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Erik,

The high pressure zone near the throat does not create distortions; the distortions (the turbulence of the wave’s front) are created right after that zone (according to William Hall, MIT, 1932, though no one said hat he is right). Also, it is very much frequency-related and at relatively low frequencies the waves propagate very linear across the horn. Do not forget that we do not have linear dependences between the horn size and the wave length…

Moreover, Erik, the AES experiment that you described unquestionably was very interesting but let look critically (my favorite mode :-) to what went on there.

You said: “two round and straight  horns where made with the same theoretical cut off frequency, something like 100Hz, they had the same mouth and throat diameter, hence the exponential horn was somewhat longer.”

There is something in this experiments that make it methodologically not kosher for me as they did not equalize the conditions. The exponential horn is longer then conical or Tractrix and if all those horns have the same mouth’s and throat’s diameters then the exponential will have larger volume of the horn bell and consequentially the larger amount of air inside of the horn. The larger amount of air produces more air mass and consequentially creates higher throat reactance. From now, it will only depend from what kind of driver the horn uses. With a driver that has a relatively low diaphragms mass (any compression driver for instance) then the minuscule change in the throat reactance will be sufficient to change the cone’s damping and certainly the sound of the entire horn. So, if we have different horns, with different air volume, use the same drivers and they are wiling to compare them, then the drivers’ resonances shell be also equalized. Here is where another problem hits – as soon you begin to EQ the driver’s resonances then the drivers begin to change sound; to change sound depends what other methods of resonance EQ you have chosen. Therefore you do not observe anymore the sound of identical horns of the different profile but rather you hear two fundamentally different sound producing sources.

I personally, dealing with horns do not use theoretical bases to observe what happen with them and I tend to use purely empirical ways. It is my believe that NO one TRULY knows what the hell it going on with horns and no one has any idea how a driver will sound when it will be horn loaded. Just listening the thing, developing better listening understanding and compiling it into more and more accurate listening evaluation techniques I think is more beneficial then knowing the theory about the horn operation.

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