What is the ideal directivity pattern for stereo speakers?

[LineArray]

Originally Posted by graaf

...tend to bring spaciousness - "images that fill a space (ASW)" (as opposed to pin points) which is a thing many consider necessary for realism of sound reproduction, for example Markus76 here, on diyaudio forum?

A solo voice with some space around is a setting which can be
reproduced quite nicely. If it is "realistic" i see no reason for artificially
"blowing it up".

Early reflections do not contribute to envelopment IMO.
Envelopment may be a desirable property, but one can have bloating
of phantom sources without creating any envelopment. That is what early
reflections tend to do IMO.

Originally Posted by graaf

yes, perhaps some early reflections may affect tonality but do You know any detailed study of this question - of which reflections (with regard to their delay, angle, frequency response etc.), how and to what degree?

If you accept "informal studies", i built lots of similar 2-ways as i was
a student and placed them in different environments.

Listening distance and near side wall reflections (above the crossover frequency
to the tweeter) were dominant factors in changeing tonality. They had to be
compensated for, in case the placement could not be changed to statisfaction
(by toeing in, moving the speakers further away from walls, listening close enough).

[speaker dave]

Originally Posted by graaf

yes, perhaps some early reflections may affect tonality but do You know any detailed study of this question - of which reflections (with regard to their delay, angle, frequency response etc.), how and to what degree?

Soren Bech of the University of Denmark studied this for a number of years and wrote several AES papers on it.

He modeled a typical speaker in a typical living room by setting up a large number of loudspeakers (early KEF uni-Q) in an anechoic chamber. The typical reflections from floor ceiling and walls were accurately modeled as to direction, delay, frequency response and strength.

Once he had the room model set up he could turn up and down each reflection to see if it was typically noticed and what its effect was.

You should read the papers but his general conclusion was that the floor bounce for sure and usually the rear wall bounce were at a noticable level (caused coloration). Other reflections were below the detection threshold, although the corner reflections will determine the bass response.

Regards,
David S.

[speaker dave]

Originally Posted by graaf

to be more precise 0.2 is just a value I have found on one of audiophile pages. On the other hand on another page I have found that 0.5 is the "optimum midfrequency RT60" for a "broadcast studio" 0.5 is also typical/recommended RT60 for small rooms according to KTH handbook,
which relies in that regard on "textbook knowledge" and results of a KTH research project "Speech and Music in Rooms". For bigger rooms like 6x8 classroom size room it is 0.8

Ideal RT must always be tied to room size. bigger rooms should have a longer RT but they won't seem more "lively" because of it.

It is better to think in terms of average absorption. For example a wide range of cinema sizes can have ideal acoustics (with a wide range of RT) if the mean alpha is 0.4.

David S.

[454Casull]

Originally Posted by speaker dave

Soren Bech of the University of Denmark studied this for a number of years and wrote several AES papers on it.

He modeled a typical speaker in a typical living room by setting up a large number of loudspeakers (early KEF uni-Q) in an anechoic chamber. The typical reflections from floor ceiling and walls were accurately modeled as to direction, delay, frequency response and strength.

Once he had the room model set up he could turn up and down each reflection to see if it was typically noticed and what its effect was.

You should read the papers but his general conclusion was that the floor bounce for sure and usually the rear wall bounce were at a noticable level (caused coloration). Other reflections were below the detection threshold, although the corner reflections will determine the bass response.

Regards,
David S.

Interesting - so it looks as though it may be better to put the woofer close to the floor in a floorstander, even at the expense of lobing with the mid...

[keyser]

Originally Posted by 454Casull

Interesting - so it looks as though it may be better to put the woofer close to the floor in a floorstander, even at the expense of lobing with the mid...

Roy Allison found it very important.

EDIT: And so does Peter Lyngdorf: http://www.lyngdorf.com/content/view/25/50/

[speaker dave]

Originally Posted by 454Casull

Interesting - so it looks as though it may be better to put the woofer close to the floor in a floorstander, even at the expense of lobing with the mid...

Yes, the Allison or AR approach still works. If the woofer is near the floor then the floor bounce frequency is high and out of the woofers range. If the midrange is well above the floor then the floor bounce frequency (for it) becomes low and below the crossover. In both cases you put the bounce frequencies out of their respective ranges.

Works great for 3 ways but not so good for 2 ways.

David S.

[454Casull]

Wait - wouldn't the ideal solution be to put a wide-angle diffuser at the floor bounce location for typical listening positions? That way one can avoid the vertical lobing by keeping the woofer close to the mid.

[keyser]

Originally Posted by 454Casull

Wait - wouldn't the ideal solution be to put a wide-angle diffuser at the floor bounce location for typical listening positions? That way one can avoid the vertical lobing by keeping the woofer close to the mid.

The floorbounce typically occurs around 300 hz. Because of the large wavelengths involved, that means you'd need a huge diffuser to do anything useful.

I think the ideal loudspeaker should do something about the floorbounce dip. The Alisson method got rid of both the floor- and back-wall dips at once. Another way to mitigate the floorbounce dip is to use vertically spaced drivers. As such line-arrays work pretty well in this regard too.

[DBMandrake]

Originally Posted by speaker dave

Yes, the Allison or AR approach still works. If the woofer is near the floor then the floor bounce frequency is high and out of the woofers range. If the midrange is well above the floor then the floor bounce frequency (for it) becomes low and below the crossover. In both cases you put the bounce frequencies out of their respective ranges.

Works great for 3 ways but not so good for 2 ways.

Do you think there is an optimum crossover frequency range (and slope) for this approach from a psychoacoustic perspective ? Eg, lets ignore for the moment the optimal crossover frequencies of specific driver combinations, and examine only the factors that affect perception of imaging and focus, interaction with the room etc.

My feeling is that for this type of design (very low woofer, very high midrange) the crossover frequency should be no lower than 200Hz but also no higher than 300Hz, and not less than 12dB/oct, but I'd be interested to see other peoples thoughts.

[ra7]

By putting the woofer very close to the floor, aren't we taking out the floor bounce? The approach to put the floor bounce out of the useable frequency range can be used in more conventional arrangements, i.e., woofer off the floor.