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In Reply to: Re: How to get the most efficiency in horns from the throat design? posted by DMoore on January 30, 2007 at 13:14:39:
The horn MOUTH size is determines the smoothness of the frequency response, NOT the efficiency.The horn mouth size is related to an antenna suffering reflections due to an impedance mismatch if the mouth is undersized. The idea is to match the mouth to the impedance of the radiation angle it exhausts (or transmits) into.
A mismatch (typically from too small of a mouth) will cause reflections back down the horn to the throat seen by the driver as changes in acoustic impedance (variations in reactance).
A general rule of thumb also described in Keele's paper is the mouth size (Sm) is commonly acepted as mouth CIRCUMFERENCE = WAVELENGTH (Fc). This is for a 1/2 space radiation angle (floor-standing, etc). The mouth can be half of that value for 1/4 space, and one-quarter the 1/2 space mouth size value for corner placement (1/8 space).
Some go even further - the 1980's Gillum/Klipsch MCM patent indicates that the mouthsize can be as small as 1/12th wavelength (Fc) when properly annulled.This would shorten the horn quite a bit. Should the throat size also be made smaller? Won't work - because other considerations of performance are at play in the throat size.
So the bottom line is that 1/4 wavelength HORN PATHWAYS are not required when other considerations are at play in the horn design. Hopefully, this does not confuse anyone.
DM
Follow Ups:
That's right. If a horn's mouth is too small, it becomes more resonant. As taper is reduced, the cross section becomes increasingly more like a pipe.Reactance annulling can be used to smooth response, but it can only cancel one node. The idea is to use the resonance of the driver in the rear chamber to counteract one standing wave node, usually the primary one. Reactance annulling cannot be used to counter all standing wave nodes.
Standing wave nodes can be countered with another method. Using careful driver placement, some antinodes can be cancelled or reduced. This is usually accomplished by offsetting the driver some distance from the end of the pipe. For several years, Martin King has been championing the idea of using careful placement to modify what standing wave nodes are energized, thereby cancelling or smoothing the antinodes.
A poster (ELENA) from Italy recently posted a design and a pdf document on the Klipsch forum MODS section that features tuned cavities in the horn pathway to reduce the reactance peaks of a reflex-ported front horn, and various tuned ports along the horn pathway (in addition to a front-port) to "fill in" the troughs.His example was a single driver short-pathway reflex-ported folded corner horn.
Same issues we are talking about here. Quite interesting.
DM
More of ELENA's design.
I should mention, ELENA's approach is necessary due to the horn's Fc being well above the Fr (back chamber resonant frequency) required to be much lower in frequency to capacitate the front reflex port which is tuned well below the horns Fc. The issue is exascerbated by the porting, in other words, as it means the driver cannot be annulled.This is not quite the same as a front-loaded of short pathway length which is capable of being annulled. Through the combination of multiple flare-rates AND reactance annulling, the reactance problem cited previously caused by the short pathway is reduced sufficiently.
Very interesting. Incorporating several resonators, each to individually annul a specific standing wave node from the horn. That's an interesting way to smooth the impedance peaks. Thanks for the link!
I don't believe it is all that unique. A number of the lowther cabinets have ported cavities adjunct to the horn path, though off the top of my head the only one i can see in my mind is ported to the outside.
How many of those Lowther-types are front-loaded reflex-ported horns?
My pleasure, sir.I thought it was a unique approach too.
Another methodology to control reactance peaks is disclosed in the 1956 Glenn (General Electric) patent # 2,765,864.A rear-loaded, short-length, unitary-pathway folded horn, this time with a purposely undersized mouth...
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