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 Why Not Other Speakers?  

      Good home stereo systems sound great if you're sitting directly between them in the direction that the speakers are facing. The sounds paths are nearly the same in each ear for each speakers and all of the sound is reaching you in phase. You can almost see the musical instruments in the orchestra and where they belong. But what if you are not sitting in the "sweet-spot" where all the phases line up and the frequency patterns are optimal? What if two or three people want are trying to listen simultaneously? Only one person can be in the optimal place at a time. What if it's a wedding reception and people are walking around?

      The whole key is the "stereo image." When all of the sounds reach you ears in phase at the correct amplitudes on a good recording, you can almost see every instrument as if it were in real 3-dimensional space. But when you are moving around, the distance from you ears to the speaker is changing. This would be fine if the frequency content didn't change with your position, but this dispersion pattern can change drastically with the angle you ear makes to the axis of a regular tweeter (figure at left). Putting two or more tweeters close together only makes the problem worse: Now the sound at any point is impossible to predict. A solution can come in several ways: A Line-source speaker, a omni-polar source speaker, and a point-source speaker are a few to consider. Each one has distinct benefits and drawbacks.

      Line-Source speakers[1]: Usually a ribbon or magnetoplanar-type speaker that uses a long (up to 7 foot), very thin element which vibrates by magnetic attraction and repulsion in a magnetic field. The magnets that create this field must be strong and are quite expensive. The element must be very precisely machined and positioned and can be very easily damaged. The since the element is a somwhat resonant metal, it can add unwanted colorations to the sound. Another problem is that the magnetic housing causes near-field sound reflections, which can cause unpredictable constructive and destructive interference at certain frequencies. These speakers traditionally rediate sound as a directional vertical bar, which helps the problem of varying your ear level vertically, but does not significantly increase the optimal listening area. These speakers cannot be used horizontally because the distinct left and right channels would not originate from one direction, but rather a range of directions. The frequency response for these type of speakers is generally midrange and higher.

      Omni-Polar Speakers: An Omni-polar speaker consists of a siamese tractrix (or two "pseudo-spheres") with a horn speaker at the top and bottom. The horn-speakers emit sound toward each other. As the sound approaches the center of the siamese tractrix pair, it is gradually bent away from the axis until it is perpendicular to the axis. This redirected sound creates a disc dispersion pattern that radiates 360 degrees radially. This type of pattern is great unless your ears are changing in distance from the ground level, i.e., if you are sitting and standing. The size of such siamese tractrices are around 6 feet tall and would be ideally quite heavy. One advantage is that midrange-tweeter horns can be used if the tractrix is large enough. This type of speaker solves only the horizontal movement problem and does not address the vertical problem.

      Hemisphereical Point-sources can be created by beaming ultrasonic frequencies at a hard, flat surface. This type of speaker, termed the HyperSonic[2] sound source makes use of high frequency beat effects. To create a 10kHz tone, it uses a 200kHz tone and a 210kHz tone together. One disadvantage is that this type of speaker still must move a large amount of air to create low frequencies, which cannot happen using small ultrasonic transducers. They also require special driving circuitry to create the beat frequencies.

      True Point-Source speakers have been the goal of audio engineers for decades, but a feasible, practical solution is very difficult to even dream up. A true point-source radiates sound 360 degrees spherically. this means that, no matter where you sit or stand, the frequency content and the phase is the same on each sphere-slice. With two or more point-sources, the stereophonic effect is even more amazing: No matter where you stand, you get a very clear stereophonic image, undistorted in phase or amplitude. Even the sound you don't directly hear, that is, the sound directed away from you, can get reflected, depending on the room's acoustics, and add to the stereophonic effect by providing depth and width, like an orchestra in a large hall. Dr. Amar Bose used this concept in his highly-accaimed Bose 901's, which use eight rear-firing speakers and one foward-firing speaker, effectively mimicing the 88% reflected sound and 12% direct sound of a typical orchestra hall.

      The only feasible true point source is the ion speaker, which uses a high-voltage ion flame to produce the sound. The sound is limited in frequency output to the size of the flame. For a 1cm flame, the frequency response is approximately 4kHz to 100kHz[3]. The problem is lowering the frequency response in order to get a more full-range sound. Before the flame can be significantly enlarged, however, the existing plasma flame creation methodologies must be made more efficient. Existing ones dissipate too much heat to be widely practical and require too much power.


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