Frontal reflections, depth of soundstage, and dipoles

[krabapple]

You'll never really know how well it has been 'reproduced', unless you were the engineer.

 

[Emlin]

You'll never really know how well it has been 'reproduced', unless you were the engineer.

As with all recordings.

 

[Bugal1998]

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

Haha yeah. Could you imagine if video display forums felt a similar need to point out that pretty much all aspects of modern video are artificial in some way, as if that somehow lessens the importance of experiencing them as beautifully or impactfully as possible. "The special effects in that movie were't really there, so I don't care if my display portays them well... in fact, I don't care if it displays them at all because they're artificial."

Not apples to apples of course, because the room is a greater participant in audio playback with loudspeakers than it is with video, but I find it to be a similary strange thing to point out.

 

[Bugal1998]

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

Right or wrong, that's how I think of it as well, though @Sokel is correct that these terms should be defined in such threads.

I speak of sounds narrower or wider than the speaker is apparent source width, and sounds out in the room and to the sides and even behind the listener as "envelopment".

 

[Bugal1998]

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.

Then I came across this paper by Dr. Matthias Johansson (co-founder of Dirac). Although most of the paper is on mixed-phase DSP, he does mention this on page 7:

View attachment 391749

And on page 11:

View attachment 391740

Regrettably, Dr. Johansson does not cite a reference for his statement. So I did a literature search and came up with nothing. I looked up both Toole and Everest and whilst there is plenty of discussion on lateral reflections, there is no discussion on frontal reflections.

I sent @Sean Olive a message (given that he and Toole did a seminal paper on lateral reflections). He could not recall any papers on frontal research off-hand, and he theorized that:

View attachment 391742

So it appears that the experts think that frontal reflections are detrimental. Lacking in Johansson's paper is information about exactly how early and how attenuated the frontal reflections are.

Now I have a question: dipoles radiate sound front and back. The rear wave would produce copious frontal reflections. Linkwitz seemed to believe in dipoles, after all he designed the LX521. My personal experience of dipoles is that they seem to produce more soundstage depth than a monopole (and yes, I have heard dipoles and monopoles side by side in the same room, although it was an informal unblinded listening session).

I have also been doing my own experiments using bookshelf speakers as ambient speakers. Using DSP, the bookshelves were equalized and delays adjusted so that they are 15ms delayed and -15dB compared to the main speakers. I then compare the effects of spaciousness with the ambience speakers placed in different parts of the room. I found that if these speakers were placed so that they fire towards the front wall, there is no additional sensation of spaciousness. Instead, what I hear is smearing. On the other hand, if the speakers are placed to the left and right of the listening position, a remarkable sense of spaciousness and envelopment is the result.

If frontal reflections do not create the illusion of soundstage depth, then what does? And, in light of this, are dipoles a fundamentally misguided design?

@Keith_W Thanks for sharing that research in the first post. I've been under the impression that reflections off the wall behind the speakers were too low in level to be perceptually relevant (dipoles and omnis as exceptions). Others profess experiences to the contrary.

I had minimal absorbtion on my front wall for the first 18 months or so, but when a center channel was added I also added absorbtion all around the center speaker. I noticed no change in depth or coloration with the central absorbtion. When asked about it, the acoustic engineer also said it should have no effect on two-channel playback.

 

[Titurel]

In my experience, the single most important factor in obtaining good depth and breadth of soundstage is having very similar FR from both speakers reaching both ears. Knowing the polar response of one's speakers is helpful for setting the correct toe in (or lack thereof).

FWIW, my room is set up LEDE, padded carpet on the floor, clouds over the listening zone, almost nothing behind the speakers. (monopoles) Out of necessity, the side wall treatments differ, absorbers on one side, diffusors on the other. First reflections are suppressed. The image (on good recordings) is deep and specific, often extending beyond the speaker positions. I'm presently listening to a recording made in Davies Hall (SF), a venue I've known from its inauspicious beginning through it most recent successful refit. The recording (of Mahler's 2nd) sounds like Davies Hall, warts and all. I've had two different dipole systems in this room and neither imaged as well as my present monopoles, regardless of their other virtues.

 

[goat76]

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

For hearing the soundstage depth, the recording is the only factor.

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.

No, it's not a flaw, and it has nothing to do with the speakers used for the audio productions...

The old recordings were more about documentation of musicians playing together in a real room, while new audio production has unfortunately mostly shifted towards a post-production approach where things in the mix are created artificially instead of capturing the real deal, to begin with.

 

[goat76]

Right or wrong, that's how I think of it as well, though @Sokel is correct that these terms should be defined in such threads.

But is there really anything that needs to be defined?

Whenever anything (instruments, room reflection cues, etc.) in any type of music mix appears to be coming from points further away from the plane of the loudspeakers, the audio production has depth. It can be anything from a recording of a guitar cab with a distance of only 1-2 feet, all the way to the reflection from the wall behind a large symphony orchestra or a distant bird in an outdoor recording. Any distance in the recording will create depth to the reproduction, so what is there to be defined?

I speak of sounds narrower or wider than the speaker is apparent source width, and sounds out in the room and to the sides and even behind the listener as "envelopment".

Yes, but I see that as a whole chapter on its own, and has probably more to do with late reflections and the diffuse field, which in turn is completely related to the listening room reflections and has very little to do with the recorded information (as long as there are no "phase tricks" done in the mix).

The size of the listening room will always be the limiting factor when it comes to envelopment (unless diffusion panels are used to "enlarge" the space somewhat), but the "small room signature" should never be the dominating factor when it comes to depth as the recorded venue in most cases where much larger in size than any listening room. So... as the recorded depth information can only come with the direct sound, the ratio of direct sound must be the dominating factor over anything the listening environment will add to the equation.

 

[Todd68]

For hearing the soundstage depth, the recording is the only factor.



No, it's not a flaw, and it has nothing to do with the speakers used for the audio productions...

The old recordings were more about documentation of musicians playing together in a real room, while new audio production has unfortunately mostly shifted towards a post-production approach where things in the mix are created artificially instead of capturing the real deal, to begin with.

When I hear vocals or instruments that project from a space in between the two speakers but at different locations sometimes as if they’re a few feet behind the plane of the speakers is that just imaging quality or is depth a separate affect?

I know the recording is important, if it’s recorded flat it will not envelope you in sound, but in my experience, some speakers portray a 3D like image better than others, and room treatments are key to being able to hear it at a convincing level. Also speaker and listening position are important.

I have some recordings that are recorded in a way that certain instruments and or vocals seem to come from speakers along the sidewall and even behind me. I only have a stereo pair. I guess this is done with phasing tricks in the recording process. Also listening to surround sound in stereo provides that kind of phantom image.

 

[Bugal1998]

But is there really anything that needs to be defined?

Only in the sense that everyday depth perception exists from our ears outward, but for purposes of discussing depth in a recording it's considered from the plane of the speakers and beyond.

And strictly speaking, envelopment also involves playback "depth", but as you said, from a different mechanism and therefore it's a different discussion. See the post immediately above as a case and point.

@Todd68 are you willing to share any specific tracks that have sound from the sides and rear? I alway enjoy new leads for engaging new music!

 

[Todd68]

Only in the sense that everyday depth perception exists from our ears outward, but for purposes of discussing depth in a recording it's considered from the plane of the speakers and beyond.

And strictly speaking, envelopment also involves playback "depth", but as you said, from a different mechanism and therefore it's a different discussion. See the post immediately above as a case and point.

@Todd68 are you willing to share any specific tracks that have sound from the sides and rear? I alway enjoy new leads for engaging new music!

You will probably laugh, Madonna the Immaculate collection is an obvious example. The whole disc sounds like it was recorded in surround sound or something. Most other times I heard it, it was just a tiny portion in the song, there’s a Pink Floyd song can’t think of the song right now, where a laughing man’s voice sounds like it’s right at your head.

As far as phantom images from two speakers from watching movies, surround sound movies played back in stereo when you have the processing down mixed to LT/RT will do it. It was helicopter sound affects in a couple different movies I watched where helicopters seemed to start from behind and “fly over you”.

I’m not sure if you would find Madonna’s music engaging, but hey you asked.

 

[Flaesh]

specific tracks that have sound from the sides and rear?

Best of Chesky Jazz & More Audiophile Tests, Vol. 2, track 47 - General Image and Resolution Test.
https://www.linkwitzlab.com/LX521/Stereo_Sound_System.htm e. g.

 

[Sokel]

The wildest example of a mainstream work that plays with any stereo effect imaginable is the original Ziggy Stardust LP.
(on the other hand the crazy producers have added a "to be played at maximum volume" note at the back cover )

 

[goat76]

Only in the sense that everyday depth perception exists from our ears outward, but for purposes of discussing depth in a recording it's considered from the plane of the speakers and beyond.

Ah, that’s how plane is spelled, not plain as I have wrote it in all the previous posts.

 

[Todd68]

Ah, that’s how plane is spelled, not plain as I have wrote it in all the previous posts.

I’m always fixing grammar errors, my writing skills are terrible, but it doesn’t stop me from posting.

 

[Todd68]

I learned that Madonna music was recorded in Q-Sound, a technique to allow surround sound type listening with just two speakers. The Pink Floyd song is called Brain Damage, from Dark Side Of The Moon. It seems Pink Floyd used these phasing tricks in some of their recordings.

Also, I forgot to mention that I owned that CD forever. And every speaker I owned played the affects at a different quality level. My set up I have now is easily the best I heard it regarding clarity of the phantom image. Though my room has changed, more treatments and the use of QRD and Polyfuser panels.

 

[Duke]

... as the recorded depth information can only come with the direct sound, the ratio of direct sound must be the dominating factor over anything the listening environment will add to the equation.

My experience has been that the spatial impression of being inside the music venue (whether the venue spatial cues be real or creations of the recording engineers or both) is not present when the direct-to-reflected sound ratio is too high. My hypothesis is that spectrally-correct, relatively late-arriving reflections arriving from many directions act as "carriers" for the reverberation tails on the recording, conveying those venue spatial cues in a way that is more natural than if they were only delivered by the direct sound.

On the other hand, (imo and ime) the first reflections are strong conveyors of the "small room signature" of the playback room, as they tell the ear/brain system how far away the playback room's walls are. So, by this line of thinking, if the goal is a "you are there" presentation, the first reflections should be minimized but the later reflections should be preserved and encouraged... which is a more nuanced target set than is described by the "direct sound ratio".

Relevant to the topic of this thread, I experimented with absorbing the backwave of dipole speakers which were otherwise correctly positioned (sufficient distance from the wall behind them). To my ears and in my opinion, the "you are there" spatial quality was no longer present when the backwave was absorbed.

 

[goat76]

My experience has been that the spatial impression of being inside the music venue (whether the venue spatial cues be real or creations of the recording engineers or both) is not present when the direct-to-reflected sound ratio is too high. My hypothesis is that spectrally-correct, relatively late-arriving reflections arriving from many directions act as "carriers" for the reverberation tails on the recording, conveying those venue spatial cues in a way that is more natural than if they were only delivered by the direct sound.

I'm not saying that you are wrong about your "carriers" for the reverberation tails theory, but as the listening room can hardly make a distinction between the recorded direct sound of the instruments and the room reverberation from the recorded room, how can the late reflections of the listening room be "carriers" of the recorded reverberation tails in specific?

Maybe I'm over-analyzing(?) your choice of word when you are talking about the tails of the recorded reverberation, but what the room "sees" when it comes to reflection-generating sound sources, is just two of them (the two loudspeakers) and the reflections in the listening room will not be able to make any distinction between the recorded direct sound from the recorded reverberation in the recording, so again, how can the late reflections be "carriers" for the tails if the reverberation in specific?

 

[tmuikku]

Here is a thought experiment:
Imagine your speakers direct sound disappeared suddenly, what would it sound like to have just the early reflections and later room sound left? Take few seconds to imagine the sound, you'd likely still localize the sound coming from front, because the first specular reflections are quite loud, any single of them almost as loud as direct sound and five of them come from front, and one from wall behind you if you have a cubicle listening room.

Now consider how "surrounding" this sound would be if this was "envelopment"? Not mucho as most of it comes from front, earlier and louder, than all later reflections so you'd localize the sound still in front of you. Luckily later reflections, that have bounced multiple times from boundaries start to be more balanced, coming from all directions, so in this sense much more surrounding as there is no distinct hot spot where they seem to emerge, but everywhere.

Now imagine you'd have a mix knob for all three, the direct sound, early reflections and later reflections. If you had only the direct sound, you'd definitely get what ever spatial cues are baked into the recording. If you turn up the later reflections, you'd get some nice spacious sound likely, "envelopment". Turn up the early reflections and you start to get effects happening in front of you, taking away from the "envelopment" and direct sound.

While this thought experiment is detached from reality, it's still fun way to think about it while playing with toe-in and positioning and acoustics in general, balancing the three.

 

[Duke]

I'm not saying that you are wrong about your "carriers" for the reverberation tails theory, but as the listening room can hardly make a distinction between the recorded direct sound of the instruments and the room reverberation from the recorded room, how can the late reflections of the listening room be "carriers" of the recorded reverberation tails in specific?

Excellent question!

The listening room cannot make a distinction between the reflections on the recording and its own native reflections, but our ear/brain systems can!

The ear/brain system can tell which first-arrival sound a given reflection goes with by looking for overtone sequences that match up. So when an overtone sequence from the reverberation in the recording arrives after multiple in-room bounces, the ear/brain system still knows which first-arrival sound it goes with, assuming it is spectrally correct and still loud enough to be detected.

And imo this is why we want to preserve the spectral balance of the in-room reflections: Once they have lost too much of their high frequency energy to be identifiable by their overtones, they cease to be "signal" and become effectively "noise". If you've ever gotten listening fatigue from listening in an overdamped room, this might be why.