A theoretical model for stereo imaging

[Keith_W]

I have a hard time putting much legitimacy into the idea of 'full-range' virtual speakers from boundary mirror images. Once frequencies polar radiations move away from onmi, it seems to me the lateral reflections, and especially the rearward reflections, probably are weak to the point of meaningless.

Bingo! Excellent point! This model does not work for traditional non-omnidirectional loudspeakers. Monopoles have very little rearward radiation, so if anything the phantom speaker would be an extremely spectrally distorted copy of the speaker. It's like pointing a torchlight towards yourself and looking at the reflection of the torch in the mirror. You see the arse end of the torch, you don't see another source of light.

I was excited to read the paper when I first got my hands on it, but by the second day my sentiment had cooled considerably. Now I don't think that the model holds up unless speakers are specifically designed to exploit this effect.

 

[olieb]

I dunno.....theory makes sense for low frequencies, but by the time we are in the top half of the spectrum, seems pretty bogus to me. Just my 2c thoughts

Mirror idea makes sense, but of course the radiation pattern has to be taken into account. Like wenn you direct a torch to the listener, there will be not much light reflected in a side or back mirror. (You are seeing the back of the speaker in the mirror.)
And all this is only valid to the degree that the walls are flat and hard reflecting like a tiled brick wall.
In this case more than 6 images must be taken into account.
When absorption or diffusion takes place the "mirror" transforms into one build from frosted, tinted or matt glass.
And each generation of reflection will get weaker and weaker (fuzzier and fuzzier).

But all these reflections do not invalidate the law of first wave front. The phantom images do not sum like linear combinations, but on a "first come - creates the image" basis.
The reflections create interference (comb filtering), image modification (apparent source width, image shift/distortion, echos) and slow decay.
So I do not buy the theory but I am still amazed by the outcome of the "auditory scene shoot out".
What is going on?
How can this

create a more convincing scene than the competitors?

 

[Duke]

If a model with ”they are here” there is a room extending behind the listening room and the front wall should be removed or acoustically invisible. There are still the reflections present in the recording and the real reflections of the sidewalls, roof and floor. But there are no reflections from the front wall. So with that model there is no use of backfiring speakers. Fixing cues that cause the event being a few meters behind the wall is the tricky one. One thing is to remove cues that disclose the speaker positions.

Most loudspeaker paradigms probably do work best when the front wall is absorptive, but there are those which make deliberate use of the front wall. Dipoles would be the most common example. Ime in order for the rear-firing energy to be beneficial, it should have approximately the same spectral balance as the direct sound, and it should not arrive too soon.

Gary Eickmeier's approach of creating a deliberate (and arguably optimized) pattern of phantom sources is imo a fascinating innovation. It's not the paradigm I use, but that doesn't lessen my respect for what he has created.

Bingo!

... I don't think that the model holds up unless speakers are specifically designed to exploit this effect.

Bingo!

Yes, if I understand correctly, it's both the specific loudspeaker design and the specific placement in a room with suitable walls. It might be feasible to simulate Gary's model with two pairs of speakers, back-to-back, and with the rear-firing pair 6 dB louder than the front-firing pair. Might need to EQ out some of the rear-firing speaker's low end because it will wrap around and potentially over-reinforce the front-firing speaker's low end.

Mirror idea makes sense, but of course the radiation pattern has to be taken into account.

Agreed.

But all these reflections do not invalidate the law of first wave front.

Yes!

How can this
View attachment 483536
create a more convincing scene than the competitors?

I would say that, evidently, Gary's "outside the box" thinking produces the results he intended. Interesting orientation of the IMP speaker as well, very strong toe-in.

Can anybody here tell whether that's the original Orion, or the Orion+, or the Orion++?

 

[olieb]

I would say that, evidently, Gary's "outside the box" thinking produces the results he intended. Interesting orientation of the IMP speaker as well, very strong toe-in.

So it seems, though I would not call it out of the box thinking, more like extreme inside the box thinking. ;-)
As they say: You have to either swim against the tide or with it, but much faster than the others.
I redid the pic with his "design" where the back drivers are 6dB louder than the one facing the listener (indicated by darker color of the circles representing those). So the "images" would have a similar amplitude than the direct radiation in spite of having longer pathway.
Hard to fathom that this actually works as intended.

 

[Curvature]

So it seems, though I would not call it out of the box thinking, more like extreme inside the box thinking. ;-)
As they say: You have to either swim against the tide or with it, but much faster than the others.
I redid the pic with his "design" where the back drivers are 6dB louder than the one facing the listener (indicated by darker color of the circles representing those). So the "images" would have a similar amplitude than the direct radiation in spite of having longer pathway.
Hard to fathom that this actually works as intended.
View attachment 483555

It reminds me of panel speakers. They have nice depth presentation when the backwave is aimed appropriately.

 

[Cbdb2]

So you need a room with perfectly flat frequency reflective surfaces and L R symmetry?

 

[Duke]

So you need a room with perfectly flat frequency reflective surfaces and L R symmetry?

I don't think speakers designed for use in an Image Model Theory set-up are currently on the market, so at this point it there would be some DIY involved.

And you bring up a good point. I would expect some EQ of the deliberately reflective energy to be beneficial, as the walls may not produce "perfectly flat frequency" reflections. And in some rooms the walls may simply be too absorptive, or non-existent on one side, or whatever.

As for how much leeway there is in the L R symmetery, I don't know.

 

[Thomas_A]

Most loudspeaker paradigms probably do work best when the front wall is absorptive, but there are those which make deliberate use of the front wall. Dipoles would be the most common example. Ime in order for the rear-firing energy to be beneficial, it should have approximately the same spectral balance as the direct sound, and it should not arrive too soon.

Gary Eickmeier's approach of creating a deliberate (and arguably optimized) pattern of phantom sources is imo a fascinating innovation. It's not the paradigm I use, but that doesn't lessen my respect for what he has created.

I get the thing with dipoles in one sense, except for the "mirror speaker behind the wall". I just can't grasp what model that would be that translates to a live event. Either I want them to be here, or I want to be there. The latter one would imply multi-channel (which I also have option for, at least surrounds and front height, and use for certain records) and the former would be speakers at the front wall or front part of the room. Another option is a dedicated center and wall speakers at specific positions on the front side walls and a processor to handle that (there is a specific system that comes to mind but I have only seen that it has been demonstrated at shows). I guess those wall speakers partly handles the mirror widening of the scene in a more exact way than reflections from stereo setups. Reports have indicated a spooky presence of live event event. In my own setup, the heavy toe in cause some interesting effects, and if having them 45° the widening collapses and gives a sharp phantom centre. Almost like mono. Turning them just a few degrees like 40° the widening effect comes back.

 

[gnarly]

Mirror idea makes sense, but of course the radiation pattern has to be taken into account.

I think the mirror idea makes sense too, or at least technically ......when both the radiation pattern, and a theoretical room construction is in place.

The reflections remind me of the way the Murphy Corner Line Array is supposed to work. Which being in a corner, is supposed to generate 3 virtual line array reflections.

I built and measured a pair of these floor-to-ceiling line arrays, and my experience was they don't work anywhere close to matching the reflection theory.
Plain ole horizontal reflections from the lines off the corner walls, swamped/masked any theoretical virtual array effects.

Setting dipoles and omnis aside, I figure it's going to be the same with the virtual mirrors being discussed for conventional speakers....that pragmatically the virtual reflections mean next to nothing, compared to direct non-virtual reflections.

Technically, i can see the mirror reflections exist;
But also technically, the lake rises when we pee in it, lol

 

[Duke]

I get the thing with dipoles in one sense...

So this is just me winging it... was hoping @geickmei would show up...

At the risk of not only oversimplifying but also misunderstanding:

Strong sidewall reflections = wider soundstage; and strong (spectrally correct and not-too-early) front wall reflections = deeper soundstage. Therefore, optimizing for both simultaneously = Gary's Image Model Theory speakers and configuration = both wider and deeper soundstage.

I just can't grasp what model that would be that translates to a live event. Either I want them to be here, or I want to be there.

I am under the impression that Gary's approach results in more of a "they are here" presentation, but with "here's" soundstage being perceived as extending well beyond the playback room's walls, and with phantom images effectively detached from the speakers.

 

[Duke]

The reflections remind me of the way the Murphy Corner Line Array is supposed to work. Which being in a corner, is supposed to generate 3 virtual line array reflections.
View attachment 483587

I built and measured a pair of these floor-to-ceiling line arrays, and my experience was they don't work anywhere close to matching the reflection theory.
Plain ole horizontal reflections from the lines off the corner walls, swamped/masked any theoretical virtual array effects.

I was suspicious that the Murphy line array's reflections would arrive too early, which ime degrades clarity. My experience (with other approaches to deliberately adding reflective energy) is that if the time gap between the direct sound and the strong onset of reflections is too small, the reflections are more detrimental than beneficial.

 

[pma]

MIM, mirror image method, is one of the two methods used in geometrical acoustics for prediction of impulse response. It is an important practical tool in analyzing the reflection paths of a room. The assumption is that sources and receivers are small. Considering the actual source characteristics is the next step.

 

[olieb]

I built and measured a pair of these floor-to-ceiling line arrays, and my experience was they don't work anywhere close to matching the reflection theory.
Plain ole horizontal reflections from the lines off the corner walls, swamped/masked any theoretical virtual array effects.

I do not see the contradiction. "Plain ole horizontal reflections" are exactly what I would call "reflection theory".

.that pragmatically the virtual reflections mean next to nothing, compared to direct non-virtual reflections.

I have no idea what a virtual reflection is supposed to be in contrast to a "non-virtual reflection".
There is just a correspondence of "reflections" off a flat wall and "virtual sources" at the symmetry location.

That does not prevent problems from reflections in corners (for such line arrays).
The black path (reflected [aka "real"] or from a mirror source [aka "virtual"] and the green path will interfere and there will be nulls and peaks at certain frequencies/wavelenghts. (And I agree with @Duke that reflections within 2-3ms will be detrimental most of the time.)

I am under the impression that Gary's approach results in more of a "they are here" presentation, but with "here's" soundstage being perceived as extending well beyond the playback room's walls, and with phantom images effectively detached from the speakers.

I tend to think so too, all these reflections in the listening room should leave a strong footprint and create a feeling of the music being transported "here". But the shootout was with orchestral music and in relation to creating an "auditory scene" which is a concept of "being there".
I am puzzled.
And the shootout was 2010, so for 15 years nobody picked it up and made a (successful) product?

 

[Thomas_A]

So this is just me winging it... was hoping @geickmei would show up...

At the risk of not only oversimplifying but also misunderstanding:

Strong sidewall reflections = wider soundstage; and strong (spectrally correct and not-too-early) front wall reflections = deeper soundstage. Therefore, optimizing for both simultaneously = Gary's Image Model Theory speakers and configuration = both wider and deeper soundstage.



I am under the impression that Gary's approach results in more of a "they are here" presentation, but with "here's" soundstage being perceived as extending well beyond the playback room's walls, and with phantom images effectively detached from the speakers.

I get that you will have get a kind of a "depth" signal with that setup and backfiring, but it will be an event where the frontal arrived music signal is equal to the back-firing sound. The event will be one group of singers and instruments which are front firing and a copy of the same group aimed backwards and back firing with the same spectral content. The effect would be the same for a studio recording and a live recording. I find it a bit artificial way to reproduce depth. Historically, there have been other backfiring solution like the Stig Carlsson speakers, but they backfired for other reasons. The were always also standing very close to the front wall; and their trick was to make the wall "invisible".

 

[Bwaslo]

There is a lot of talk about correct or accurate image reproduction amongst us audio types. Here are some thoughts I've had on the subject in reading this thread, also related to the method I use for dealing with a room:
_________
I think that as a priority for survival, our perception of sound direction would have evolved to be more important than things like sensing tonal balance or waveform accuracy. For instance, knowing what direction a possible predator is coming at you from would take sonic priority over whether the sound it makes has a depression in the mid-range!

If that’s true, perception of sound direction should be very significant and general awareness of where sound fields emanate from should be an important factor in making them
sound realistic. Probably more so than a sense of spectral balance.

Sounds from live music being performed in a space spreads out from the performers toward boundaries and obstructions, reflects, and comes at a listener in that same room not only directly but from multiple directions. That is very different than having sounds come from only two spots somewhere in front of the listener. For a stereo system listener to feel like he shares common space with the performers, he needs to have reproduced sounds come from diverse directions. In stereo that effect is provided with help of reflections in the room. Sounds sensed from only two spots in front can at best give an illusion of an event heard through a window. Reflected sounds, real or electronically enhanced, can be configured to provide an impression for the listener of sharing space with the recorded performance.

The perception of image, space and immersion with a stereo system is only an illusion. There are only two discrete signals to work with, from which a brain (with help from the audio system including the room) conjures up an impression of something dimensional and (sort of) real -- because that’s what a brain is used to doing with sound fields. But that’s almost like trying to take two spots of color to represent a 2D photograph based on them. The effect could only be remotely approached as some kind of illusion.

There’s nothing accurate or ideal or pure about a perceived image or “soundstage” or any depth produced in stereo. Spectral balance and timing of the initial arrival of the sound might be evaluated for accuracy (and might affect the illusion). But any “they are here” or an “I am there” perceived experiences can’t really be accurate. At best, you can get an impression of you and the performer sharing some contrived space, but not together in your actual room nor in the actual venue of the original recording. It’s JUST an illusion, there’s not enough information in only two originating voltage signals do do more than suggest possibilities of some space. You can use the two signals to make an illusion of space, but don’t expect accuracy, and do expect to be providing much of the meat of the illusion yourself in how you configure things.

Speakers get placed to adjust room reflections for improving the illusion (subjective) of sonic space generated by the system. That’s ideally done for least disturbance of the timbre from too-early reflections. It’s all part of engineering the illusion, and doing it by speaker placement or adding room treatments is no more valid (nor pure!) than instead doing it electronically or with secondary radiators (ambiance or “splash” radiators, preferably delayed). The option of using secondary radiators, with processed delay and/or convolution by decay patterns, is much more flexible and can even be used to improve sound in less than ideal room arrangements. You don’t have to just take-what-you-get.

The room where my audio system is happens also to be our family room and where a lot goes on, it isn't an audio temple and isn't allowed to look like the control room of a nuclear power plant. In order to be compatible with life in general here, the seating places and the speaker placement (and size) are fixed. I use compact constant directivity speakers and hidden subs (to minimize too early and unbalanced reflection off a large floor-to-ceiling window), placed very near to the front wall, and that can give a rather dry sound. Using secondary radiators with dsp electronic processing to them does wonders in this case.

 

[Thomas_A]

Another thread though but ”spatial maxima” occur at like 22-24 degrees and 50-60 degrees. So speaker position at around +/- 23 degrees and wall reflections at +/-50-60 degrees ideal?

 

[Juhazi]

I'm sitting here listening to my dipoles... Setup is symmetrical along the long wall. Distance between speakers is less than to listener. "Wall" behind the speakers is windows partly diffused by curtains and plants. Stereo image is always between the speakers (except Pink Floyd ) and virtual mono is coherent, but spaciousness is very pleasant.

I have heard many dipoles in more damped rooms, but the magic doesn't happen. I suppose Linkwitz's recommended setup being suboptimal. First arrival wavefront is dominant for freq balance and imaging (including first refl. nulls), late arrival makes spaciousness of some kind. Omnipoles or Eickmeier IMP like speakers send more energy backwards and sideways so they create wider more diffuse imaging and are perhaps more sensitive to placement and wall material (I have omnis at my summer cabin but have not tested them at home). The "new" theoretical model of Eickmeier from 1989 is outdated, we know much more about sound perception nowdays (read Griesinger, Lokki et al.)

 

[Digital_Thor]

I think Beolab 90 did it right, by actually controlling each driver to achieve some sort of collaboration between dispersion, driver placement, individual level and EQ + an actual consideration to the given speaker and its specific best possible sound in accordance to its placement in the room and your listening position.

 

[Juhazi]

Plus the 90 has different modes of operation

B&O Tech: Introducing the BeoLab 90 Concept

#63 in a series of articles about the technology behind Bang & Olufsen products

www.tonmeister.ca

 

[Vuki]

I've never heard soundstage recreation to improve when loudspeakers get closer to the walls. Really, in 40 years building, listening and writing about loudspeakers and hi-fi I've never ever expirienced that.