I wanted to incorporate some kind of air pressure startle this year.  So I threw this together today on the cheap.  Sure I could have just used a solenoid and an air line but that didn't give me the pressure I wanted.  This will blow your hat off from 5 feet away!

This could be an air cannon with a few modifications but I was going for NO sound and all pressure.  So you hear a rush of air and but after it hits you (so you don't really hear it at all).

Hardware:
2" galvanized end cap
2" x 6" galvanized nipple
2" galvanized connector
2" x 6" galvanized nipple
2" x 1" galvanized reducer bushing
1" x 2" galvanized nipple
1" x 1/2" galvanized T connector
1" x 2" galvanized nipple
Orbit Watermaster 1" Sprinkler Valve
1" x 3/4" reducer
3/4" x 1/2" reducer
1/2" x 1/4 reducer
1/4" quick connect air fitting
24vAC wall wart ($1.49 - Hosfelt Electronics)
Teflon Tape
Plumbers Wrench
 

Recipe
Well just connect the parts in the order they appear as shown in the photo.  Use lots of tape to stop any leaks.  Experiment with different size openings to get the air blast force you want.  I found that dropping the opening size to 1/2" at the last second created a long straight blast of air that traveled up to 6 feet with force.  This should be a good waist high out of nowhere type of startle.  Even knee high would be great.
 

Supplies:
All parts and fittings - Western Supplies
Sprinkler Valve - Menards
Power Supply - Hosfelt Electronics

Inspiration:
Concussion Canon - Deathlord