This was a simple build brought on by the need to keep a coworker awake...through fear!

Step 1: Stuff I Used...

I found an air horn at a swap meet for next to nothing and realized how simple it would be to make it useful for pranking people.
One air horn. One air wand. A coupling and a 2 inch long piece of threaded pipe.
From there just a couple wrenches and Teflon tape to seal the connections.
No actual sizes are given because there are so many possibilities. I think my parts were just 1/4 inch course thread....

Step 2: 10 Minutes Til Fun

I took the rubber end off the air wand and attached a coupling to the end of the wand. The coupling went into my threaded pipe and finally into the air horn. All assembled with the teflon tape. Pretty simple and basic!

Step 3: No Pic Necessary

It must be pointed out, (because some people need it to be) that this build can be dangerous and unsafe! If your "mark" is of poor health, it could easily cause cardio issues up to and including death.
These air horns are also pretty loud! They were designed for trains, large vehicles, or even as alarms. They have a decibel rating at 120 and higher. This can cause temporary or permanent auditory failure (makes some one go deaf).
When/if you use this never direct the bell (open end) towards a person and make sure they are healthy enough to take a scare (funerals are expensive at the very least).
I built this and showed its functions as proof to my coworker that he wasn't going to like the way I was going to save his job (sleeping gets you walked out).
I think this Halloween I will use it to scare some people as well.
I think this took me about 10 minutes to throw together from stuff I got at the swap meet (maybe 15 bucks..?) but I'm sure it'll be worth much more in laughs alone!
Enjoy and be safe!