Buzzer horn beeper horn

Horn for small buzzers, commonly 5v or 12v. This makes these small buzzers significantly louder. 

Example buzzer:

https://thepihut.com/products/buzzer-5v-breadboard-friendly

Two buzzer diameters available - 9.25mm and 12mm.

Buzzer push-fits into the end. Versions with and without a mounting block are included.

Quick and dirty Fusion3d models included for you to adapt as required. Roll back the F3d timeline to remove the mounting block if required.

Print without supports is possible, you may wish to add variable layer height to decrease the layer height / increase the detail around the horn outlet. You could alternatively chop off the horn outlet by 1mm or so to lessen the overhang: it will still sound fine. 
 

Experiment with printing wide-end or narrow-end on the bed, depending on whether you want better internal or external surface finish.

The horn is an Exponential Horn

https://en.wikipedia.org/wiki/Horn_loudspeaker

The buzzer and horn dimensions are parameterised, but I couldn't work out a parametric exponential calculated design in F360, so instead plotted the exponential curve graph, saved as screenshot and imported as canvas. You may need to tweak the canvas position and scale (maintaining ratio) to adjust between new buzzer diameters.