I've been wanting to compile some of the current research on reflections into one thread for awhile as a reference and of course some debate as I know there are differing opinions on some of this and I admit some of this going to be my own editorializing based on personal experience with various speaker designs. Much of this is dealing with music listening in stereo, I realize in a home theater or studio, fewer reflections is seen as beneficial by many people. It's also important to note that there isn't really an agreed upon standard for what exactly is "wide" or "narrow" dispersion but these globe plots of the BMR and LS50 Wireless 2 clearly show the extremes:
Current Research
There is countless evidence that the majority of people prefer reflections when listening to music as it gives a sense of space and sounds more like live music. Dr. Toole also speculates that since Stereo can't possibly replicate a live performances 3d sound field that reflections may help to fill in the gaps and simulate it a bit better. Further research by Toole and Sean Olive have shown that listeners not only prefer reflections but that "Early Reflections" have a higher importance than later arriving reflections and that they also need to closely match the direct sound for the highest preference in listening tests. This research is the basis for the early reflections curve in the CTA-2034 measurement spec. I would go a step further and say the while the average of all the early reflections is important, I like to see that all of the early reflections are close to that average. Dr. Toole also considers sidewall reflections to be more important than vertical reflections but there isn't much research on that and I'll show some research later that shows that we probably can't neglect vertical reflections.
Ultra Wide Dispersion Speakers
I personally think the research that shows people generally like wide dispersion has been taken too far in some cases and some speakers have prioritized "wide dispersion" at the expense of other qualities in a speaker. The most extreme examples of course are speakers showcasing the RAAL 64-10 ribbon tweeter which have the widest dispersion of any speaker on the market but are also limited vertically due to their geometry. Now I admit I have limited experience with this tweeter because I auditioned them in a smaller room where wide dispersion maybe didn't matter as much but my big takeaways were that they sounded pretty normal regarding spaciousness except it was very obvious they were lacking vertically when switching back to my LS50. The LS50 sounded "bigger" because of it which was pretty surprising considering the 64-10 is known for wide dispersion and the spaciousness that comes with it. This comparison was the 1st moment that I realized that maybe vertical dispersion is more important than conventional wisdom gives it credit for.
Dr. Toole's study that was the basis for comparing speakers in mono actually also shows something interesting about dispersion and whether ultra wide dispersion is a goal worth pursuing. Aside from showing that listening to the speaker in mono makes it easier to discern differences in sound quality, another argument can be made that when listening to just 2 speakers in stereo, these spatial differences largely disappear. This is more along the line of what I hear in my own room listening to the "narrow dispersion" LS50s in 2 and 3 channel stereo for music, they don't seem to be lacking in spaciousness at all to my ears.
Ideal Dispersion
Based mostly on my thoughts between the RAAL 64-10 and the KEF LS50 I have been wondering if there is maybe an "ideal" dispersion considering both the horizontal and vertical planes. I've heard people debate that the widest dispersion is the best or even that we should be matching the dispersion pattern to the room dimensions. Neither approach has ever made sense to me, it seems like we should be trying to simulate the dispersion patterns of real vocals and instruments as best we can to get closest to the ideal speaker directivity, assuming of course that your goal is to get closer to a live acoustic performance in your room. I've never noticed that live music in smaller venues sounds bad in one room compared to another, they all sound natural to my ears. Musical instruments are obviously complex and no speaker is going to simulate all instruments but generally the sound is radiated fairly evenly from wherever the sound originates since they will be complex patterns from multiple point sources. The human voice also has a fairly even polar pattern and most notably isn't extremely wide horizontally or vertically.
Vertical Reflections
Another interesting debate is about the importance of vertical reflections where there is surprisingly very little research done on. Dr. Toole briefly mentions a study where he states "...intuition is rewarded in that the dominant audible effect of the lateral reflection was spaciousness (the result of interaural differences) and that of the vertical reflection was timbre change (the result of spectral differences)" but it isn't clear if the reflection was a perfect timbral copy of the direct sound or if it was distorted to match what would be the typical ceiling reflection from a speaker with vertically arranged drivers.
I've only found 2 other studies that give some information on vertical reflections. The first one is:
"The Effect of a Vertical Reflection on the Relationship between Preference and Perceived Change in Timbral and Spatial Attributes" where I'll quote the abstract:
This study aims to investigate a vertical reflection’s beneficial or detrimental contribution to subjective
preference, compared with perceived change in timbral and spatial attributes. A vertical reflection was
electro-acoustically simulated and evaluated through subjective tests using musical stimuli in the
context of listening for entertainment. Results indicate that the majority of subjects preferred audio
reproduction with the addition of a reflection. Furthermore, there is a potential relationship between
positive preference and the perceived level of both timbral and spatial differences, although this
relationship is dependent on the stimuli presented. Subjects also described perceived differences where
the reflection was present. These descriptors provide evidence suggesting a link between timbral
descriptions and preference. However, this link was not observed between preference and spatial
descriptions.
The interesting thing to me was even with a large dip in the vertical reflection, a little over half of the participants still preferred the tracks with the reflection, also 59 out of 78 of the total ratings were positive. This is the measurement of the reflection:
The other study is called: "Influence of first reflections in listening room on subjective listener impression of reproduced sound" This study compared the effects of absorbing various first reflections and comparing the results. Basically, the fewer reflections you absorb the wider the soundstage and envelopment but you lose clarity. Absorbing reflections is similar to them being absent so it's interesting that removing the ceiling reflections lessens image width and envelopment just slightly less than removing sidewall reflections. Here are a few of the key graphs: (Rs is absorption of sidewall, Rf= ceiling)
Also found in the thesis paper from the above study, another study was referenced: "Auditory envelopment" (Furuya, Fujimoto, Takeshima & Nakamua, 1995) :
All experiments utilised musical stimuli and the reflection and sound field were electro acoustically simulated. In relation to this thesis investigating a singular reflection, experiment one demonstrated that as the delay time of a singular reflection increased, the sound image grows vertically in size. Regarding auditory envelopment, experiments two and three show that as long as the ratio of lateral and vertical energy remains constant up to 200ms, envelopment becomes stronger as energy arriving from above increases. The author does note however that lateral arriving energy must be the “predominant factor to perceive envelopment”.
Small Room vs Big Room:
I mentioned that I was previously in a small room (11' wide with 8ft ceilings and about a 10' listening distance) and just moved to a concrete loft that is a similar listening distance but is 15' wide with 12' ceilings. It is somewhat surprising but it didn't make the speakers sound smaller, if anything they are more spacious and the room sounds like a concert hall. I have almost no absorption in this room at the moment, just a few love seats so that may be a factor. I also feel like the sound is more clear, I speculate that I'm hearing the direct sound more clearly with the early reflections that are now arriving a bit later due to the longer distances from the walls and ceiling and then the later arriving reflections are much lower in level so the overall sound remains clear.
Conclusion
I don't think definitive conclusions can really be drawn yet until we get some more research on these issues but it is food for thought. I'd like to see the effect of vertical reflections when the reflection isn't distorted as it was in one of the studies I showed. I'd also like to see how listeners perceive overall spaciousness when listening to speakers with differing horizontal and vertical dispersion.
My personal experience with multiple listening tests (sighted and blind, all level-matched and instantly A/Bing) is that my ears agree with most of the research I have posted. I have found that once at least 2 speakers are playing, I don't notice much difference in spaciousness but I do notice a difference when vertical response is very limited. This tells me that all early reflections need to be considered when going for that live spacious sound, not just the sidewall reflection. I also believe that since early reflections seem to be mostly what matters regarding spaciousness, the large difference shown in the globe plots between a wide and narrow dispersion speaker may not perceptively sound that different when the 1st reflections are within a decibel or so in amplitude. More in the preference realm but I also find listening to music in 3 channel stereo with the center about 3-5db lower than the fronts is a great way to get even more spaciousness while also reducing the stereo imaging effect which makes music sound more like music and less like headphones.
Current Research
There is countless evidence that the majority of people prefer reflections when listening to music as it gives a sense of space and sounds more like live music. Dr. Toole also speculates that since Stereo can't possibly replicate a live performances 3d sound field that reflections may help to fill in the gaps and simulate it a bit better. Further research by Toole and Sean Olive have shown that listeners not only prefer reflections but that "Early Reflections" have a higher importance than later arriving reflections and that they also need to closely match the direct sound for the highest preference in listening tests. This research is the basis for the early reflections curve in the CTA-2034 measurement spec. I would go a step further and say the while the average of all the early reflections is important, I like to see that all of the early reflections are close to that average. Dr. Toole also considers sidewall reflections to be more important than vertical reflections but there isn't much research on that and I'll show some research later that shows that we probably can't neglect vertical reflections.
Ultra Wide Dispersion Speakers
I personally think the research that shows people generally like wide dispersion has been taken too far in some cases and some speakers have prioritized "wide dispersion" at the expense of other qualities in a speaker. The most extreme examples of course are speakers showcasing the RAAL 64-10 ribbon tweeter which have the widest dispersion of any speaker on the market but are also limited vertically due to their geometry. Now I admit I have limited experience with this tweeter because I auditioned them in a smaller room where wide dispersion maybe didn't matter as much but my big takeaways were that they sounded pretty normal regarding spaciousness except it was very obvious they were lacking vertically when switching back to my LS50. The LS50 sounded "bigger" because of it which was pretty surprising considering the 64-10 is known for wide dispersion and the spaciousness that comes with it. This comparison was the 1st moment that I realized that maybe vertical dispersion is more important than conventional wisdom gives it credit for.
Dr. Toole's study that was the basis for comparing speakers in mono actually also shows something interesting about dispersion and whether ultra wide dispersion is a goal worth pursuing. Aside from showing that listening to the speaker in mono makes it easier to discern differences in sound quality, another argument can be made that when listening to just 2 speakers in stereo, these spatial differences largely disappear. This is more along the line of what I hear in my own room listening to the "narrow dispersion" LS50s in 2 and 3 channel stereo for music, they don't seem to be lacking in spaciousness at all to my ears.
Ideal Dispersion
Based mostly on my thoughts between the RAAL 64-10 and the KEF LS50 I have been wondering if there is maybe an "ideal" dispersion considering both the horizontal and vertical planes. I've heard people debate that the widest dispersion is the best or even that we should be matching the dispersion pattern to the room dimensions. Neither approach has ever made sense to me, it seems like we should be trying to simulate the dispersion patterns of real vocals and instruments as best we can to get closest to the ideal speaker directivity, assuming of course that your goal is to get closer to a live acoustic performance in your room. I've never noticed that live music in smaller venues sounds bad in one room compared to another, they all sound natural to my ears. Musical instruments are obviously complex and no speaker is going to simulate all instruments but generally the sound is radiated fairly evenly from wherever the sound originates since they will be complex patterns from multiple point sources. The human voice also has a fairly even polar pattern and most notably isn't extremely wide horizontally or vertically.
Vertical Reflections
Another interesting debate is about the importance of vertical reflections where there is surprisingly very little research done on. Dr. Toole briefly mentions a study where he states "...intuition is rewarded in that the dominant audible effect of the lateral reflection was spaciousness (the result of interaural differences) and that of the vertical reflection was timbre change (the result of spectral differences)" but it isn't clear if the reflection was a perfect timbral copy of the direct sound or if it was distorted to match what would be the typical ceiling reflection from a speaker with vertically arranged drivers.
I've only found 2 other studies that give some information on vertical reflections. The first one is:
"The Effect of a Vertical Reflection on the Relationship between Preference and Perceived Change in Timbral and Spatial Attributes" where I'll quote the abstract:
This study aims to investigate a vertical reflection’s beneficial or detrimental contribution to subjective
preference, compared with perceived change in timbral and spatial attributes. A vertical reflection was
electro-acoustically simulated and evaluated through subjective tests using musical stimuli in the
context of listening for entertainment. Results indicate that the majority of subjects preferred audio
reproduction with the addition of a reflection. Furthermore, there is a potential relationship between
positive preference and the perceived level of both timbral and spatial differences, although this
relationship is dependent on the stimuli presented. Subjects also described perceived differences where
the reflection was present. These descriptors provide evidence suggesting a link between timbral
descriptions and preference. However, this link was not observed between preference and spatial
descriptions.
The interesting thing to me was even with a large dip in the vertical reflection, a little over half of the participants still preferred the tracks with the reflection, also 59 out of 78 of the total ratings were positive. This is the measurement of the reflection:
The other study is called: "Influence of first reflections in listening room on subjective listener impression of reproduced sound" This study compared the effects of absorbing various first reflections and comparing the results. Basically, the fewer reflections you absorb the wider the soundstage and envelopment but you lose clarity. Absorbing reflections is similar to them being absent so it's interesting that removing the ceiling reflections lessens image width and envelopment just slightly less than removing sidewall reflections. Here are a few of the key graphs: (Rs is absorption of sidewall, Rf= ceiling)
Also found in the thesis paper from the above study, another study was referenced: "Auditory envelopment" (Furuya, Fujimoto, Takeshima & Nakamua, 1995) :
All experiments utilised musical stimuli and the reflection and sound field were electro acoustically simulated. In relation to this thesis investigating a singular reflection, experiment one demonstrated that as the delay time of a singular reflection increased, the sound image grows vertically in size. Regarding auditory envelopment, experiments two and three show that as long as the ratio of lateral and vertical energy remains constant up to 200ms, envelopment becomes stronger as energy arriving from above increases. The author does note however that lateral arriving energy must be the “predominant factor to perceive envelopment”.
Small Room vs Big Room:
I mentioned that I was previously in a small room (11' wide with 8ft ceilings and about a 10' listening distance) and just moved to a concrete loft that is a similar listening distance but is 15' wide with 12' ceilings. It is somewhat surprising but it didn't make the speakers sound smaller, if anything they are more spacious and the room sounds like a concert hall. I have almost no absorption in this room at the moment, just a few love seats so that may be a factor. I also feel like the sound is more clear, I speculate that I'm hearing the direct sound more clearly with the early reflections that are now arriving a bit later due to the longer distances from the walls and ceiling and then the later arriving reflections are much lower in level so the overall sound remains clear.
Conclusion
I don't think definitive conclusions can really be drawn yet until we get some more research on these issues but it is food for thought. I'd like to see the effect of vertical reflections when the reflection isn't distorted as it was in one of the studies I showed. I'd also like to see how listeners perceive overall spaciousness when listening to speakers with differing horizontal and vertical dispersion.
My personal experience with multiple listening tests (sighted and blind, all level-matched and instantly A/Bing) is that my ears agree with most of the research I have posted. I have found that once at least 2 speakers are playing, I don't notice much difference in spaciousness but I do notice a difference when vertical response is very limited. This tells me that all early reflections need to be considered when going for that live spacious sound, not just the sidewall reflection. I also believe that since early reflections seem to be mostly what matters regarding spaciousness, the large difference shown in the globe plots between a wide and narrow dispersion speaker may not perceptively sound that different when the 1st reflections are within a decibel or so in amplitude. More in the preference realm but I also find listening to music in 3 channel stereo with the center about 3-5db lower than the fronts is a great way to get even more spaciousness while also reducing the stereo imaging effect which makes music sound more like music and less like headphones.
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