Frontal reflections, depth of soundstage, and dipoles

[sweetsounds]

There are multiple threads on ASR about reflections, including:

Perceptual Effects of Room Reflections - @amirm
Research on Reflections - @aarons915

However, it is mostly lateral reflections which are being discussed. There seems to be good science on lateral reflections, so we will NOT be discussing that here. Instead, what I would like to discuss are frontal reflections.

For a long time, I believed that frontal reflections are just like lateral reflections, in that within a certain window of time and amplitude, they create spaciousness. I simply transposed my understanding of lateral reflections to frontal reflections and used the same target - ETC should show a peak which is >-15dB with respect to the main impulse within the Haas fusion window of 20ms.

Without confusing you more, I am afraid, that the effect of front- and back-wall reflections is not a simple answer.
All data in the articles and experience point to contradicting data: bipolars and omnipolars can have great spatiality, as can have monopoles in absorptive environments. The reverse can also be true.

In my experience is

Different from good speaker sound, which is "just" a flat frequency response, spatiality involves time signature, time phase, directivity and time difference between the ears. Even if the freq response is flat, it says little about perceived spatiality.

My explanation is the following:
- depth and width are psycho-acoustic interpretations of the brain to conclude from echos and timing differences, where the source of a sound is.
For human survival, the location of a sound was a life-or-death information.
- the key is the sound reaching your ears with the Haas window of ~40ms.

- during recording and mastering delayed sounds in the mix create the "natural depth". However, as
- are the reflections coherent to the source? i.e. does the reflection come from the same direction within a small delay from the initial sound? This is bipolar/omnipolar, which adds early reflections/echos which the brain interprets as depths. Here the room+speaker add "replay depths", which is not on the recording, but the ears interpret it the same as depth.
- if the reflections have a very different time and spatial signature (e.g. 2nd order reflections from the front- or 1st order from the back walls), the perceived depth decreases. The brain seems to interpret the sound field then as a diffuse field, meaning it can't conclude on the location of the sound.
- purely late, diffuse reflections coming from all directions (incoherent) are not detrimental. The brain has already concluded, where the sound must be coming from.

All of that might not help you, except that adding more coherent and consistent sound reflections in the Haas window (or eliminating incoherent sounds) will improve spatiality.

 

[Bugal1998]

This is not true, I have a huge soundstage including depth, speakers are 15cm from the wall.

I suspect many listeners can't get beyond the sighted bias of seeing the wall so close to the speakers, and moving the speakers into the room reduces a visual issue.

There's a stereophile test track that has spoken callouts from various places in a church, when I close my eyes it gives a pretty convincing--though not perfect-- illusion of life sized voice coming from waaaaaaay behind the front left corner of the room. When I open my eyes it sounds like a miniature voice coming from within the visual boundery of the room.

 

[Sacha]

At the risk of oversimplifying, imo soundstage depth has to do with getting the spatial characteristics of the recording to dominate over the spatial characteristics of the playback room, which in turn can mean suppressing or manipulating the early in-room reflections.

This was an awesome succinct answer, I would just add that I would reckon other indicators of less-than-ideal speaker performance could also conceal existing depth and soundstage details in the recording such as: distortion, resonances, crossover issues etc.

 

[Bugal1998]

At the risk of oversimplifying, imo soundstage depth has to do with getting the spatial characteristics of the recording to dominate over the spatial characteristics of the playback room, which in turn can mean suppressing or manipulating the early in-room reflections.

I've read (possibly in Toole's 'Sound Reproduction'?) that delayed reflections contribute to a sense of depth by offering additional distance cues [Edit: see @jeffaegrim's post below for more specifics on the quote]. Anectdotely, when my speakers were in a highly damped space, the sound was big and 'in your face', often feeling like it was originating from a place between the listener and speaker. Everything was experienced closer to the listener; it sounded big and impressive, and I enjoyed it!

When sidewall reflections were reintroduced (but still treated to keep the early reflections in check) along with many more late reflections, the perception became one of increased distance from the listener, with the experience of sound originating at or behind the speakers. While initially less impressive, I quickly came to have an overall preference for the spaciousness and energy of the livelier space.

I have also read that front/rear reflections narrow the perception of width due to narrower angle of separation between the reflections, pulling the sound closer to mono. The same issue allegedly exists with ceiling reflections. I've also read that ceiling reflections aren't perceptually important because our binaural hearing ignores them, and reflections off the wall behind the speakers are too low in level to be perceptually meaingful.

Maybe all points are true depending on the room? I just don't know.

 

[Bugal1998]

I find it immensely enjoyable when the sensation of acoustic space changes dramatically from one recording to the next. When that happens seems to me it must be the recording's spatial cues, rather than the playback room's spatial cues, which are perceptually dominant, because the playback room's acoustic signature does not change from one recording to the next. Hardcore subjectivist Harry Pearson advocated for pursuit of the "you are there" experience, and I think he got that right.

I agree with this! And with most things in life I think there's a balance, which you also alluded to... perceptually dominant doesn't have to mean "completely eliminated".

My unproven hypothesis is that the in-room reflections act as "carriers" for the reverberation tails on the recording, ideally delivering them spectrally intact (or close enough) from many directions, and thereby effectively presenting the ambience information on the recording. At the same time, imo it is desirable to minimize the "small room signature" of the playback room.

Whether it's true or not, this does seem to align with my personal experience. A room with managed early reflections and late reflections left intact allowed much more ambiance to be heard, along with impressive track-to-track spatial differences. I hear far more variation between recordings in such a room than a deadened room or headphones have ever revealed to me.

Also, there seems to a presumption among many that soundstages should be as wide as possible. That's why they set up their systems to sound as wide as possible, and are actually listening to the acoustic properties of their rooms rather than the recording. In doing so they also sacrifice depth.

Perhaps, but width and depth need not be mutually exclusive. The system with the widest apparent source width I've heard also has the most depth and reveals the most track to track spatial variation.

Interestingly, in my system, if the tonal balance is shifted toward higher frequencies I hear more depth (to a point) and spatial variation between recordings. When the tonal balance is tilted toward reduced high frequencies all recordings sound more spatially alike than different.

I will admit to prioritizing width in my own system--even if it trades depth (which I haven't noticed)--simply because it creates an overall more enjoyable experience with typical content.

 

[jeffaegrim]

This is not true, I have a huge soundstage including depth, speakers are 15cm from the wall.

I completely agree. My speakers are front ported and as close to the wall as physically possible. They are wedge-shaped, so there is technically some more space between them and the wall, but one part of each speaker is almost touching it. I’ve tried every configuration in my room and the soundstage/depth doesn’t improve when I move the speakers further away from the wall, I just get worse SBIR. Them being further back in turn lets me have a bigger stereo triangle, which proved to be favourable. Manufacturer apparently even recommends corner placement.

I have usually referenced the Genelec speaker placement guidelines in my decisions irt wall distance and imo they limit the placement options in a typical living room quite drastically. Unless you’re pulling the speakers way out, might as well leave them close to the wall. This alone should provide a stereo image with appropriate depth. Everything else can probably be attributed to the recording and to a smaller part the speakers. I’d make an exception for rear ported speakers. I often hear the recommendation of leaving the length of the back port as the distance to the wall at minimum.

There is a Toole quote on apparent source depth: "The sense of depth is very imprecise, but most of the evidence points to a proportion between the direct sound and delayed reflections and reverberation. This information is in recordings and the times involved are so large compared to those created by small room reflections that they are not likely to be a substantial factor. Play with a good stereo upmixer to appreciate what augmented lateral reflections can do for a sense of space and envelopment, including depth." Linkwitz holds a similar view.

(Somebody noted in a different thread that the Genelec guide presupposes a heavily treated „studio“ environment and may not be directly applicable to a normal room. I am not sure whether this assertion is true but I wasn’t provided with enough information to conclude that it is.)

 

[goat76]

I suspect many listeners can't get beyond the sighted bias of seeing the wall so close to the speakers, and moving the speakers into the room reduces a visual issue.

That is what I suspect too. Of course, a disturbance from a too-close-to-the-wall placement of the speakers can overshadow things in the recording, but that should have about an equally bad effect on everything and not just on the perceived depth of the recording in particular. How close a loudspeaker can be placed to the wall mostly depends on how much bass reinforcement is needed, and how the particular speaker is designed as there are speakers that are meant to be placed close to the wall.

It's usually easier to hear the depth of the recorded sounds if we shut our eyes preventing us from focusing on the wall in front of us, which otherwise will mess with the actual depth in the recording. I have sometimes thought about what a highly convincing three-dimensional picture on the whole front wall would do to the perceived depth of different recordings, would our hearing be tricked to hear more depth than what is actually on the recordings, or would it simply give us the true depth of the recordings when we no longer perceive the front wall as close as it really is?

 

[Bugal1998]

I have sometimes thought about what a highly convincing three-dimensional picture on the whole front wall would do to the perceived depth of different recordings, would our hearing be tricked to hear more depth than what is actually on the recordings, or would it simply give us the true depth of the recordings when we no longer perceive the front wall as close as it really is?

Clever idea! World be an interesting experiment.

 

[Sokel]

I'm afraid that as in many threads we have to first define what "depth" is.
I read some post about it and my poor english don;t help probably.

As far as I know is not only when all the sound or some clues come behind the speakers.That's not depth,that's positioning.
Depth is when co-existing with the rest of the work which can very well come from all around,above,below,front,between and behind the speaker.The later is depth.

Edit:By the way,what's this fixation with only the front wall and side walls?When it comes to image in general ceiling is very important too.
Think about that even some of the nicest speakers are not as good both at horizontal and vertical dispersion.And depending the distance ceiling reflections can combine at interesting areas.
Even my ceiling which is 3.2 m did not escape treating.Killing and widening the front wall is never enough.

 

[Mikig]

I've been experimenting with reflective/absorbent cylinders for a while now and I have to say that the result is very pleasant. They are a sort of boost of the stage in front of me, which becomes more stable, more articulated and projects towards me.

 

[Bugal1998]

Edit:By the way,what's this fixation with only the front wall and side walls?

The sidewall fixation is a consequence of our ears being the sides of our heads. We're more sensitive to lateral refelctions.

 

[Sokel]

The sidewall fixation is a consequence of our ears being the sides of our heads. We're more sensitive to lateral refelctions.

Yes,we are and their importance has been documented.
My point is that we must not ignore the sources of the rest of reflections.Depending the angle and the distance ceiling reflections can be earlier than the side ones and they can too come from a side enough angle.

At my case that's certainly true,side walls are at 5 m and the ceiling only at 3.2 m for example.

 

[Bugal1998]

I've been experimenting with reflective/absorbent cylinders for a while now and I have to say that the result is very pleasant. They are a sort of boost of the stage in front of me, which becomes more stable, more articulated and projects towards me.

Absorbing early reflections had the same effect for me; a bit more precision and significantly more projection into the room.

How are you using the reflectors?

 

[Bugal1998]

Yes,we are and their importance has been documented.
My point is that we must not ignore the sources of the rest of reflections.Depending the angle and the distance ceiling reflections can be earlier than the side ones and they can too come from a side enough angle.

They were actually discussed a bit earlier in the thread. For what it's worth, my ceiling has as much absorbtion and diffusion as my sidewalls combined; mostly due to a slightly lower than optimal ceiling height (8.7' or 2.65m).

 

[Mikig]

Absorbing early reflections had the same effect for me; a bit more precision and significantly more projection into the room.

How are you using the reflectors?

I looked at the ASC website to get an idea of how to position them. I saw that they use them starting from the treatment of the front wall only, to the lateral ones to arrive at a sort of "circle" around the listener. As far as I'm concerned, while in the large listening room I have them in front, on the side and behind, in the small one, for now I only have them along the front wall. On the corners, I have two corner panels, then I use two cylindrical ones immediately next to the corner ones, which are more or less behind the line of the speakers, and a tall one in the center of the wall. But I'm still doing a lot of tests, to find the situation that satisfies the most. One thing is certain: between having them and not having them there is a good difference.

 

[goat76]

I'm afraid that as in many threads we have to first define what "depth" is.
I read some post about it and my poor english don;t help probably.

As far as I know is not only when all the sound or some clues come behind the speakers.That's not depth,that's positioning.
Depth is when co-existing with the rest of the work which can very well come from all around,above,below,front,between and behind the speaker.The later is depth.

Edit:By the way,what's this fixation with only the front wall and side walls?When it comes to image in general ceiling is very important too.
Think about that even some of the nicest speakers are not as good both at horizontal and vertical dispersion.And depending the distance ceiling reflections can combine at interesting areas.
Even my ceiling which is 3.2 m did not escape treating.Killing and widening the front wall is never enough.

Whenever there are sounds in a music mix appearing behind the plane of the two loudspeakers in a stereo setup, the recording has depth.

It can for example be a typical recording of classical music where everything appears to be at some distance behind the loudspeakers, or it can be a mix of closely recorded sounds in combination with sounds that appears to be coming further away as is common in a typical recording of jazz. So as long as there are any type of sound in a mix that appear to be coming from a further distance than where the loudspeakers are positioned, the recording has depth.

 

[Mikig]

sulla spaziosità e sul palcoscenico: sui miei Epsilons c'è un tweeter posteriore, e ho letto nella brochure che questo è stato posizionato sul retro dell'altoparlante proprio per aumentare la sensazione di spazio e aria. Penso che su alcuni Snell ci fosse anche un conducente posteriore.

 

[krabapple]

Also, there seems to a presumption among many that soundstages should be as wide as possible. That's why they set up their systems to sound as wide as possible, and are actually listening to the acoustic properties of their rooms rather than the recording. In doing so they also sacrifice depth.

Properly, all recordings have different soundstages and depths thereof, and a decent system should reflect that.

Many, many, many recordings -- most of the ones most people listen to -- have no real 'soundstage' at all. It is a creation of the engineer. There was no 'room' in the first place.

 

[Sokel]

Whenever there are sounds in a music mix appearing behind the plain of the two loudspeakers in a stereo setup, the recording has depth.

It can for example be a typical recording of classical music where everything appears to be at some distance behind the loudspeakers, or it can be a mix of closely recorded sounds in combination with sounds that appears to be coming further away as is common in a typical recording of jazz. So as long as there are any type of sound in a mix that appear to be coming from a further distance than where the loudspeakers are positioned, the recording has depth.

Yes,these are choices.
And recordings are the most important factor as I already wrote at the first page of the thread.

The best of them combine these virtues and oddly enough some of these are 60 years old.
Is it the flaws of the time back then that created this effect?Is it pure skill?
I was amazed seeing the big powered(!) speakers of the venues that these works were made.

Sadly lately this is either a lost art or the fixed flaws straightens things to a degree that stuff is like a flat painting.
Very few,usually small ensembles,usually made in purpose with boring music demonstrates it.

Edit: speaker link

 

[Emlin]

Many, many, many recordings -- most of the ones most people listen to -- have no real 'soundstage' at all. It is a creation of the engineer. There was no 'room' in the first place.

How the soundstage got there is irrelevant to me. The important thing is that it is there and can be reproduced.