What is the ideal directivity pattern for stereo speakers?

[Rudolf]

Originally Posted by markus76

I had the same perception in a similar test but why did you expect it?

From the child`s experience: Walking down the church hall up to the organ gallery and discovering that the notes came from different parts of the organ. Playing at the other end of the corridor, while my mother was playing the piano in her room. Listening to self-made omnis from varying distances.
To me it is an iron law: If you leave the vicinity (nearfield) of the sound source and let the room do (too much of) its work, you loose the detail and the resolution, but you gain envelopment and the sources become bigger.

Of course it was only recently that I became aware in some detail how these things work in conjunction with acoustics and the human brain.

Rudolf

[erjee]

Originally Posted by dantheman

The Toole/Olive study with 4 speakers in 4 different rooms was informative in the subject. The detrimental earlier reflection(if they can be called that) didn't influence which speaker was preferred in every room negatively or positively. I think we are essentially overanalyzing this of course. They have effects..good and bad, but overall is the speaker measures smooth/flat and set up with some care, it's going to make an enjoyable noise with enjoyable material.

Oh, no offense intended Dave! I thought it sounded funny so I had to write it. Probably should have thrown a smiley on there, but the deadpan can be more effective sometime.

Someone mentioned accuracy a few pages ago. We will never be close to accurate for the original event, but we can get close to the recording. If there were one "ideal" pattern, we know what we are stuck with still.

Dan

I mentioned accuracy in post 1469.

Sharp imaging, I think, is the key issue is this whole directivity discussion. Sharp imaging is required in the production end, as most recordings are done with a microphone placed close to the instrument and positioned in the acoustic space afterwards. It is not the 'original event', but a production.

As we know from this discussion: magnitude, time-delay, direction and number of reflections have influence on imaging and envelopment, exact numbers are unknown. I would say that at the reproduction end the loudspeakers must be able to deliver the same sharpness in imaging as the production end, just like a 1080P HD movie doesn't come to it's full potential on a 20 inch SD television. Omni's, dipoles and conventional boxes are all perfectly capable of delivering this sharp image, but in general they need more distance to the walls to deliver the same sharpness as the narrow directivity waveguide speakers can deliver. So, room size dictates the directivity pattern.

Remains spaciousness and ASW. Both should, in my opinion, be controlled at the prodcution side. It can be done, a nice example is this one:

[markus76]

Originally Posted by Rudolf

From the child`s experience: Walking down the church hall up to the organ gallery and discovering that the notes came from different parts of the organ. Playing at the other end of the corridor, while my mother was playing the piano in her room. Listening to self-made omnis from varying distances.
To me it is an iron law: If you leave the vicinity (nearfield) of the sound source and let the room do (too much of) its work, you loose the detail and the resolution, but you gain envelopment and the sources become bigger.

Of course it was only recently that I became aware in some detail how these things work in conjunction with acoustics and the human brain.

Rudolf

Not sure what you describe is the same scenario. Equidistant stereo triangle delivers sharp center phantom images. Now add ipsilateral reflections and the center becomes ambiguous. Why is this happening? Instead of two false pinna cues we now have 4 but they are still symmetrical. Seems like the two pairs of reflections are perceptually summed and increase the opening angle of the stereo triangle which creates the very same sensation of a "missing" phantom center.

[ScottG]

Originally Posted by markus76

..Equidistant stereo triangle delivers sharp center phantom images.

Now add ipsilateral reflections and the center becomes ambiguous.

Why is this happening?

Instead of two false pinna cues we now have 4 but they are still symmetrical. Seems like the two pairs of reflections are perceptually summed and increase the opening angle of the stereo triangle which creates the very same sensation of a "missing" phantom center.

Unless the loudspeakers are close to the side walls, I've not heard center become "ambiguous", or even altered much in any fashion (..again, assuming equidistant speakers that are equidistant to their ipsilateral wall.)

So, for me and those I've tested - it isn't happening.

Now alter the direction of the loudspeaker (i.e. "toe-out"), and yes - center *can* start to become more ambiguous.

Is this a function of the ipsilateral wall, or that of the driver + baffle? Considering I've used heavy insulation to test this on the wall I can say without a doubt that it's the loudspeaker, NOT the wall.

Assuming a member has access to loudspeakers, it's not difficult to test any of this.

Of course none of this is to say that the wall doesn't contribute as well in these circumstances, rather that the manner of it's contribution is an alteration that doesn't really impact it's clarity with respect to position.

Now the *contralateral* wall, that can often "stretch" and image somewhat - in a similar manner to "apparent source width" for acoustics. Of course it also has other negative qualities with respect to cross-talk, tending to "shrink" sound-stage width to the boundaries of the loudspeakers depending on the horizontal polar pattern of the loudspeaker.

[ScottG]

Originally Posted by Rudolf

..If you leave the vicinity (nearfield) of the sound source and let the room do (too much of) its work, you loose the detail and the resolution, but you gain envelopment and the sources become bigger..


Rudolf

Yes, this is function of cross-talk generally.

I'm not sure about "envelopment" however - I suppose it would depend on your definition.

[Bob Richards]

Originally Posted by DBMandrake

If you're relying on a high degree of rejection of the ipsilateral wall reflection due to it being at or near the 90 degree notch of the dipole, what happens for listeners who are at a different distance from the speakers ?

Surely this makes the forwards/backwards seating location very critical, due to the sharpness of the notch in the polar pattern of a dipole. Sit at just the right spot and you'll get almost complete attenuation of that first ipsilateral reflection, (maybe a good thing, if your tastes are to minimize side-wall reflection) but sit a couple of feet closer or further back and now you'll get far less attenuation and the overall presentation will change, perhaps dramatically.

Not only that, as you move further forward from the side-wall reflection "dead zone" the reflections will receive an additional 180 degree phase shift in addition to the path length time delay, while if you move back from the "dead spot" they will be in phase, meaning that being slightly ahead or behind the ideal spot will sound very different to each other.

Compare that to a more conventional well designed "directional" or CD design, where there are no sharp notches in the polar pattern, and no abrupt change or reversal in phase of the reflections.

I'm struggling to accept that a very deep and narrow notch in the polar response of a speaker is a desirable thing, especially if you are trying to increase sweet spot seating location radius, not reduce it.

The fact that there is a constant phase difference of 180 degrees at all frequencies on either side of the notch is worrisome to me too - nobody has yet provided a convincing (or any) explanation why we should not be concerned about the fact that some reflections depending on incident angle are getting an additional 180 degree phase rotation - something that all the diagrams with nice little arrows gloss over.

Maybe at high frequencies there is enough random phase decorrelation provided by the room geometry and path length that the treble reflections from the room corners will come back sufficiently scrambled in phase that it doesn't matter, but at midrange frequencies, no, there will be in and out of phase coherency issues.

As I've stated before, I don't think you can place a dipole on a continuum between "less directional" and "more directional", and extrapolate the directivity performance characteristics of a dipole to a "more directional" monopole. A dipole is a special case which must be treated as such, and can't be approximated in directivity by any monopole, equal DI figures notwithstanding.

A dipole with a DI of 4.8 dB does not behave the same way as a monopole with a DI of 4.8dB, so a monopole with a DI of say 8dB is not just the same thing further along a "more directional" linear scale...

I see my dipoles as being effectively controlled directivity below about 500HZ due to the cancellations at their sides, which makes embedded timing cue info less damaged by lateral room reflections in the 100HZ - 500HZ region, which I like when I'm sitting in the sweet spot. I noticed that in my room anyway, when I'm significantly outside of the sweet spot, the rear emission reflections off the front wall become a bigger part of what I hear, which makes the speakers sound more like ambience speakers, which may be preferred for non-critical listeners anyway. They're not perfect, but I still like the set of tradeoffs. My openbaffle dipoles give a very enjoyable listening experience. Coarse, I'm also using an inter-aural cancellation circuit, so lower mid timing cue info can really come to life.

[Bob Richards]

Originally Posted by markus76

Not sure what you describe is the same scenario. Equidistant stereo triangle delivers sharp center phantom images. Now add ipsilateral reflections and the center becomes ambiguous.

Why is this happening?

Instead of two false pinna cues we now have 4 but they are still symmetrical. Seems like the two pairs of reflections are perceptually summed and increase the opening angle of the stereo triangle which creates the very same sensation of a "missing" phantom center.

Inter-aural crosstalk screws up our ability to use lower midrange timing cue information embedded in a recording almost completely (if there is any). Sidewall reflections that might generate a sense of spaciousness in the upper frequencies (above 1kHZ), may compound the timing cue image information problem in the lower mid freqs., to the point of weakening the overall sense of center image. That's my present belief. When using my inter-aural cancellation circuit, I've found in many different rooms over 25 years, that sidewall reflections are very detrimental to it working well.

[boris81]

Originally Posted by markus76

Equidistant stereo triangle delivers sharp center phantom images. Now add ipsilateral reflections and the center becomes ambiguous. Why is this happening? Instead of two false pinna cues we now have 4 but they are still symmetrical. Seems like the two pairs of reflections are perceptually summed and increase the opening angle of the stereo triangle which creates the very same sensation of a "missing" phantom center.

I observe the same phenomena. Furthermore, I find that strong contralateral reflections (with extreme toe-in) tend to exagerate the center phantom image while shrinking the width of the soundstage.

I don't think that we can fully understand how this is happening without answering the fundamental question "why do we hear a center phantom image in stereo".

[WithTarragon]

Originally Posted by boris81

........

I don't think that we can fully understand how this is happening without answering the fundamental question "why do we hear a center phantom image in stereo".

Well, that one is fairly simple. This was nicely described in some papers by Ben Bauer that were published in JASA in the early 1960's. The topic was called the "theory of stereophony".

[markus76]

Originally Posted by ScottG

Now alter the direction of the loudspeaker (i.e. "toe-out"), and yes - center *can* start to become more ambiguous.

Is this a function of the ipsilateral wall, or that of the driver + baffle? Considering I've used heavy insulation to test this on the wall I can say without a doubt that it's the loudspeaker, NOT the wall.

What property of the speaker would cause this?