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In the Forum: Horn-Loaded Speakers
In the Thread: Jessie Dazzle Project
Post Subject: On second thought...Posted by jessie.dazzle on: 7/28/2008
Klaus,
I should start by saying that I'm not a scientist and not an engineer. I arrived at the following conclusions as a result of a little research and some thinking about the problem. It is possible that I am incorrect.
I don't know how you "see" a driver's membrane, but a lot of people see it as a sort of piston. I feel this view is incorrect and that the membrane, even in the case of a sub-woofer, does not pump and displace air like a piston, it excites air. There is a big difference.
I explained something about this view in a previous post (a long time ago), and after answering your post, started thinking more from this perspective.
So I will now add this to my last post.
Acoustic pressure : Translated from French Wikipedia (I like the directness of the French definition) :
"...In air, sound waves are transmitted in the form of a longitudinal oscillation about a normal value, which entrains the compression and expansion of the "particles" making up that medium (air). The value of this oscillation is called acoustic pressure..."
Our concerns for a structure capable of resisting pressure need not go beyond this definition.
What am I saying... I think it is incorrect to place the focus on a construction that would best resist the expansive forces radiating form within the horn; it seems more appropriate to develop a construction that moves the natural resonant frequency of the horn's structure as far as possible away from (above or below) those frequencies that the horn will be asked to reproduce as sound (2nd harmonics included).
So we are not so much trying to insure against the horn bursting into pieces, as we are insuring against it's wall bursting into song. High material density and mass are desirable because they make for a relation with the air (having relatively low density, and low mass), such that when excited by the membrane of the driver, very little of this energy is transmitted to the horn's wall. The horn's wall is thus less likely to resonate and less likely to interfere by contributing as a second source of "excitement" to the air. The resulting sound will therefore be less "polluted".
Acoustic impedance : Translated from Wikipedia (again, minimalist and to the point) :
"...Acoustic impedance = The capacity of a material/structure to resist being set in motion when subjected to sound waves..."
Making a laminate by alternating between two materials, one having high acoustic impedance, the other having low, I suspect might give best results as a barrier, stopping the passage of energy from the inner to the outer wall of the horn, but it seems to me that the primary objective remains minimizing resonance of the inner wall. This can be done either by means of damping the inner wall, absorbing and converting the energy to heat, or by reflecting and projecting more of the energy forward.
I am in fact now leaning more toward using foam only as a means of adding stiffening ribs (having generous section depth) to a much more dense lamination than would be the case if going with sandwich construction. The result would be heavier horns, but as long as the parts can be moved without going to extremes (cranes), heavy is good.
Kaus wrote:
"...i was planning to make most of the fibers 90° to the axis of the horn...what do you think of using biaxial inlays(fabric visible, then -45°/+45° and then 0°/90°)..."
I don't know what you mean by the term "inlays", but in my view there is no need to go crazy with fiber orientation. I would consider it largely sufficient to use a biaxial 90° weave (carbon or glass), alternating each layer at 45° to the previous.
I hope this does not confuse you or slow your progress.
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