By setting the cutoff frequency higher, you'll get a shorter horn with a wider angle and vice-versa. At around 150Hz with your throat and mouth you'll reach 90 degrees so you can choose any frequency below that.
If this is a freestanding horn that will open onto a flat baffle you may want to go for 90 degrees at the mouth. This isn't strictly necessary and a lesser angle may be fine. It will come down to getting the bandwidth you want from the size you're prepared to tolerate. You might try choosing the throat and cutoff you want and letting the mouth then be as large as you can afford.
If you compare the beamwidth and group delay between different sizes it can help to see why and where the tradeoffs are.
Another way to look at this is that 90 degrees will be the full tractrix and less than 90 degrees will mean a small mouth (of course you might choose a smaller angle if it fits your needs). A small mouth is going to create ripple in the response plot and with the acoustic impedance of the horn. A small amount of ripple is OK but you shouldn't accept too much.
If this is a freestanding horn that will open onto a flat baffle you may want to go for 90 degrees at the mouth. This isn't strictly necessary and a lesser angle may be fine. It will come down to getting the bandwidth you want from the size you're prepared to tolerate. You might try choosing the throat and cutoff you want and letting the mouth then be as large as you can afford.
If you compare the beamwidth and group delay between different sizes it can help to see why and where the tradeoffs are.
Another way to look at this is that 90 degrees will be the full tractrix and less than 90 degrees will mean a small mouth (of course you might choose a smaller angle if it fits your needs). A small mouth is going to create ripple in the response plot and with the acoustic impedance of the horn. A small amount of ripple is OK but you shouldn't accept too much.
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Hi HP8903B,
Given:
S1 = 20.27 cm ^ 2
S2 = 3848.45 cm ^ 2
F12 = 150.00 hertz
Then for a tractrix horn, the axial length and mouth flare tangent angle will be:
L12 = 85.83 cm
Fta = 73.52 deg
The half-angle at the horn throat will be:
AT = 3.99 deg
You might like to try experimenting with the Horn Segment Wizard - see attached screenprint.
Kind regards,
David
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