High Efficiency Speaker Asylum

RE: Doubling rate for a 1/2 wavelength (WL) exponential bass horn design vs 1/4 WL

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Paul Eizik

A

In Reply to: RE: Doubling rate for a 1/2 wavelength (WL) exponential bass horn design vs 1/4 WL posted by Rafaro on December 11, 2014 at 11:16:10

Rafaro

I don't understand what you mean by "doubling" in your question. A horn is most efficient when the length is equal to 1/2 of the wave length of the lowest target frequency, and you have estimated this correctly as 195" for a 35 Hz horn. It's not clear where the 22" dimension comes from in the current context of a 1/2 space or 1/4 space horn.

Paul

Follow Ups:

RE: Doubling rate - Rafaro 18:12:53 12/12/14 (2)
Doubling rate=At what axicentric distance from the throat does the volume of the horn doubles. For example often in horns there are two non expanding walls such that the volume is strictly limited by the two walls that are expanding. So in these cases we speak of doubling rate as a way of knowing the rate of a horn`s volumetric expansion. For example an exponential horn designed for 30 Hz with a length of 113" in this setting ( of 2 parallel walls) Doubles its volume every 24" while a horn designed for 60 Hz with a length of 56" doubles at 12". Knowing the throat area (based on the driver) and the doubling rate and needed length (1/4 WL or 1/2 WL) the mouth area will then be automatically determined. This is a simple and useful way of understanding horn design. Never before getting into 1/2 WL exponential horn design I am unsure of the needed rate of volumetric expansion (doubling rate). I am NOT referring to 1/2, 1/4, 1/8 space here I am strictly referring to the need for expansion rate in 1/2 wavelength exponential horns.

Rafaro
- RE: Doubling rate - Paul Eizik 10:47:23 12/13/14 (1)
  Rafaro
  
  In horn design, the horn length and mouth area are dominant factors, along with the expansion rate used to connect them together. In a full size horn (as used in the midrange) the horn length for maximum efficiency will equal 1/2 of a wavelength for the lowest target frequency, and the circumference of the horn mouth will equal 1 wavelength (a round horn assumed). Maintaining a cross section constant with the expansion rate chosen becomes a concern in a folded bass horn where two of the walls will be parallel, as you observe. The cross section of the horn does not necessarily dictate the size of the mouth, though they are related by the expansion chosen. The results of downsizing the mouth area, or the horn length, are predictable, as is choosing an expansion not optimum for a given application: for example a tractrix bass horn as Dinsdale recommended in his Wireless World horn articles from the 70's. See mine and Bruce Edgar's critiques of the Dinsdale articles on this forum: On Dinsdale on Horns from april 2003.
  
  Paul
  - RE: Doubling rate - Rafaro 15:11:42 12/17/14 (0)
    Solved the question..... need to design a 17 Hz 1/4 WL horn if want 1/2 WL response at 35 Hzs. Mouth size can be Perimeter = 35 Hz WL but length of 17 Hz WL =185". Doubling rate 42 inches that of a 17 Hz 1/4 WL horn. The small mouth P= 35Hz WL will keep the fundamental, 2nd and 3rd harmonic at about same level at 35 Hzs with little output below that freq.
    
    Thanks
    Rafaro

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