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Why am I (currently) building what I build ?
At first, it was only a gut feeling that not all reflections are bad by nature, although too many people still believe this today. Then I experienced myself that wide and uniform dispersion are desirable properties; something that had been missing in my audio voyage most of the time !
Reason and rhyme was provided by Wolfgang Klippel [1], [2] with his work around 1990, although I found his papers only after my first "new" experiments. These had been inspired by Siegfried Linkwitz, who provides a wealth of complementary and additional knowledge and experience.
“Room Reflections Misunderstood ?” [3], for example, is a major supporting pillar in this picture.
Klippel's work is also summarized in Toole's book [4].
In a nutshell, Klippel’s findings suggest that a certain increased amount of indirect/reflected sound (5 dB for music) had been rated positively in a big study mainly for the sensations of “pleasantness” and “naturalness”. The main contributor to both sensations was the sub category “feeling of space”.
When you go to a classical concert you hopefully find during the event that the air is full of sound, which is produced by 15+ first violins and 15+ second violins as one (!) example. Such an auditory experience is at least close to goose bumps. It is for certain reasons highly satisfactory; for people, who like music anyway, even if classical music is not on their regular playlist. So much highs, full of soft and velvety energy, cannot be reproduced at home (actually not a lot of a big live un-amplified concert can be reproduced at home not only because of the sheer energy and dynamics). But the speakers that come closest to this very feeling and sensation, do have the properties mentioned above.
Another resulting desirable behavior is that the speakers disappear. Partly because of the polar response and partly because of the amount of reflections they create. Ideally, they don't provide any clues where they are, so you cannot really locate them and your brain can concentrate on enjoying the music. You hear less from the speaker and more from the auditory scene, although this might sound contradicting to some people. This is a bit like in electronics: Sometimes adding an active component lowers noise instead of increasing it.
In total, a higher auditory satisfaction is the consequence of the mentioned attributes.
Now how do dipoles, which actually have a pretty narrow dispersion with -6dB @60° and no radiation @90°, fit into the picture ? It must be their rear radiated sound that produces the required reflections and interestingly this does not make a huge auditory difference compared to omnis.
Other constant/controlled directivity speakers such as boxes with a low crossed tweeter in a waveguide are not able to create these sensations. At least not to a satisfying extent because they produce too little reflections in typical small rooms. They might have other applications such as sound reinforcement where you are listening to such speakers from a greater distance.
Speakers, which have a constantly changing/alternating radiation pattern (directivity sawtooth at cross-over points) such as regular consumer box speakers, can normally be disregarded right away. If implemented well, their responses look pretty +-45° off horizontal axis but not beyond. Although it has to be said that regular speakers with relatively small dimensions (compared to wave length) can sound reasonably good. In this respect, typical bookshelf speakers have an advantage over the bigger floor standers if not played in a bookshelf, but free standing.
Update 17-May-2012:
In terms of speaker building this year we have one of the lousiest springs ever (felt anyway). But at least there is some time to put some thoughts down on "paper"....
Now, why do I believe that Constant directivity is the way to go ?
First and foremost, the claim that the reflected sound is a delayed and attenuated copy of the direct sound [3] supports the precedence effect, provided that the speakers are set up correctly. The precedence effect can be divided into three sub phenomena:
Terminology used by...
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W. M. Hartmann [5]
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JASA [6]
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1. localization phenomenon
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fusion
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2. Haas Effect
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localization dominance
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3. De-reverberation
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lag-discrimination suppression
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In addition, constant directivity results in at least two more effects, which directly influence the speaker/room/listener interface:
a. The Initial Time Delay Gap (ITDG) is constant.
So, the perception of distance or the intimacy feeling does not vary with frequency no matter how far away the listener is located from the speakers. With box speakers there are more and stronger reflections in the bass than in the highs because the bass is typically omnipolar. As a result the ITDG is longer in the highs. Hence tweeters appear closer to the listener or they seem to somehow stick out. This effect is maximized by horn tweeters, which I perceive as "blowing into the face".
Check out this nice animation.
b. The critical/reverberation distance of a CD speaker is also constant.
With consumer box speakers, the critical distance is smaller in the bass than in the highs because, again, the bass is typically omnidirectional. So you hear more bass reflected from the room compared to the highs. The critical distance increases in the highs as the directivity index rises and there is more direct sound and less reflections. This creates an imbalance and the tonality / timbre of a speaker varies from room to room and with the listening distance.
Only beyond the critical distance of the tweeter the room finally dominates completely.
In the above considerations the term box speakers include speakers with a monopole bass and a tweeter in a big waveguide although latter features CD behavior.
For best results, a speaker should exhibit CD behavior over the entire audio spectrum or at least above the transition zone/Schröder Frequency.
Only acoustically small omnis, bipoles, dipoles and cardioids are practical solutions for typical listening rooms. They are perceptually neutral transducers even though the on-axis frequency response may need alteration for a balanced sound.
[1] Multidimensional relationship between subjective listening impression and objective loudspeaker parameters, Acustica 70
[2] Assessing the subjectively perceived loudspeaker quality on the basis of objective parameters, 88th Convention, AES, preprint 2929
[3] Room Reflections Misunderstood?, Siegfried Linkwitz, AES Convention Paper, presented at the 123rd Convention, October 2007, Preprint 7162
[4] Sound Reproduction, The Acoustics and Psychoacoustics Of Loudspeakers And Rooms, first edition, 2008, pages 457 - 461
[5] William M. Hartmann, "Listening in a Room and the Precedence Effect", Chapter 10 in "Binaural and Spatial Hearing in Real and Virtual Environments" R. Gilkey and T. Anderson (Eds.), Lawrence Erlbaum Associates, Hillsdale, NJ, 1997
[6] Litovsky, R.Y., Colburn, H.S., Yost, W.A., and Guzman, S.J. (1999). "The precedence effect". The Journal of the Acoustical Society of America (JASA) 106: 1633–16.
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