A theoretical model for stereo imaging

[MKR]

According to @geickmei the speakers need to have aimable rear drivers and variable loudness control. I can not think of a single commercial speaker like that. Maybe prototypes and DIY versions exist.

It is rare, but both @Duke and Soundfield Audio extensively use rear firing drivers. To great effect with Soundfield per my personal experience. And I have no doubt Duke’s implementation also very good as he is an excellent speaker designer.

 

[Duke]

Like everyone else here I'm seeing Gary's work through my particular lens, which is probably more reflection-friendly than most.

... I don't think the theory works for three reasons.

1. Image modelling only applies to the statistical region of small rooms, above transition and modal. In the transition region, what we hear is the result nonminimum phase response of wavelengths that approach the dimensions of the room, while the modal region comprises a spatial map of minima and maxima due to wavelength being much larger than the room dimensions. From an image perspective, it is as if the images in the transition and modal regions are partial, broken and forced both inside and outside the room.

I'm under the impression that most of our sound image localization happens well north of the transition region, while in the transition region and below we're mostly getting envelopment cues rather that image localization cues. So it seems to me that an Image Model Theory set-up would work well in the frequency region that matters most for sound image localization (soundstage size).

2. True radiation patterns of physical instruments are incredibly complex, while synthesized sounds are a-spatial...

I don't know enough about how an Image Model Theory set-up places the sound images of differently-recorded physical and synthesized instruments to comment.

3. The requirements to have significant space behind and around the speakers (outside of creating trouble with SBIR) is highly impractical, either at home, a studio or most performance venues.

My guess is that the specified speaker positioning is a critical factor, which obviously would limit the situations in which an Image Model Theory set-up is practical. On the other hand, for the person who has free reign and who would like to maximize soundstage size with convincing phantom sources while making the actual sources (the speakers) "disappear", this might be just what they're looking for.

 

[Petrushka]

Hmmmm.

I wonder if you could get the 6dB by placing eight speakers facing the rear, and one facing forward.

 

[Duke]

Hmmmm.

I wonder if you could get the 6dB by placing eight speakers facing the rear, and one facing forward.

At most practical listening distances imo that configuration results in a direct/reflected sound ratio that is too low. And those rear-firing arrays will have a very different frequency responses than the lone front-firing driver (due to destructive interference), unless they are EQ'd.

One driver (or one coaxial or one array) facing forward and two facing rearward would be much more consistent with Gary's 6dB paradigm, but imo it might still make sense to equalize the two rear-firing drivers. The 8-to-1 ratio of the Bose theoretically results in an 18 dB difference (in practice it's less than that because of destructive interference as the wavelengths become short relative to the dimensions of the array).

Note that at low frequencies the outputs of the drivers (or coaxiasl or arrays) will probably tend towards omnidirectional, so imo some reduction in the low frequencies for the rear-facing drivers (or coaxials or arrays) would probably be called for to maintain good tonal balance.

Sounds interesting, Duke, I hope you're able to demonstrate this method to numerous listeners at shows. I really believe this kind of thing is the solution to getting live sounding music in practical home living spaces.

BEAUTIFUL execution of the concept! As they say, I LOVE what you've done with the place! And, thank you very much for letting me know it was useful to you.

I wanted to do something similar after seeing @Duke's designs. Like a small Genelec speaker, highpassed at 1kHz and reduced in level, tilted up.

I think that would work very well. If possible, "shield" the little Genelecs, as seen from the listening area, such that none of their side radiation has a line-of-sight path to your ears and therefore arrives too early.

It is rare, but both @Duke and Soundfield Audio extensively use rear firing drivers. To great effect with Soundfield per my personal experience. And I have no doubt Duke’s implementation also very good as he is an excellent speaker designer.

Thank you! Again giving credit where credit's due, AJ was using highly adjustable, fairly wide-band rear-firing drivers before me. (But I didn't copy him.)

 

[Curvature]

I'm under the impression that most of our sound image localization happens well north of the transition region, while in the transition region and below we're mostly getting envelopment cues rather that image localization cues. So it seems to me that an Image Model Theory set-up would work well in the frequency region that matters most for sound image localization (soundstage size).

This is a really good point. IMT can be adjusted to have slightly different explanatory principles below the transition region. That's one objection down.

On my other two objections, my final comments about the lack of a perceptual model concern something I've been thinking about for a long time:

• Recordings can sound really good, even though they are compromised in a spatial sense.
• Speakers can sound really good, even when suboptimally positioned or designed.

IMT doesn't have enough explanatory power to address either point, I think.

Somewhat separately, I'd like to refer to architectural acoustics, where this modelling method used.

There are applications (for reverberation modelling) where a 3D model is used, and what's seen is a multiplication of images that accounts for later and later reflections. The initial model ends up exploding in complexity when you start drawing mirror images of virtual sources as well.

I think IMT acknowledges this complexity in the articles implicitly, but certainly doesn't dive into it. There is probably something here worth thinking about. Not sure exactly what at the moment.

 

[Curvature]

I think that would work very well. If possible, "shield" the little Genelecs, as seen from the listening area, such that none of their side radiation has a line-of-sight path to your ears and therefore arrives too early.

I was going to hide them behind the TV to ensure that only late reflections hit the listening position.

 

[Bwaslo]

I was going to hide them behind the TV to ensure that only late reflections hit the listening position.

Yeah, but you can probably save some $$ by using some waveguides with dome drivers rather than Genelecs. The requirements aren't that precise, and at least on mine only doing upper frequencies seems to work well enough. Oddly enough, the sound seems to get more full at lower midrange (where the guide has no output!) when the splash levels are set too high, psychoacoustic illusion I guess. But having a digital delay on the signal seems kind of important to keep the ambience uncorrelated with the mains.

 

[pma]

Speaking about room acoustics, to me the most valued publication is this one:

Acoustics of Small Rooms

Much time is spent working out how to optimize the acoustics of large rooms, such as auditoria, but the acoustics of small rooms and environments can be just as vital. The expensive sound equipment of a recording studio or the stereo in a car or living room is likewise rendered useless if the...

www.routledge.com

 

[krabapple]

ok so not exactly
a speaker he made according to his theories beat out (A) a linkwitz orion with no sub or EQ (lol) and (B) a terrible Beringer speaker

Except, it's not a terrible speaker. It's a champ in its category (budget 'monitor') , as is the passive version. Maybe you're prejudiced against the name?

in a test with 13 listeners. I'm no expert, but it seems to me they did a commendable job trying to produce a controlled test, so that's good.

The test unequivocally can't support this claim, however. And that's what I was asking about.

What speaker blind tests produce 'unequivocal' results? There's always someone who can equivocate.

 

[krabapple]

The BOSE 901 at those old times was not bad but also not perfect as far I remember. The backfired speaker sound depends much on the back surfaces whether damped or reflective. So the sound was not perfect compared with todays high quality speakers. For me to broaden the acoustical scene the distance between the stereo speakers can be increased without the need of early reflections. But anyway this all is subject for personal taste.

there's a limit to how far you can separate the front L/R before there's a 'hole' in the middle.

and AIUI 'broadening' is usually due to reflections from the side walls. Front wall reflection add 'depth'. But I could be wrong.

Thank you! Again giving credit where credit's due, AJ was using highly adjustable, fairly wide-band rear-firing drivers before me. (But I didn't copy him.)

as in, Florida AJ? I used to see him all over audio forums, fighting the good fight. Whatever happened to him?

 

[kyuu]

What speaker blind tests produce 'unequivocal' results? There's always someone who can equivocate.

He didn't say anything about the test producing unequivocal results (or not). He said that the test unequivocally cannot support the claim being made.

 

[Flaesh]

to me the most valued publication is this

I've been recommending this book to everyone for a long time. But just the other day I realized that the authors' last names mean "small" and "silent.)
But I don’t remember the book promoting +6 dB reflections and the like.

 

[MarkS]

you really like it when the soundstage width extends beyond the speakers. ... suppose you read Floyd Toole's book and learn that this happens because strong early same-side-wall reflections expand the Apparent Source Width in the direction of those reflections.

I'm not sure what Floyd Toole actually writes about this in his book, but this is definitely not the whole story.

Listen to out-of-phase pink noise in stereo on narrow directivity speakers far from side walls, and you will hear the sound coming from well beyond the left edge of the left speaker and the right edge of the right speaker.

This effect has nothing to do with wall reflections. Wall reflections actually reduce the effect (by messing up the left-right cancellations that are tricking your brain about where the sound is coming from), and so is less prominent with wide directivity speakers.

 

[Duke]

I'm not sure what Floyd Toole actually writes about this in his book, but this is definitely not the whole story.

Listen to out-of-phase pink noise in stereo on narrow directivity speakers far from side walls, and you will hear the sound coming from well beyond the left edge of the left speaker and the right edge of the right speaker.

This effect has nothing to do with wall reflections. Wall reflections actually reduce the effect (by messing up the left-right cancellations that are tricking your brain about where the sound is coming from), and so is less prominent with wide directivity speakers.

Image broadening because of out-of-phase signals is not the same thing as image broadening because of a strong sidewall reflection. Neither precludes the other.

 

[Thomas_A]

Phase effects should be a different thing. The acoustic mirror of a speaker in the direction of the reflection is another effect.

 

[Thomas_A]

Not sure about the back-wall firing stuff. It has been used for various reasons, one was using tweeter for diffuse sound to compensate for the usual loss occurring in the highest octave. It gives effects for sure, but I can't really see the depth unless signal is delayed (or speaker far out in the room). I still think it's best having frontal arriving sound as "pure" as possible, meaning the front wall should be acoustically invisible.

 

[MarkS]

Image broadening because of out-of-phase signals is not the same thing as image broadening because of a strong sidewall reflection. Neither precludes the other.

Yes, exactly. Your post seemed to imply that sidewall relection was the only source, and I was pointing out that there is another source. I'm glad that you agree.

 

[RexrothPigeon]

What speaker blind tests produce 'unequivocal' results? There's always someone who can equivocate.

You misunderstand. Go back and reread the claim I was responding to and then review the test. The test is not testing for that.

 

[Thomas_A]

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.

 

[gnarly]

In this model, speakers in a room can be thought of as a lamp in a room where every wall is covered in a full length mirror. Whilst a hall of mirrors will generate an almost infinite number of reflections, it is different with speakers. Only the first and second order reflections are important for imaging, because the other reflections die off and become more diffuse in time. It is argued that the position of these phantom speakers (my term, not his) is responsible for apparent soundstage width (ASW) - again my term and not his. All imaging takes place within the area bound by the two actual speakers and the six phantom speakers as shown.

It can be seen that the radiating pattern of the speaker, and the position of the speakers relative to each other and to the room, has a major influence on the strength of the phantom speakers.

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.

I've been taught in prosound sub-deployment classes, to visualize all the virtual subs that get formed with perpendicular boundaries.....due to the more-omni-than-not sub radiation. Which makes sense.
What doesn't make sense to me, is how can there be a virtual speaker in play, without near full strength radiation going directly towards the perpendicular boundaries (forming the mirror images.)

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