The  Squawker

Chapter 7


Here is shown a sequence of photos taken of the devices that had to be concocted in order to couple an ElectroVoice 1824M midrange driver with a 0.90625 (29/32) inch diameter orifice to a rectangular horn with a throat of dimensions 1.5625" by 0.46875" ( 1 9/16  by  15/32) inches.  The transition also had to conform to an exponential flare of 200 Hz.  and over a length of 0.75 inches.  In actuality, it came out to 183 Hz.

The photos here were taken in mid December, 2000, some 22 years after the fact.


In the above photos we see the 4 blocks of wood which make the mould.  The plug, as I call it, is inserted in the geocenter of the rectangular form and the space was filled with plaster.  When the plaster set, the 4 blocks were removed.  The plaster cast had to be cut in half with a razor saw to remove the plug; the two plaster pieces were then glued together.






Two views of the "plug" that made the throat in the plaster cast.  Note the cuts left from the razor saw.  


Figuring out the curve was a lot of fun; remember, it's logarithmic to the base, e.  Also, there were no personal computers back then so I resorted to a book of logarithms left to me by my father; six place common logarithms from which I derived the natural by changing the base.  Taking three courses in calculus in one semester, at night, came in handy.

Please bear with me whilst I reminisce.  I got married in 1968 and moved to N.Y. state, about 60 miles north of the Big Apple.  In 1969 I began employment with a large company and worked about 10 hours a week in overtime.  I went to college at night, taking 3 courses in calculus simultaneously.  I still managed to do the assignments and get A's and B's, work days and some overtime, fiddle with my first horn project and keep up with the social obligations of a newly married couple living in close proximity to both our families.  I was 26 then and at that age one is still invincible.  Things that were no big deal then are today.  At 58 now, things that would take moments to do then, take days now, if they get done at all.  If you've ever had the feeling that time seems to move faster as we get older, it's because we slow down with age.  Anyway, enuff of that.  So, here I am, more than 30 years later blowing the dust and cobwebs of old drawings and notebooks.  I cannot express my gratitude enough for your reading these pages.  Thank you.  :)


These are the patterns from which the wood horn was cut.  The longer of the two, the bottom one,  is about 27 inches long and the shorter one is just a little over 26 inches.  The horn mouth is about 17.5 by 6.  These patterns also account for the thickness of the wood used for the horn. 



If ya have to build a set of these horns, then do it for the achievement.  Building something from drawings done by someone else isn't the same as doing it yourself, from scratch.  It's like climbing a mountain.  A helicopter will get you to the top but ya cheated.  


The rear of the midrange horn showing the box which houses the plaster cast.  Affixed to the rear of that box is an adapter to hold the driver.  Originally, the driver was screwed into a threaded piece of wood screwed to the rear of the box but with the fiddling around with the third set bass horns, the threads in the wood kinda wore out.


The crossover seen is second order Butterworth and the coils are all hand wound on homemade forms.  Originally, the capacitors were of the oil filled bathtub variety but have long since been replaced by mylars.  The intent now is to replace this with an electronic crossover, still unfinished as it has adjustable time delay circuits to compensate for the time delay between the midrange & woofer and also the midrange & tweeter.




Chapter 8

Horn Theory - Chapter Index