about the subject based on his experiences, which I assume were largely obtained in his home listening room (see Figure 2 for a hint). It seems that researchers have come up with values anywhere between 1msec and 10msec, based on Taylor's paper. I suppose one could go out to 10msec (locate the speakers about 1.5m away from the rear wall) to err on the safe side. Needless to say, dipoles need sufficient "room" to obey these limit AND for the listener to be away from the wall that it behind them as well. In fact SL lays out some minimum room dimensions in his paper (see Figure 3).
Following the sentiment above and regarding LATERAL WALL reflections, it would seem that as long as they are delayed past the XXX msec threshold then they are no worse than any other type of reflection. For SL the side walls in his listening room were a problem because they were "too close", and so he came up with the idea to toe in the speakers to aim the 90-degree null at the spot on the wall where the reflection was strongest. But it seems that this would be completely unnecessary in other settings for which the room is sufficiently wide. Too bad we cannot just ask him about it.
Here are the references from the Taylor paper:
References
[1] F. Toole, Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms.
Focal Press, 2008.
[2] H. Haas, “Uber den einuss eines einfachechos auf die horsamkeit von sprache,” Acustica, vol. 1,
p. 4958, 1951.
[3] ——, “The influence of a single echo on the audibility of speech,” Journal of the Audio Engineering
Society, vol. 20, pp. 146–159, 1972.
[4] M. Barron, “The subjective effects of first reflections in concert halls: The need for lateral
reflections,” Journal of Sound and Vibration, vol. 15, pp. 475–494, 1971.
[5] S. E. Olive and F. E. Toole, “The detection of reflections in typical rooms,” Journal of the
Audio Engineering Society, vol. 37, pp. 539–553, 1989.
[6] S. Linkwitz, “The challenge to find the optimum radiation pattern and placement of stereo
loudspeakers in a room for the creation of phantom sources and simultaneous masking
of real sources.” Audio Engineering Society, Oct. 9–12, 2009, paper 7959, presented at
the 127th Convention, New York. [Online]. Available: http://www.linkwitzlab.com/AES-NY'09/The Challenge.pdf
Below you can find a link to an online pdf of SL's AES paper on the subject. It seems that on page 3 he uses "relevant observations from designing and evaluating loudspeakers" to assign the 6ms threshold, that is to say his own opinions and feelings about the subject based on his experiences, which I assume were largely obtained in his home listening room (see Figure 2 for a hint). It seems that researchers have come up with values anywhere between 1msec and 10msec, based on Taylor's paper. I suppose one could go out to 10msec (locate the speakers about 1.5m away from the rear wall) to err on the safe side. Needless to say, dipoles need sufficient "room" to obey these limit AND for the listener to be away from the wall that it behind them as well. In fact SL lays out some minimum room dimensions in his paper (see Figure 3).
Following the sentiment above and regarding LATERAL WALL reflections, it would seem that as long as they are delayed past the XXX msec threshold then they are no worse than any other type of reflection. For SL the side walls in his listening room were a problem because they were "too close", and so he came up with the idea to toe in the speakers to aim the 90-degree null at the spot on the wall where the reflection was strongest. But it seems that this would be completely unnecessary in other settings for which the room is sufficiently wide. Too bad we cannot just ask him about it.
Here are the references from the Taylor paper:
References
[1] F. Toole, Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms.
Focal Press, 2008.
[2] H. Haas, “Uber den einuss eines einfachechos auf die horsamkeit von sprache,” Acustica, vol. 1,
p. 4958, 1951.
[3] ——, “The influence of a single echo on the audibility of speech,” Journal of the Audio Engineering
Society, vol. 20, pp. 146–159, 1972.
[4] M. Barron, “The subjective effects of first reflections in concert halls: The need for lateral
reflections,” Journal of Sound and Vibration, vol. 15, pp. 475–494, 1971.
[5] S. E. Olive and F. E. Toole, “The detection of reflections in typical rooms,” Journal of the
Audio Engineering Society, vol. 37, pp. 539–553, 1989.
[6] S. Linkwitz, “The challenge to find the optimum radiation pattern and placement of stereo
loudspeakers in a room for the creation of phantom sources and simultaneous masking
of real sources.” Audio Engineering Society, Oct. 9–12, 2009, paper 7959, presented at
the 127th Convention, New York. [Online]. Available: http://www.linkwitzlab.com/AES-NY'09/The Challenge.pdf
I have read a lot about this issue, but can't give specific references. One approach to the perceptual aspects would be to look at Geddes' and Toole's writings. Toole does not like to suppress sidewall reflections as they are felt to enhance spaciousness. Geddes disagrees (I think! Hate to put words into people's mouths).
I have long suppressed sidewall reflections. I think (especially for a solitary listener), that this promotes a "you are there" perspective rather than "they are here." In my thinking the reverberation in the recording comes to the fore with this approach rather than the signature of the room.
Bill
I have long suppressed sidewall reflections. I think (especially for a solitary listener), that this promotes a "you are there" perspective rather than "they are here." In my thinking the reverberation in the recording comes to the fore with this approach rather than the signature of the room.
Bill
The paper below quotes Toole & SL but then SL quotes RT ("Richard Taylor has analyzed the required room dimensions and speaker setups to obtain a >6 ms delay for the first order reflections"). When I read the Tooles book I can't find 6ms, its actually less. Tooles chapter 6, "precedence effect" (<1ms) tells us where the sound is coming from and "fusion zone" (1-30ms) is when all sound is integrated (no direction). BTW I use the 6ms because I read it somewhere and it seemed to work. There may be other studies.
https://faculty.tru.ca/rtaylor/publications/reflectgeometry.pdf
.
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This was something SL wrote many times but here is one example
"The loudspeakers must be set up at least 1m
distant from left and right side walls and 1m from
the wall behind them. The resulting time delay of
about 6ms is necessary for the ear-brain
perceptual apparatus to separate direct from
reflected sound streams. With neutral excitation
of the room response, the listener can then
withdraw attention from the room and process
primarily the direct sound streams from the
loudspeakers. This is similar to not hearing the
ticking clock or to the cocktail party effect where
attention and hearing is drawn to information
streams of interest. Everything else is moved
beyond the acoustic horizon as in survival mode"
from here https://www.linkwitzlab.com/IJAETv2n2a2-Linkwitz-1.pdf
I scanned the paper, reading up to that passage, but I did not find any mention of a source for this claim by SL. Now I regard him very highly, but I think we need to be careful not to make some hard and fast rules about how loudspeakers should be set up or minimum distances to boundaries, if those ROTs are based on one man's observations in his living room. This is why I am looking for the source of this number (6msec or 1m distance). From all I can tell, SL himself made it up based on his own observations. And if that is true that is fine, I just wish he had been more clear about it.
I have a copy of Toole's book. It would have been nicer to get a chapter or page number... I will look it over tonight.
OK, I found that the Taylor paper references Figure 6.16 in Toole, so I poked around that section of the book. Unfortunately all the figures have too coarse a time resolution to make out any differences of 1 or 2 msec. It seems that 5msec is about right, on average, but this depends on the level of the reflection, etc. Maybe this is just best left up to user preference?
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