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In the Forum: Horn-Loaded Speakers
In the Thread: The Ridiculous Japa-Brazilian horn.
Post Subject: Resonance DampingPosted by jessie.dazzle on: 3/4/2007
After looking at some photos of Stock Altec horns, though they appear reasonably stout, I can believe they would still resonate when excited by sound pressure alone, as they are still on the thin side if left as bare metal.
See them here :
I would guess these horns (the cast ones) are aluminum : Aluminum and softer metals will have a duller ring, but they will still ring if the proportion/ratio favors audible resonating.
As you suggest, these horns could be damped by putting their surface in contact with a gelatinous coating... Achieving the same effect as when you hold a musicians triangle by your fingers and not by the string.
What is really happening here is sort of the opposite of pushing the resonance up in frequency (as is the case when adding more of a stiff material)... Instead, the resonating energy is immediately converted to a tiny amount of heat energy via your malleable finger tips.
I recently discovered an excellent damping material. The French are absolutely robotic about roll-up security window coverings... Which are vigorously retracted at the crack of dawn up into metal cylinders, which resonate like hell... There is a Spanish company that offers a solution in the form of a semi-cured, gooey rubber sheet with an adhesive backing on one side ; it is intended to be stuck to the insides of the cylinders... This stuff is a bit more than 1/4" thick, and heavy as hell (I carried a roll of 2 square meters home on the subway... exactly like transporting an entire cow in the form of freshly ground beef... pure dead weight. I am certain it would immediately silence even the most ringy horn (or bell). This would be more cost effective than making a horn from a mass of solid metal substantial enough to achieve the same effect.
Along these same lines, some of the large multi-cell horns from Vitavox/Philips were hallow fabrications designed to be filled with a damping material... Usually either sand, or mix of wax and tar.
As for doping/damping speaker membranes... In that case you are not only converting the tiny resonances of the speaker membrane to heat, but you are also adding considerable mass to the moving assembly.... Thats all mass that must be accelerated, stopped, and accelerated again... Think of an internal combustion piston engine ; good ones have light-weight reciprocating parts so they don't rip themselves to pieces at higher piston speeds (titanium connecting rods, light-weight pistons, etc). Back to speakers : From a pure physics point of view, the lightest, stiffest moving mass possible should be the goal. However, it is not so clear cut as this ; Paper for example can be beat for stiffness/lightness, but it would seem only at the expense of tone. In the case of a well conceived driver, I can't see that doping would result in anything other than an attenuation of detail. If it seems to result in an improvement, I would be suspicious of the design of the driver.
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