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Old 25th November 2005, 08:27 AM   #11
el`Ol is offline el`Ol  Germany
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An alternative that is often used with high-Qts dynamic drivers are resonating side walls made of thin wood (not exactly parallel). Never heard about anybody doing this with ESLs but a try would be cheap and easy.
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Old 25th November 2005, 02:19 PM   #12
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Calvin,
By citing the obvious challenges of backwave absorption, IMO you are too readily dismissing the many problems with dipolar drivers that potentially could be eliminated with a 360 degree cylindrical ESL. We live with these problems and we are too ready to forgive them as something we just have to live with, or go to great lengths to compensate for. Before I go on, please know that my beloved Quad ESL-63s are dipoles, and represent a high-water mark in dipolar design that I respect, but Walker’s efforts to reduce fundamental dipole problems were heroic, and not simply implemented. If the backwave reflection could be stifled, then we are ready to remove a number of dipolar problems. Although I am not prepared to disclose all my work yet, let me just say that there are other absorptive techniques besides fiber fill “stuffing”, including time and frequency selective mechanical dissipative filters that can be used inside the cylinder. Walker started looking at these approaches decades ago.

I was motivated to consider a 360 degree ESL when I was stunned by the sound from the MBL 101 speaker, a 360 design that “shocked and awed” listeners at a Stereophile show with its panoramic imaging and soundstaging.

Here are some dipolar problems that *potentially* could be solved by a tall cylindrical drum with ESLs on all surfaces:

A dipole will exhibit a wavelength/frequency dependent radiation pattern which is a function of panel dimensions. On the “boresight” axis, the highs will rise, and this increase is a complex interaction of the wavelength with both the vertical panel dimension and the horizontal panel dimension. Phase response will not be flat if the frequency response is not flat; so a square wave or impulse transient is smeared in time. Segmentation or electrical filtering becomes necessary, but these measures pose new problems, and rarely are exactly compensatory for the original problem. A cylinder approaches the ideal of a frequency-independent emitter at any listening angle. Phase response and resultant transient would be outstanding. All ESL drivers contribute to a effective pulsating line source.

Dipolar off-axis frequency/phase response will be variable at every angle, in both vertical and horizontal directions, and zero at degrees (a potential advantage). The inevitable room reflections will have a different character than the on-axis response. Not so with a 360 cylinder.

Dipole cancellation kills the bass as a function of wavelength interaction with both panel dimensions. Efforts to boost the bass to compensate for this cancellation can result in large ESL diaphragm excursions that invite overload and arcing. Magnetic driver subwoofers are often employed (as I do with my Quads) but there are new sonic compromises with this approach. A cylinder pressurizes the air in one polarity at every angle. The speaker doesn’t have to work so hard. With either type of speaker, dipole or cylinder, the fundamental panel resonant peak has to be dealt with. Electronic EQ can be used, and if the fundamental equations are understood, can exactly compensate for the bump, at least in theory, the same as can be done with a cone in a box.

The whole dipolar speaker vibrates in opposite reaction to the diaphragm’s forces on the air. This is a big "hidden" problem. Consequently, there is secondary emission from the whole speaker, and frequency dependent Doppler intermodulation, since the diaphragm vibrates within a moving reference system. Mass loading helps, but the dipole is fundamentally imbalanced. All radial force vectors in a cylindrical driver cancel to zero, so there is no net force to push the speaker in any direction in reaction to the diaphragm’s motion. Forces exist perpendicular (radially) to the center line (“squeezing” and “negative squeezing”), but it is possible to brace for these relatively easily compared to the dipole.

I’m not saying it’s easy, but the potential to eliminate the above dipole problems (and these really are BIG limitations) makes the cylinder worth considering, and makes trying to find answers to the backwave absorption a worthy endeavor, IMO.
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Old 25th November 2005, 02:33 PM   #13
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Quote:
I think that if recordings are made with directional microphones then playback ought to be directional as well. No proof about that it's just my impression.
Moray,

I've heard this belief stated occasionally through the years. IMO, I don't think it holds water. If it were true, you'd need a cardioid speaker for some recordings, and an omni for others. True dipolar (“figure of eight") microphone patterns are less commonly used than cardioids in commercial recordings. Recordings are often made with all kinds of microphones. Multi-tracked studio recordings often combine several microphone polar patterns at the same time. And some the finest classical recordings ever made used three spaced omnidirectional mikes.
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Old 25th November 2005, 03:06 PM   #14
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Dispersion pattern is the function of output vs angle.
If we are listening at a fixed position (which is mostly the case) the dispersion pattern doesn't matter. We can't hear it. We only encounter the secundairy effects of it which is more or less reflection of the walls, a rising freq. response related to beaming effects.
Disadvantages (probably more): A lot of indirect sound (like BOSE), more area which has to be driven by the amplifier.
You have to absorb the sound in the middle of the cylinder because sound of the back of the cylinder will get through the front adding a delayed sound which is absent in the original sound.
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Old 25th November 2005, 03:19 PM   #15
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You have to absorb the sound in the middle of the cylinder because sound of the back of the cylinder will get through the front adding a delayed sound which is absent in the original sound.
Of course! That was the whole point of the discourse above. (Did you read it?)

Quote:
If we are listening at a fixed position (which is mostly the case) the dispersion pattern doesn't matter. We can't hear it. We only encounter the secundairy effects of it which is more or less reflection of the walls, a rising freq. response related to beaming effects.
Again, you are certainly right, but those secondary effects are not so nice and are worth trying to eliminate. And, at least in my case, I want a broad sweet spot to allow more than one listener at a time. Beaming dipolars won't allow that. Just last night, three of us were listening to old jazz and blues recordings on a pair Quad 63s. We had to keep swapping the sweet seat among us. Besides, I don't like to be locked to any one spot even when listening alone. I have been known to move around playing "air guitar" when no one else is looking. It would be nice to have full spectrum response around the room.
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Old 25th November 2005, 03:35 PM   #16
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That's why I like the martin logan speaker.
Partially cylindrical shape and dipole at the same time!
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Old 25th November 2005, 03:57 PM   #17
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I like ML speakers very much too. If the Quads didn't exist, I would be looking seriously at current ML models. But the partial cylinder approach that ML (as well as Sound Labs) uses only partially addresses the grievances that I listed above for dipoles. While highs are better dispersed, there is still frequency dependence in the radiation pattern that has had to be compensated for (even for sitting on axis). Dipole cancellation still exists. Reactive forces still exist in the frame and stators. The curvilinear approach, with diaphragm tension much higher in the vertical direction, causes the diaphragm to want to cave inward. ML compensated for this with many support strips and doesn’t allow excursions to become too high (bass is made by cones in most models).
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Old 25th November 2005, 04:28 PM   #18
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Hi,

As you said, marton logan avoids some problems by choosing for the hybrid concept.
The reactive force problem is probably encountered by clamping the durved panel firmly in the frame, mimicking the strength of a full cylindre. Unfortunately most of the logan vibrations are caused by the woofer.
While a cylindre can solve some reactive force problems, vibrations across its surface can still be present. Since bracing of a cylindre is more complicated you will probably have to consider this. A large metal sheet with poor internal damping is a problem wether being a cylindre or not. I think the concept of a cylindre will have its benefits but will be a very difficult to make. I've never seen such a thing. Maybe building some extra rear- or side-firing panels will suit your taste? ( a bit soundlab like)

regards, MJ
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Old 25th November 2005, 04:42 PM   #19
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Yes, MJ, you recite a good list of things to be concerned about. In my on-going design efforts, I started wih plastic stator supports such as egg-crate light louver panels (with wire mesh electrodes) to reduce metallic panel flexure. Much more to do...
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Old 25th November 2005, 05:38 PM   #20
Bazukaz is offline Bazukaz  Lithuania
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I have just built a little round cylinder (~5 x 25 cm) , but there are seriuos problems with diapragm tension.
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