I am very attracted to the idea of making a large mid-high horn for a BMS 4592 ND in order to cover the frequency range between 300-20,000 Hz with ONE high quality (coax) driver
(for home use, not PA).

In order to do so I need a horn that:

- has a cut-off about one octave below the crossover frequency, that is, about 150 Hz
- has a “quick” expansion rate (and hence a big mouth) in order to have as little distortion at the throat as possible (I will happily sacrifice some efficiency for lower distortion:-)

Now, if I try to calculate the contour of such a horn using the exponential calculator on the Melhuish website, I get a horn with Length = 98 cm and Mouth size = 4352 cm2

However, if I look at well-respected exponential horns (TAD or Bill Martinelli’s) they seem to have a MUCH larger mouth relative to their length compared to the horns I get using the calculater at the Melhuish site.

Since, I believe that I really need a large mouth in order to make the horn work from 300-20,000 Hz, I would like to change the “expansion rate” of the horn. I consider a horn about 110 cm long and with a mouth size of 7200 cm2 (120 cm wide [app 4 feet] and 60 cm high [app 2 feet])

So, my question is:
Can that be done by inserting a "constant" in the exponential contour formula, or..?

Any advice is more than welcome! Thanks!

Regards
Peter

• Rapid flares work best

• http://www.soundpractices.com/junk.htm

In Reply to: horns+air non linearity equation posted by toxicport.e on October 11, 2003 at 04:21:33:

Hi Mike,
Borwick's book is in fact pretty good. The wonderful chapter on transducer drive mechanisms (by Stanley Kelly) and the even more wonderful chapter on electrostatic loudspeakers (by Peter J. Baxandall) alone are worth many times the price of the book. (My copy is the first edition; the 2nd edition contains only minor additions.)

But even good books could sometimes be improved by careful proofreading. The formula you cited has become an "old rule of thumb", but the reason is mainly careless copying, not faulty physics. First, the intensity should be under a square root, as it was in Beranek's book (Equation (9.33), p. 275). However, the multiplier 1.72 (1.73 in Beranek (whether it's 1.72 or 1.73 could be explained by slightly different values having been used for the characteristic impedance of air)) is wrong--it should be 1.22. Apparently, Beranek while manipulating the equations accidentally dropped the the square root of two which correctly appears in the denominator of his Equation (9.31) (1.73/Sqrt(2) = 1.22). Beranek's slightly erroneus equation (9.33) has been copied to countless other places. The correct form is as follows:

D2(%) = 1.22*f/fc*Sqrt(It)*10^-2 .

This equation appears in slightly different but correct forms in Thuras et al., "Extraneous Frequencies Generated in Air Carrying Intense Sound Waves (JASA, Vol. VI, pp. 173-180 (January, 1935)), and also in Olson (Eq. (7.20), p. 224).

It seems that the air overload phenomenon was first theoretically investigated by B. Riemann (1860) and Y. Rocard (1933). If one of the more serious enthusiasts on here (Dennis? Steve? Bruce? anyone?) happens to have copies of the papers by these early investigators, I would be very interested in getting a copy.

Trad

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Right now I think Ron is going to help me get a pair of Altec A7s up and running. This should keep me busy for a little while, but I must get a pair of Pi's. I have put this off too long.

Oh yeah, as soon as I get that done I've got to jump back on the Group Build and get an amp done. Shit, then there's the basement to finish, complete with dedicated music room.....Colin